Systematics in Palaeontology

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Systematics in palaeontology

THOMAS NEVILLE GEORGE

PRESIDENT'S ANNIVERSARY ADDRESS 1969

CONTENTS Fossils in neontological categories I98 (A) Purpose and method x98 (B) Linnaean taxa . x99 (e) The biospecies . 202 (D) Morphology and evolution 205 The systematics of the lineage 205 (A) Bioserial change 205 (B) The palaeodeme in phyletic series 209 (e) Palaeodemes as facies-controlled phena 2xi Phyletic series . 2~6 (A) Rates of bioserial change 2~6 (B) Character mosaics 218 (c) Differential characters 222 Phylogenetics and systematics 224 (A) Clade and grade 224 (a) Phylogenes and cladogenes 228 (e) Phylogenetic reconstruction 23 I (D) Species and genus 235 (~) The taxonomic hierarchy 238 5 Adansonian methods 240 6 References 243

SUMMARY A 'natural' taxonomic system, inherent in evolutionary change, pulses of biased selection organisms that themselves demonstrate their pressure in expanded and restricted palaeo- 'affinity', is to be recognized perhaps only in demes, and permutations of character-expres- the biospecies. The concept of the biospecies as sion in the evolutionary plexus impose a need a comprehensive taxon is, however, only for a palaeontologically-orientated systematics notional amongst the vast majority of living under which (in evolutionary descent) could organisms, and it is not directly applicable to be subsumed the taxa of the neontological fossils. 'Natural' systems of Linnaean kind rest moment. on assumptions made a priori and are imposed Environmentally controlled morphs, bio- by the systematist. The graded time-sequence facies variants, migrating variation fields, and of the lineage and the clade introduces factors typological segregants are sources of ambiguity into a systematics that cannot well be accommo- in a distinction between phenetic and genetic dated under pre-Darwinian assumptions or be fossil grades. Trends in variants are not to be formulated by a Linnaean nomenclatural confused with trends in transients; and se- method. quential demes may (in time and space) show The fossil evidence of 'affinity' is best served trend reversals as a sign ofnongenetic (phenetic) by a system of classes defining intrinsic relation- selection. The morphospecies of the local ships that are phylogenetic. Multiple demic palaeodeme is then to be distinguished from variants, clinal bioseries, variable rates of the holomorphospecies of the lineage segment,

Jl geol. Soc. vol. xz7, I97X, pp. 197-245, x9 figs. Printed in Northern Ireland. 1

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both in kind and in taxonym; and the associa- disjunction as an incipient sign of cladal tion of variant morphotypes in a unitary deme branching presents other problems in nomen- is no justification for a splitting of the deme into clature and in the definition of taxon-range. several morphospecies. Accelerated and retarded rates of bioserial Criteria of cladal disjunction are multiple. change in sub-parallel lineages further com- They may rest in one or a few 'differential' plicate systematics notably in mosaic evolution. characters, or (in a 'numerical' taxonomy) on A basic distinction between Linnaean (morpho- an Adansonian equal-weighting of 'all' char- typic) and evolutionary systematics rests in the acters. A consequence is to impose on an relevance of the question that asks whether the inferred phylogeny taxonomic incompati- class determines the characters or the charac- bilities deriving from contrasted interpreta- ters the class. tions of homology and homoeomorphy. Clinal

I. Fossils in neontological categories

A) PURPOSE AND METHOD TAXONOMY, the construction of an information-system, is a practical art. There can be as many kinds of taxa as there are purposes for which they are devised; and for its purpose one kind of taxon, based on whatever chosen criteria, is more acceptable than another only because it is more useful, more informative, and not because it enshrines a deeper cosmic truth. Such empiricism is generally taken for granted when emphasis is put on functional reaction rather than on static anatomy: members of a benthic community are classed as epifauna or infauna; stenothermal animals are distinguished from eurythermal; some plants are hydrophytes, some xerophytes, some halophytes. It has nevertheless been regarded as a superficial empiricism, local and tem- porary, if it penetrates no further and does not ask what biotopic elements are burrowers, what animals are heat-tolerant, what plants live in the desert; or, when it asks, does not expect a precise and definitive answer. No doubt, it may be conceded, an individual organism cannot be completely isolated from its environment but has a multitude of responses to external stimulus that partly determine its nature, not least in physiological and metabolic process; but it has for centuries been the analytical practice, explicit or implicit, for its structure to be distinguished from its function, especially when as a fossil it is dead. In an abstraction of organism from context, anatomy and morphology become dominant. Identification, specificity, then presupposes a different kind of classification from the simply functional, and the 'same' kind of organism--a Kantian kind---can be both epifaunal and eurythermal, xerophytic and halophytic. In classical example, fish-like aquatic vertebrates (a class so called already circumscribed on some taxonomic basis) then include many kinds of organisms, not only skates and plaice, sharks and salmon, but also dolphins and sea-sirens; and in a change of method by a shift of emphasis from behaviour and 'adaptive' profile to what is considered to be a more significant morphology, skates and plaice are separately distinguished and linked skates with sharks, plaice with salmon, and x98

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 Systematics in palaeontology dolphins and sea-sirens are separated from all 'true' fishes and linked with dogs and elephants (the nouns, the names of the organisms, the colloquial taxa, presupposing yet other taxonomic assumptions or criteria). (See Blackwelder I964.) The concentration on anatomy and morphology, formally elaborated by Lin- naeus but begun long before his day, implies or seems to imply that there are kinds of classification, expressing principles deeper than those of empirical convenience, it is the purpose of the systematist to construct. The strength of the construct, as it has been assessed in the stages through which systematics has evolved, lies in the principles the systematist invokes to justify his systematic method, notably by referring to the way in which he has not subjectively or arbitrarily imposed his system on his material but has found his material imposing its system upon him. When the method is apposite the system is then discovered; it is not invented; it is revealed as the 'systema naturae', intrinsically superior to other kinds of systems (which may be used to supplement it) by reflecting the inherent characters of the organisms classified. For over two centuries the systematist in his formal search for a 'natural system'--it has sometimes had the air of a search for the philosopher's stone--has followed many paths. The bias of his search has been overwhelmingly neontological, and in great part the systems that have emerged have been expressed in neontological format, geologically timeless. Fossils have found only an incidental place in most of them, as the dead complements of living organisms. They still continue to be forced into the frame of a Linnaean taxonomy ill devised for their reception, although it is notorious that the more they are known in their time- sequence and their facies distribution the less comfortably they fit into any neontological scheme. An analytical appraisal of a systematics appropriate to fossils, in a review of method and principle, is perhaps not misplaced.

(B) i'.INNAEAN TAXA Linnaeus, a hundred years before Darwin, knew only a timeless biology, in which fossils occupied but a minor and incidental part. He laid down the principles of his systematics in the Aristotelian belief, a belief almost taken for granted in its I8th- century context, that his taxa, his universals, represented or should represent the 'natural' affinities and the degrees of affinity of organisms constructed to a Plan it was his task to discover and express (see Mayr x968 , p. 546). His 'natural' system, in so far as it truly reflected the 'natural' affinities of many distinct kinds and groups of organisms, was thus (so he assumed) not external, not artificial and arbitrary; and when he organized his taxa into categories of increasing rank, he regarded the hierarchical system he devised as expressing, however imperfectly, the pattern of the Plan. It is noteworthy that to him the genus was the primary taxon, the major component of the Plan: his species were sharply delineated sets in which the attributes of the genus were variously displayed: the genus determined the characters, not the characters the genus: the genus had an existence independent of the taxonomist, and was real in a Scholastic sense. (See Fig. I.) Linnaeus was a man of his time, at once a leader in his field and a channel for I99

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the currents of contemporary thought; and although he was (for his day) excep- tional in the sustained consistency of his system and of his nomenclatural notation, he formulated what had been maturing in the minds of biologists for many decades, and crystallized a method that was to persist for many decades to come. For a hundred years before 1859 his basic assumptions, his ingrained essentialist idealism, remained unchallenged, while his nomenclatural scheme, particulate and hierarchical in its set theory, rapidly permeated the corpus of taxonomics, finally to become an intrinsic element of a formal and universal biology.

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genus b " . .. ~. , .... ,. q ,_t~-'sb55 familyX tnrneazaqram arrmlry is . sp.'t qeffds n MORPHOTAXA measured by relative proximit,/

F xo. I. Linnaean-type taxonomics. The diagrammatic representation is of the contents of sets in a Linnaean system based on degrees of "affinity' whose subjective assessment justifies the set boundaries. For instance, the cluster of species 1-6 determines the contents of genus a, and cluster x3-x8 of genus d; and correspondingly an "affinity' gap helps to distinguish the two genera. In practice, 'affinity' is not always easily measured, and species 15 of genus d is further removed from species 18 of the same genus than it is from species 6 of genus a. A similar difficulty at a higher level is seen if genus f is included with genus a in family X, and is separated from genus k of family Y; and the justification for including species 33 in one or other of the genera m and n, and for the separate recognition of family Y, is conventional, a pointer (on Linnaean principles) to uncertainty or ignorance of the true bases of 'affinity'.

It is true that long before Darwin many biologists--Adanson, Cuvier, de Can- dolle, St Hilaire, Lamarck---differed from Linnaeus in their manner of interpreting the notional Plan, and thus in the details of their classificatory schemes; but few of them were moved to question the basic assumptions or the class structure of the Plan, or to challenge its particulate nature as seen in objectively identifiable finite delimited taxa. By i859 a Linnaean systematics had permeated biology, and the language of Linnaeus's system had become the language in which all biologists, including palaeontologists, thought and wrote, Darwin with them. Although the whole basis of the system, not only in its gratuitous apriorism but even more radically in its static timelessness, was overthrown when the Plan (so far as a Plan was presupposed) was seen to rest on affinities that were 'essential' only in being evolutionary, the Linnaean taxonomic categories were in their form and their circumscription inherited unchanged by the new regime--and after two

200

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 Systematics in palaeontology hundred years they still remain in their traditional vestments protected in inalien- able possession by the Rules of Nomenclature. (See Cain I958, 1959; Mayr 1968. ) A Linnaean taxon thus has the quality (when it is truly identified) of inherent permanence: it comprises all the individuals or subsidiary taxa that 'ought' in their nature to belong to it. At all levels the systematist's percipience is tested when in assessing the 'natural' affinities of individuals or species or genera he may be misled by partial or superficial similarities that hide deeper differences, and may then be compelled to extend the range or change the balance of the criteria on which his affinities are judged: but that is a measure of the skill of the systematist, not a criticism of the system. When one systematist differs from another in his classification it is because emphasis is placed on different characters as signs of 'affinity' by men who have not yet discovered or demonstrated the 'true' criteria: in due course the dolphin is 'proved' not to be a fish, the thylacine not a wolf. In such a system the meaning of 'affinity' is central. It permeates method, and is embodied as grades of 'affinity' in the taxonomic hierarchy of classes (sets) and in the hierarchical nomenclature. Neither Linnaeus nor any of his pre-Darwinian successors gave an explicit definition of the term, perhaps not needing to do so when they presupposed a Plan of unchanging categories; but what they meant, and what the actual nature of a Linnaean-type taxon is, are clear enough in their practice: the affinities within a species or a genus are recognized by a weighted gradation of characters in discriminatory morphological comparison. In the result, while they differed greatly amongst themselves in the emphasis they put on different characters, and while most of them often linked function to structure in an assessment of morphological importance, they all established morphotaxa by private assessment of the status of 'essential' structural features in the field of a comparative morphology. (See Cain & Harrison 1958.) The technique persists. In the static morphological climate before 1859 each Linnaean-type taxon, created by Linnaeus himself or by his successors, was discrete, isolated, precisely defined, morphologically circumscribed in its nature. In the static timeless climate of post-Darwinian neontological systematics, taxa, however refined, tend to be endowed with the same real existence and the same permanence, and to imply amongst their members an intrinsic 'natural' affinity (which may allusively be said to be phylogenetic but which in fact is rarely so). It must, of course, be accepted that morphology, extended into a comparative morphology with some sophistication to include chemical and genetic morphology, is necessarily a central part of any taxonomic system, which is always a construction on the basis of observed structural data: but how the morphological evidence is manipulated is not compellingly prompted by assumptions, however oblique, that the data themselves determine a priori the lines along which the system is to be synthesized and the kind of taxonyms that 'best' represents them. A major criticism of Linnaean-type systems lies in their implicit claim to a superiority over other kinds of systems in being inherently 'natural'--a claim that, even when it is professedly rejected, is built into the Linnaean nomenclature of a present-day neontology.

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C) THE BIOSPECIES In partial reversion to a functional assessment of 'affinity' (fortified indirectly by genetic-structural support), a currently-preferred kind of neontological taxon 1 is the biospecies. One of the most obvious and directly recognizable signs of'natural' 'affinity' is provided by organisms that themselves declare the affinity in their behaviour; and members of a community that in the natural state freely interbreed to produce fertile offspring may then properly be regarded as belonging to the same species (or conversely, the particular kind of taxon, the biospecies, may be defined as containing such members). Linnaeus and other pre-Mendelian systematists were aware and sometimes took note of the panmictic evidence, even if they had no knowledge of genetics; later neontologists have formalized the method by expressing it in neo-Mendelian terms. The functional criterion is explicit in a mixed community (a biotopic association) of biospecies, when the contrast to free interbreeding is the reproductive isolation, the discreteness, of the species-unit from all other similarly recognized species-units; and the biospecies, thus delimited, may in extended concept be looked upon comprehensively as inherent in the collective gene-pool of all its members (see, for instance, Mayr I969, pp. 25ff.). The reality of the biospecies as a taxon is not be be denied; and it may be accepted that biospecies were equally real and comprised comparable species at any moment in past geological time. Nevertheless, the biospecies, when appeal is limited to the direct evidence, is a highly restricted taxon, both in its kind and in its range, for it includes only the individual deme (cyclically, in the way 'deme' is defined as a gamodeme). Demic range is generally very small in relation to conceptual species range, and although there are chains of peripheral contacts between neighbouring demes in species of extended (and continuous) geographical distribution, the evidence of conspecific unity, of an all-embracing gene-pool with potentially free gene exchange throughout the species, ceases to be direct when demes whose members are not widely migrant are at some distance from one another; and the species is additively integrated by the systematist to include all the demes that inferentially fall into it. It then ceases to have the immediacy of recognition that makes it strictly a biospecies, and the vast majority of individuals ascribed to it cannot be shown to have the affiliation postulated. When species-range is broken by clinal disjunction the 'species' lacks the integration of even an additive continuity of demes. Mayr (idem), while asserting that the boundaries of the biospecies are 'defined objectively', makes the leap from evidence to inference not only amongst widely distributed contemporaneous

1 There are in systematics various kinds of taxa, which should be distinguished. Thus in Lin- naean usage a genus is one kind of taxon, a species another; but they are inter-related, a genus as a set including one or more species, and as taxa they fall into the same hierarchical set of taxonomic divisions. A biospecies is however an incommensurably different kind of taxon from a morphospecies, which is again different from a chronospecies, although in Aristotelian verbal terms they are all species. Canis lupus is one taxon, Canis vulpes another, nomenclaturaUy of the same kind on whatever basis they are identified.

202

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(topoclinal) forms, but also amongst sequential (fossil, chronoclinal) forms-- clinal effects in space and in time being then directly comparable, as Newell on other grounds long ago emphasized--although manifestly individuals that are not contemporaneous can never be shown to interbreed. The topocline of the ring species, however, reveals the dangers in the extrapolation of evidence from the observable to the inferential (see Fig. 2). Thus in the

t9poclinal speciation, classification, and ph,/Ioqen,/ (based on Parus)

/-\\ ,/ ~.. tundra overlaP9inq ranqe with \ / __ "~.,~ little or no h,/bridisation

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desert~\ ' -k _.....L-____~) mounta ns /.,,; ,subspectes ":'' d : ,2'1 ",~ ..... ~.._~-~S~(;dcT&si;;7 ,4:,: . , : j,,'- \~ -~-~-,-,-- ..... I -Zi~ !' i: !i desertX--~h~-brid -zone-i-L-, -~..._ __ "1 '!1' lhlbrid,, ,' zone. Y'1:/see possible phyloqeny: ~'~ "-~:I-,~¢'-!-I I I I-/-T-lq- i I1,...... i.l:l.i .'. /

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FIG. 2. The extended biospecies. The distributional range of forms in continuous genetic profile is physically contained by environmental controls to whose variant selective pressures the divergence of 'subspecies' is ascribed. The clinal gradient may be relatively gentle over wide areas, but where it is locally steep rapid phenotypie change occurs between neighbouring demes in 'hybrid zones'. At the terminals of the long cline, in the zone of overlap between subspecies a and subspecies d, differentiation may be so great as to result in complete reproductive isolation with full speciation. The cline, a product of migration and differential adaptation, takes some time to be established, and the resultant contrasts in the end forms are the products of incipient clado- genie (chronoclinal) divergence precisely analogous to the sequential changes occurring in fossil lineages (see p. 226), although the details ofphyletic branching may be unknown.

well-known example of the great titmouse (Parus) the continuity of range from eastern Asia to western Europe is accompanied by a geographical differentiation of 'subspecies', recognized not on functional-genetic characters but on morphological characters, that reflects differential selection pressure (in locally 'adaptive' features of proportion and plumage) expressed in a series of forms considered (because of the grading) to be all of one biospecies. But only the accident of rapid post-Glacial migration of European forms to China, where in a recently established zone of overlapping range the will and the ability to interbreed may be

203

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 T. N. George observed, allows the inference to be tested. As it happens, hybridization of the 'subspecies' (European major and Chinese minor) appears to occur in the zone of overlap, but on a much reduced scale. Since it is not completely extinguished, major and minor are technically members of a single biospecies, if only as genetically peripheral members between which gene-flow persists although it is no more than a trickle. In the ring species of the gulls, on the other hand, the terminal members (Larus argentatus and Larusfuscus) of the chain of 'subspecies' in the northern hemisphere do not interbreed in their zone of overlap: they are on the criterion truly different biospecies, although they display unbroken topoclinal intergradation. Moreover, there are no natural means of determining if the graded differentiation of the two species is not pulsed by intermediate biospecies, truly distinct on the criterion from both argentatus and fuscus; nor is there a measure of correlated morphological difference on which to assess the onset ofbiospecific difference. Any long topoclinal gradient, whether it closes as a ring or not, is likely to display wide differentiation by distance, and to present the taxonomist with the same uncertainty, through a lack of either functional or morphological evidence, in identifying successive biospecies and in applying a specific nomenclature to them. It is not easy, therefore, to accept Mayr's contention that the palaeontologist is misguided if he does not apply the concept of the biospecies because he cannot test reproductive isolation in fossils. Mayr (i969, p. 28) has suggested the possibility that biospecific discontinuity (as reproductive isolation) is in living organisms accompanied by a correlated morphological discontinuity, and that the 'competent systematist' can place in parallel the two modes of classification by inter- changing or alternating his criteria; but, as Parus and Larus demonstrate, there is much uncertainty and no explicit sign to help him in his doing so. It is conceivably possible for him, with diminishing assurance, to transfer the biospecies concept to fossils in a second-remove extrapolation of morphological analogues from living to fossil organisms, when the living and the fossil forms show only minor morphological differences (that is, differences not going beyond those found or expected in living nearly-related graded or topoclinal biospecies) (compare Walker 1969, p. 8). But, even if the tenuous validity of the morphological parallel is granted, the linked criteria of sterility and structure, which give the method its only strength in a separation of one species from another, are wholly excluded from application to fossil forms that are not closely analogous (at a sub-generic level) to living forms whose biospecific relations have been genetically tested. In practice, the method fails to touch fossils almost all of which lack such close living analogues. A definition of a 'natural' species in terms of the biospecies, however acceptable it is for living organisms, is therefore effectively outside the province of palaeontology; and while it may be fully conceded that every fossil once fell into its own biospecies, and that morphologically recognized conspecific forms amongst fossils may well have fallen into a single biospecies, the inferred biospecific relations are secondary, derived and consequential, and do not provide the credentials of the fossil taxon. It is true, in an epitome of a genetic system, that 'twins are not twins because they are similar, but are similar because they are twins'; but among

204

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(D) MORPHOLOGY AND EVOLUTION Palaeontology complements neontology as a branch of biology. Fossils were once- living organisms: how knowledge of their relationships is organized, how they are classified and named, must conform to modes that reciprocally incorporate a neontological systematics. It is salutary and instructive to read again the Presiden- tial Address given by Bather (1927) just over forty years ago, in which under the title 'Biological classification' he attempted to reconcile a static with a dynamic system, and to penetrate a future in systematics that would emerge out of a Linnaean into a Darwinian world. His interest was in the impact of phylogenetic theory on a timeless Linnaean scheme of discrete categories; but the encumbrances he found in trying to graft the continuity of evolutionary change onto a compart- mentalized morphology are shown in the ambiguity and ambivalence that run through his text both in the way he thought of his fossil groups and in the traditional terms he used to discuss them. Many of his difficulties remain. He could (if he were alive) still insist that the effects of an amplifying fossil record should impinge more directly on a systematics that does not yet take geological time adequately into account or accommodate the continuum of lineages whose secular changes (and only incidentally whose momentary neontological ends) are the stuffofwhat has to be systematized. He could still point out the need to analyse much more deeply the implications of phylogenesis in classification and nomenclature, and to explicate the transition taxonomics of I859 in terms of the grades and stages of evolutionary series---even if the manner of the analysis must continue, until the transition is made, to be riddled with a Linnaean terminology and bemused by its theoretical overtones.

2. The systematics of the lineage

(A) BIOS RIAL CH GE A living organism is an intcgratcd unit gcnctically bclonging to its dcmc and activcly adaptcd to its ccodcmic cnvironmcnt. As an individual it is mcrgcd in its dcmic matrix, and is itsclfnot of taxonomic significancc unless it is incidcntally sclccted in typological mcthod as a samplc of its population. A fossil organism is ncvcr found as an intcgratcd unit, and ncithcr its gcnctic structurc nor its dcmic affiliation can be dircctly dctcrmincd. As an individual it is a mcmbcr of an cvolving lineagc whosc clemcnts arc scqucntial dcmic generations in which thc individual is anonymous, and it is of cvcn less taxonomic significancc than its living countcrpart. The difficultiesof linking the individual fossilor thc individual palacodcmc to its lincagc are notorious. Comparative morphology is a slender thrcad on which to 205

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hang ancestor-descendant relationships if the time-sequence ofmorphs is not known (and it is even weaker when contemporaneous living forms are compared with one another on a neontological scale of 'primitive' and 'advanced' characters). The palaeodeme of dead fossils lacks the visibly interacting association of members of a live population and rests only on a thanatocoenous proximity of similar forms. Changes of rock-facies are accompanied by the migration of lineage segments which then are insecurely identified as belonging to the lineage. A lineage as an uninterrupted chronoclinal deme-sequence is thus conceptual, never actual; and to use the term 'deme' for a close association of similar fossils is to transfer to a palaeontological construct the attributes of a neontological entity without proof of the propriety of doing so: a palaeodeme is acceptable as a demic analogue only because in morphological synthesis it normally falls into a unitary variation field like most living demes. In a statistical (biometrical) assessment of the palaeodeme and the lineage, a convenience is the use of characters ('biocharacters') abstracted from the structural elements of the unitary organism because they can be measured. Some of the characters may be closely correlated (length of femur and length of tibia in tetrapods), others may show only low correlation (number of ribs and length of shell in spirifers) ; some of the characters (as they are phenetically expressed) may vary little about the mode presumably because of strict genetic control (stipe growth in monograptids), some may be 'plastically' responsive to environmental pressure (shell form in oysters); but what emerges from the study of every sufficiently well-known lineage is that palaeodemes in sequence show sustained shifts in variation field and mode of each biocharacter, and that evolution may be regarded as the composite summation of the shifts with time (see, for instance, Ziegler I966 , fig. 5, P. 534). (See Figs. 3a,b.) The shifts place the character-stages in bioserial sequence. In a lineage sufficiently prolonged, later stages may show a variation-range overlapping insignifi- cantly or not at all the range at an earlier stage; and in the bioseries no single individual at one stratigraphical horizon may be similar (for the isolated character) to any individual at another horizon--as stages I, 2, and 3 of Fig. 3 a illustrate. The contrasts may then appear to justify the giving of different names to the

FIO. 3. The complexities of the lineage. The single biocharacter may show smooth gradation with time (Fig. 3a); or in very rapid change over a short time-span (Fig. 3b), not adequately represented by closely successive palaeodemes, it may appear to display discontinuous seriation and to imply cladal branching. When several characters are used in palaeodemic definition, the field of variants may be wide (Fig. 3¢) if there is low correlation between the characters, and may offer inducement to the 'splitter' to break the unitary deme into groups (m and n) along any arbitrary line. If there is high correlation (Fig. 3d) the tight variation field offers no such inducement. The composite variation field based on a number of biocharacters migrating through time is the lineage (Figs. 3e, 3f)- When after a lineage interval there is little or no overlap of variation range in bioserial members or in palaeodemes, the continuous sequence may be broken into arbitrary stages ( I, 2, 3 of Fig. 3a) or species (r, s, t; x, y; x, z of Figs. 3e, 3f)" 2o6

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(a) stead,/ seriation pulsed seriation stage 3 ../'~ stage3~'~ N ..,,"x, T /'-'x ~/-"N,,~t aqe 2 //~stage 2 I

/'-'x stage I j//"'~taqe I bioserial stages bioserial stages

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stages--a practice that is helped if there should be gaps in the preservation of intermediate stages in a disjunct lineage. Usually a 'normal' rate of sedimentation in the rock-facies to which the lineage is adapted results in a spacing of bioserial stages; but a condensed stratal sequence in which evolutionary change appears to be accelerated may result in a very rapid replacement of stages; and when the condensed sequence reaches the limit of non-sequence and the lineage becomes disjunct the replacement of stages may appear to be saltafive in a deceptive 'macromutation', as Brinkmann's ammonites long ago showed (see Fig. 3b). Biocharacters in combination, in integrated bioseries, give a comprehensively multidimensional solidity to the lineage. If the characters are weakly correlated (Fig. 3 c) the scatter of variants is wide and is an inducement to the taxonymic segregation of morphotypes (m and n) and to the artificial clustering of morpho- typic forms in differentiated 'species' all comprised within a single palaeodeme. If the characters are closely correlated (Fig. 3d)--the degree of correlation being observable but not explicable in phenotypic fossils whose genetical structure cannot be analysed--there is less inducement to taxonomic adventure; and when the correlation is unity, the graphical representation then being linear as the variation field is reduced to nil, one individual looks exactly like another and there is but a single morphotype to embrace all the members of the palaeodeme. Difficulties of nomenclature arise when the continuity of the lineage is fairly complete and the successive palaeodemic variation fields are adequately known (Fig. 3e). With sustained modal shift, and when palaeodemic variants are not too widely scattered, overlap of the variation fields is reduced to nil and no member or a later palaeodeme falls into the same morphological set as any member of an earlier. In compulsory Linnaean heritage the two palaeodemes may then be said to fall into different 'species' (or, arbitrarily, when there is partial overlap of variation fields, into different 'subspecies'). This is taxonomic confusion. The definition of a taxon in the continuum of the lineage cannot fit into a Linnaean-type system, whose criteria apply to timeless particulate sets. If such randomly named 'species' are separated in a lineage sequence, the intermediate transient palaeodemes find no place in the classification or the nomenclature, although any one of them has equal claim with any other to taxonymic recognition. The lineage as a continuum matches in every detail the extended cline of a biospecies (which as a cline of expanding range is itself a brief lineage); and although it lacks genetic criteria it combines with the cline in revealing the utter incongruity of applying a Linnaean-type systematics to organisms in either chronoclinal or topoclinal sequence. The incongruity has long been recognized and understood (see for instance George I953, p. 3o; George 1956; Westoll 1956) but it is an incongruity that will continue to confuse systematics so long as a Linnaean taxonomic system is imposed on fossils by legislative decree. A cladogenic branching of divergent lineages adds further nomenclatural complication if Linnaean-type taxa are erected to include the cladogenes. So long as an overlap of variation fields persists between the potentially independent series differentiation is not complete and collateral and ancestral palaeodemes may 2o8

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 Systematics in palaeontology be considered to fall into the same species; 1 but when there is no overlap the collateral divergents, morphologically (biometrically) segregated (and in so far implying full allometry), belong to different species established to denote the biometrical differences. * Thus in Fig. 3f, deme 7 and deme I I are initiating divergence, but they have a wide zone of overlap that does not warrant full specific distinction; whereas their respective descendants, deme 8 and deme I2, are biometrically circumscribed independent morphotaxa which considered alone could be called separate species. But in chronoclinal and not collateral relationships both deme 8 and deme I2 overlap in variant range their common ancestors that go back to deme 6; and a single chronospecieswspecies x--includes deme 6 and deme 8, and deme 6 and deme 12. Not until divergence has been long sustained can species y and species z (as cladogenic and sequential taxa) be unambiguously distinguished from one another and from species x.

(B) THE PALAEODEME IN PHYLETIC SERIES A thanatocoenous palaeodeme, a local phenon, is an accident of preservation, its members winnowed usually from a much larger clinally extended demic association by environmental factors including ecological controls on the living organisms (some of the controls still recorded in the rock matrix, many of them briefly ephemeral and unrecorded) and depositional controls on the manner of fossilization and preservation. Such a palaeodeme (almost every palaeodeme) is thus a sample-- it may be a large sample, but always only a small proportion of the forms that might have been preserved--of a notional phenon biometrically defined by the range of extreme variants along its biocharacter-axes. The field of variation of any deme, plotted by as many biocharacters as lend themselves to measurement, contains all the demic members, most of which are clustered (but not necessarily symmetrically) about the mode (Fig. 4). If the biocharacters are not closely correlated, however, any one individual may lie near the mode on one biocharacter-axis, and far from it on another; and if the axes of variation may descriptively be regarded as extending from the mode in negative and positive directions (without any imphcations that negative means 'retarded' or positive 'advanced' in stages of character development, or of evolutionary trend), the individual may be on the positive side of the axis for one character, on the negative side for another. It may then synthetically be regarded as the sum of its properties at its loci on all the character axes. In such depiction of a palaeodeme, the pictograph is formalized to illustrate variant distribution, and rests on factual data provided preferably by direct measurement, but at least by objective assessment. It is not a device for promoting

1 To avoid circumlocution Linnaean taxonomic terms ('species', 'genus', 'family') are constantly used for convenience, although a main purpose of the address is to criticise the applicability of Linnaean concepts and taxonomics to fossils: but the varied usage of the terms, commonly non-Linnaean, should be evident from the contexts in which they occur. z A biometrical (and nomenclatural) distinction between species does not demand that the overlap of palaeodemic fields of variation shall be nil. Any perscribed degree of limited overlap may arbitrarily be used as a measure. Nil overlap is illustratively convenient in the text.

209

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hypotheses and is not a construction to express desired relations between its constituents. Consequently for any individual member of the deme there are as many point-loci on the pictograph as there are biocharacters measured; and the pictograph carries in multiple representation a biometrically synthesized deme whose individual members are anonymous. No individual member, even if it lies near the mode on most axes, is therefore more than an allusive illustration of the demic characters, or can except in unfounded and subjective emphasis be regarded as a 'type'. The deme as a phenon, an integrated association of members the forms of which are a product of genetic potentialities and environmental encouragement, may then be expected to display changing fields of variation in response to increasing bias in selection pressure. If in a notional field of possible variants the bias increasingly encourages some and discourages others, there will be a shift of modal cluster in the favoured direction along the biocharacter-axis, although there need not be a reduction in variant range (Fig. 5). If the less favoured phenotypes suffer intensified discouragement---if, that is, they are 'plastically' moulded along preferred lines in ontogeny or extinguished before maturity--the variant range is reduced by the elimination of extreme forms, and the phenon becomes more re- strictedly uniform. Collateral demes, of broadly the same genetic constitution and the same variant potentialities but clinally at a distance and then subject to differential selection, may in biometrical terms be significantly different in the balance of variants they include and in the range of variants along the biocharacter-axis. Under stringent differential selection they may be represented by contrasted variation fields having little or no overlap and prescribing what appear to be separate morphospecies (a3 and a4 of Fig. 5), although none of them need contain a variant form that is not potentially present in the 'standard' field of species a.

(C) PALAEODEMES AS FACIES-CONTROLLED PHENA The 'standard' variation field is conceptual. But while no environment in nature has such an extreme of tolerance that it allows the maturing of an indefinitely extended range of phenetic variants in a deme, additively the summation of a multiplicity of variants in rapidly fluctuating environments gives some approxi- mation to the generalized field of available forms amongst which local and tem- porary selection can operate. Such fluctuations are, at least in some facets, well seen in cyclothemic deposits in which a sequence and a recurrence of intergrading facies are characteristic. The Carboniferous rocks, in which phena of anthracosiid mussels are relatively abundant, provide fruitful examples of both process and method, exemplified particularly in the work of Eagar (for instance, I953, I960) and recently systematized by Weir (see i968, pp. xxxviii-xlv). The subtleties of environmental control on phenotypic development are at best difficult to determine in present-day biotopes; they are virtually impossible to determine in the remnants of palaeo-biotopes seen in thanatocoenous assemblages, when little more evidence remains to be interpreted than can be got from the lithology of the rocks of which the fossils are constituent elements. In such an

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VARIATION FIELDS AND ECOLOGICAL SELECTION OF PHENOTYPES

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anthracosiid environment as the Carboniferous swamps it is probable that lithological contrasts between bottom muds and silts were far less important to the mussels as agents of selection than the physical and chemical factors of the water regime; and while the petrological and geochemical properties of the rock matrix give some hint of environmental differences they never offer a direct 'explanation' of the forms of demic variants or of the modal shift in variation fields. Nevertheless, the essence of the ecodemic change, correlated with environmental change, is conveniently analysed in a model that assumes a linear correspondence between a phenotypic cline and the graded operation of a single environmental factor. An example is to be found in the brief marine incursions that periodically interrupted non-marine deposition of the Ammanian Coal Measures in cyclothems implying pulsed subsidence. The factor, as the marine bands illustrate in their lateral variation, was not of the same intensity or duration either over the whole region of its influence or at any one locality during successive rhythms; but for the purposes of the model it may be supposed to have had similar effects on local palaeodemes (of the same or different ages) when its impact was of the same order of intensity. Thus, if the factor was salinity, a range from high (sea-water) to low (fresh-water) as successive beds were deposited would be accompanied by a train of variants (phenotypes) revealing a biometrical modal shift in response to the environmental change. Similar effects may be postulated for each of many other factors--sediment grain-size affecting burrowing habit, for instance, or food- supply, or water turbidity, or temperature, or currents, or aeration, or depth. In an actual stratal sequence 'all' factors no doubt contributed in varying degree to influence and direct the net selection of preferred phenotypes, and it is un- likely that they did so in exactly repetitive relative strengths in successive cyclothems. Factorial change in the model (Fig. 6) is thus oversimplified. It is recorded in two-dimensional histogram form as a biased selective preference at any one horizon for some variants as against others (compare Fig. 5), the suppressed variants at the same time remaining 'available' in gene-pool store for selection when the impress of factorial change is reversed (when the next cyclothem is initiated by whatever means). The pattern of successive palaeodemes within any one cyclothem thus shows an oscillation in fields of variation from one extreme of phenotypic concentration to another. There are obvious consequences that throw light on taxonomic techniques adopted to systematize notably the anthracosiids but also other groups of variable fossils; and the consequences probably affect all evolutionary series.

F I G. 5. Differential selection. A tolerant environment allows a wide spread of variants (as in speciesa), which with a tighter selectivity is likely to be restricted according to a biased discrimination. No two environments at a distance are identical: contrasted preferential selection may then result in variation fields of two palaeodemes of the 'same' species having little or no overlap, and in a consequent morphological distinction between 'species' a3 and 'species' a4. But while comparison of a3 and a4 in isolation supports the distinction, all the variants of both 'species' fail within the variation field of a.

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In the illustration (Fig. 6a), three species (a, b, c) may be recognized if independence of variation range is a criterion. They are morphospecies of which a and c (in their extreme palaeodemes) doubly justify their separate naming in being very useful stratigraphically. The 'intermediate' species b, a long-range form, is less useful, with recurrent palaeodemes falling into its field in successive cyclothems (deme 19 is virtually identical with deme 3)- Demes (II, 14, 2i), that partly overlap the range of a strictly delimited species a, may be referred to 'subspecies', or, if the trinomial is to be avoided, may be linked with the species by the use of 'aft.' (implying lineage-sequence). Other demes are less happily 'identified'. Deme 5, for instance, overlaps in variant range both species b and species c; and deme 16 similarly overlaps a and b. Each may then be said to include representatives of two species, and (as in Fig. 3 c, p. 2o7) the unitary variant field may be split down the middle to accommodate the nomenclature. And at any one horizon (Fig. 6b), with intensification of factorial influence in one or other direction, topoclinal change matches chronoclinal, and localities sufficiently distant may be typified by members of different 'species' as signs of ecological differentiationmspecies as biometrically (morphologically) distinct as those in stratal sequence. The model is over-simplified. A multitude of factors operating jointly and per- mutatively determines the characteristics of the palaeodeme at any one locality, and in refined biometric measurement the individual deme is likely to be unique-- there is never an exact repetition of circumstances in the chronoclinal environment or an exact repetition therefore of demic variant constitution. Moreover, a second effect of environmental selection, not observable in fossils but without much doubt always emergent, lies in changes in the demic gene-pool, both in genetic proportions and in genetic mutations, and the selective preference of changing environments becomes continuously modified as the pattern of available variants changes with time; for there is, as all extended clines of living organisms show, a genetic differentiation amongst contemporaneous forms despite free gene-flow diluted only by distance. Fossils add a further complication in rarely being abundantly represented in every bed of a continuous stratal sequence; and one of the effects of too great a change in the local environment is a chronoclinal disjunction of the lineage through emigration. There is then no assurance that returning

F I o. 6. Differential selection in a rhythmic sequence. A repetition of discriminatory environments, characteristic for instance of the Carboniferous cyclothems, operating in rhythms of biased pressure on potentially widely variant palaeodemes, suppresses some kinds of variants at one moment, encourages them at another. It is then possible, by isolating particular palaeodemes whose variational fields show little overlap, to distinguish one 'species' from another (a, b, c) because of their morphological independence. When there is recurrence of the 'same' environment, there is recurrence of the 'same' species: demes I i, x 4 and 21 all 'belong' to species a, 3 and x9 to species b. When morphospecies are thus founded, demes of overlapping variation range then include members of more than one species (deme 16 contains members of a and of b; deme 5 ofb and oft), despite the inherently unitary character of the deme. The same kind of differential selection can operate when lateral (topoclinal, facies) change is the analogue of cyclothemic change (Fig. 6b).

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 T. N. George demes are in direct descent from any earlier identifiable deme, or that phenotypic similarities between diachronous demes are anything more than analogous. Nevertheless, the model illustrates evolutionary mode. Its fictional simplicity should not hide the likelihood that in a fossil group like the anthracosiids, which in their Ammanian evolution permute character expression through no great range, the recurrence of similar forms, or the minor differences between successive and collateral demes, or the overlapping variant range of one deme with another, or the patterns of pictographic variation-fields, reveal in part the net effects of a multifactorial selective environment on potential demic variants recurrent in their availability for moulding and selection in broadly recurrent rhythms. Evolution in the comprehensive lineage sequence is then analytically to be distinguished from phenetically fluctuating variation; and trends in evolution are not the long-term equivalents of trends in variation on biocharacter-axes. The paths that evolution follows are strewn with locally encouraged palaeodemes of very many kinds whose differences and similarities are ecophenotypic and only incidentally chronoclinal, although they are the potential sources from which sustained lineages may emerge. The taxonomic organization of such variants and transients is not easy on traditional lines. If each palaeodeme is regarded in isolation as biometrically defined by its parameters, it can be given a species name precisely of Linnaean type; and as a morphospecies it may in Linnaean fashion be precisely distinguished from any other deme differing, however slightly, in variation field. But such a practice leads on the one hand to an endless regression in refinement, almost every palaeodeme, no matter how imperfectly preserved it may be, being biometrically distinguished from any other; or on the other hand to an internal splitting of demes (deme 5 of Fig. 6) that makes the morph and not the deme the criterion of reference. The sequence of demes in this kind of lineage (which differs in no fundamental way from any other kind of lineage) frustrates, except in mechanical application, the use of Linnaean-type species in taxonomics, and emphasises the impropriety of Linnaean principles in attempts to bring system to their classification (see George 1956, p. 134; Pastiels 1964).

3. Phyletic series (A) RATES OF BIOSERIAL CHANGE Most biocharacters, except those closely correlated, show independent rates of bioserial anagenesis; and many lineages, perhaps all, show staggered progression because their characters display differential evolutionary rates. A simple change in allometric gradient, a relative acceleration or retardation of character development (commonly in matched ontogeny and phylogeny, as Robb's illustration of the equids well revealed), is exemplified in loss of fused septa in amplexoid corals, in degree of coiling in nautiloids, in elongation of shell in burrowing bivalves, in acquisition of ribs in spirifers, in angularity of cone in snails, in migration of mouth in heart urchins. There are few, if any, instances in evolution of the sudden appearance of wholly novel 'macromutant' characters. Except in a persistent rigid demarcation of morphic elements as taxonomic differentiae--a relic of Linnaean 216

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ONTOCENETIC AND BIOSERIAL,ALLOMETRY F I O. 7. Rates of change in bioseries. Differential rates of evolutionary change in as- sociated integrated biocharacters--an aspect of allometric growth--are characteristic of most lineages. They are expressed in migrant variation fields which, in the diagram, indicate fairly close ontogenetic correlation between the biocharacters a and b. Illus- tratively, a shows evolutionary acceleration relative to b. In ancestral palaeodeme I, b develops relatively faster than a. In its descendants, with relative acceleration in a, there is a rectilinear (isometric) relationship between a and b in deme 2, and a develops faster than b in deme 3. Profiles, in shell-shape for instance, exemplify the changes: morph X illustrates a sample of deme 2, morph Y of deme x, both morphs having the same early-neanic stage in ontogeny.

attitudes--the cline as a demic sequence of intergrading forms, collateral or sequential, exhibits in every detail the signs of differential rates of biocharacter change. (See Fig. 7.) The lineage as a synthesized integration of bioseries reveals the independent rates, especially when there is alternating parallelism (an aspect of 'programme') in iterative evolution, with divergence in the parallel stocks marked by one character, convergence by another, in contemporaneous onset (see George I962, p. 46). The alternation is itself indirect evidence of the repeated emergence of variant forms hidden in the potentialities of the gene-pool and awaiting selection by an encouraging environment, as well as of the independence of the bioserial rates of change. The independence also gives meaning or explanation to the multiplicity of stocks evolving from common ancestral forms that provide the evidence for adaptive radiation (see p. 226), and to the involutions of form, process, and taxonym that are embraced by the concept of mosaic evolution (see p. 220).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 T. N. George The lineage when there is a steadily gradational demic sequence is not readily divided into segments of distinctive kinds (species) especially when a tolerant environment allows a wide range of variants at successive horizons; but 'accidental' breaks (preservation failure) in the continuity of the lineage may result in conveniently segregated contrasts in variation fields to allow 'species' without overlapping variants to be created (and, if as in neontology the gaps are large enough, genera, families, orders, classes). The criterion for taxonomic distinction in such circumstances is manifestly subjective--the size of the gaps. The 'lumper' is doing exactly the same thing, adopting the same principles, as the 'splitter', the only difference between them being in the coarseness of the nets they use when the 'lumper' is prepared to accept a longer gradient between his taxa than the 'splitter'. The convenience of the gaps is wholly negative. If there were no gaps the continuum of the preserved lineage would impose upon the taxonomist the need to enlist other criteria if he should wish to name lineage segments in an artificial fragmentation of the continuum (including the lineage segments he is happy to name when preservation failure provides him with the gaps to ease the fragmentation). Even so, when the gaps are small (Fig. 8a) the range of variants within a species may be greater than the difference between extreme variants of neighbouring species (species c and d, or g and h, for instance, of Fig. 8a. The continuity of the unbroken lineage, or of the broken lineage displaying only comparatively small gaps between the species, gives no opportunity for a further taxonomic fragmentation, and all the species belong to a single genus (W of Fig. 8a). Sets of species, seemingly abruptly segregated by pulsed bursts of bioserial anagenesis, promote genera that give the impression of being 'natural' in the self-evident reason for their establishment (Fig. 8b). The gryphaeoid oysters described by Hallam (1968) well illustrate procedure. Grades of coiling, relative proportions, and the development of a sulcus, are convenient measures of both lateral (ecological, topoclinal) variants and sequential (chronoclinal) transients, that allow a taxonomic distinction to be made between successive species (or, when the transients are only slightly different, of subspecies), with repeated support for the taxa being given by geographical or chronoclinal (zonal) gaps. The sudden (or at least geologically abrupt) change in form including incurvature and shell thickness, from the flat earliest-Liassic oysters assigned to the genus Liostrea to the later coiled forms assigned to Gryphaea is postulated by Hallam to reflect a single genic mutation environmentally encouraged in adaptive response of the mutants to stability in a muddy matrix; and the suddenness of the change combined with a sparsity of intermediate transients is the justification for separating not only the species Liostrea irregularis from the species Gryphaea durnortieri, but also the genus Liostrea from the genus Gryphaea. On his premises, Hallam's taxonomic system is exemplary.

(B) CHARACTER MOSAICS It does not need to be said that the kinds of categories that emerge in any ordered system reflect the bases on which the order is either discovered or imposed; and the terms used no less carry their implications of theory and are not always 218

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BIOSERIAL RATES OF CHANGE AND CLASSIFICATION

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V,- ~species Q A B C D E F G H L M N 0 P 0 R S bioch~racters biocharacters (a) (b) rnnnor variations in rates of bioserial chanqe pulsed bursts of bioserial chanqe F I o. 8. The lineage as linked bioseries. Independent rates of bioserial change in the lineage are illustrated in Fig. 8a, in which D is sluggish in comparison with the other characters; F shows relatively rapid anagenesis to species f and then slows down; C shows retardation, H shows acceleration, between species d and e. When, in Fig. 8b, one or other biocharacter shows exceptionally rapid acceleration over a brief interval, especially if intermediate stages are poorly preserved fossil, successive species (m and n; p and q) appear to be widely distinct in a biocharacter (M, Q.) that may then lend itself to use as a 'differential' or 'diagnostic' character in the separation of major taxa (genera in the diagram). The gaps in the diagrammatic lineage sequence are intended to represent gaps in the fossil record: they allow ready segregation of successive demes into species and genera. The widths of the species-bands are intended to indicate the morph ranges in the lineage-segments preserved.

readily transferred from one kind of system to another. The recognition of differential characters, and the applicability of the Aristotelian term 'differentia', pre- suppose the inherent 'natural' categories of a Linnaean-type system and are (in the experience of the Linnaean systematist) in neontological heritage. Implicitly they put out of court the view that the crossing of a taxonomic threshold is no more than the crossing of a verbal hurdle: rather, they insinuate that a differential character, if (at whatever level) it is truly differential, is to a degree a barrier to free evolutionary passage from one taxon to another (see Simpson I96I, p. 56). Thus the much-discussed definition of vertebrate classes has been riddled with assumptions that, in using terms now unquestioned if not sanctified, veneer and penetrate theory even when they are repudiated; and Linnaeus was right, at least in the perpetuation of his dictum, when he said that the taxon determines the characters by which it is identified. So, in neontological usage, organisms are classed as reptiles because they have the differentiae of reptiles. Mammals and 219

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 T. N. George reptiles are mutually exclusive classes, as their respective characters reveal to the experienced eye. Most aquatic vertebrates are fishes not because they swim: they swim because they are fishes, swimming being one of the differentiating characters of fishes. The hidden expectation behind such cyclical taxonomics lies in the systematist's having or needing to have an esoteric acquaintance with the characters, the differentiae, of an already known or an already existent class. In a post- 1859 world the expectation vanishes, but the neontologically-founded class names continue to inject an ambiguity into discussion, notably in the concept of mosaic evolution. The mosaic is the association in the single organism of characters of differentiated classes: Archaeopteryx has the feathers, the wish-bone, the pelvis of a bird, but the long tail, the sacrum, the metapodials, the teeth of a reptile; a seymouriamorph would be regarded as a reptile because of its single condyle, its clavicle, its pha- langes, its vertebrae, if it did not also have the otic notch, the skull-bone pattern, the lateral-line system of an amphibian; an ichthyostegid possesses the tetrapod pentadactyl limb, the interclavicular bone, the free skull-hinge of an amphibian, but is a fish in the lepidotrich tail-fin supports, the opercular bones, the lateral-line canals. Each of these forms combines in itself some of the differentiae of two classes, and is both bird and reptile, or reptile and amphibian, or amphibian and fish, a mosaic of additive characters got from two morphological sources (see, for instance, Carroll I97O , p. 3o3). In an evolutionary context the reality, or a description of the reality, is very different. The long lineages from fish to mammal are very imperfectly preserved; but is is to be supposed that if they were fully known they would be seen to be gradational in a multiplicity of bioseries showing the sort of variable and partly independent rates of change that mark most phylogenetic stocks. The expectation always is that at every stage of anagenesis some characters are relatively accelerated, some retarded; and if the retarded characters are called amphibian (the characters then defining the taxon), and the accelerated characters reptilian, that is a matter of convention and elicits neither surprise nor special mention. Thus Kermack (i967, p. 24i ) dissected the multiple factors and grades of evolutionary change from Mesozoic reptiles into mammals, in characters including mandible, ear-bones, jaw-suspension, teeth, palate, by arbitrarily defining a mammal as a tetrapod characterized by a jaw with dentary-squamosal hinge, no matter what other 'differentiae' (no longer used for differentiation) a particular form may exhibit; and similarly Carroll (I97O , p. 3o6) regarded the palate as the only satisfactory differentia that can be used to separate amphibians from reptiles. (See Fig. 9.) (See Carter I965; Ktihne i968. ) The recognition of mosaic can be extended both upwards and downwards on the hierarchical scale. Vertebrates are sometimes considered to carry relict signs of echinoderm larvae; persistent annelid features characterize the arthropods; some late-Cainozoic nautiloids (Aturia) have 'ammonoid' septa; Hipparion is a Pliocene equid with complex hypsodont teeth but with three functional toes in each foot, Thoatherium is a Miocene monodactyl litoptern with simple brachydont teeth; the platypus is a mammal in its milk glands, a reptile in its oviparity; Azyogograptus

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has a pseudomonograptid its single stipe, a pendent dichograptid first theca; Brachythyris ovalis is a brachiopod with the ribs of a spirifer but the short hinge of a martiniid. Any organism that displays a combination of'primitive' and 'advanced' characters in its anatomymalmost every organism in more or less degreemis a witness to this descriptive kind of mosaic evolution; and, in an Aristotelian sense, there are no two kinds of characters, one differential, the other accidental. In complementary phrasing, differential rates of bioserial anagenesis lie at the root of cladal branching; and every clade is a mosaic of grades. The character mosaic in Archaeopteryx or Ichthyostega is a neontological incident. The diversified lineages that make present-day organisms the divergent terminals of a host of clades allow the sharpest distinction to be made between the major taxa; and so long as taxonomy remains under neontological domination the taxa are truly described as differentiated by absolute criteria. But the criteria are absolute by definition, only because of the static timeless nature of neontological comparison. When the lineage is animated by time and becomes a continuum of integrated time-transects of an evolutionary plexus, there is a fading of the absolute criteria as they are absorbed into the gradualness of the transition from one class to another. In a 'struggle' for survival Seymouria, scarcely class-conscious, found only a nominal threshold to cross in order to become a reptile. This is yet further comment on method: time-transect taxonomy of the momentary present is not compatible with chronoclinal taxonomy, not merely because the taxa of the one kind are differently measured from those of the other, but also because the principles on which the taxa are established are not transferable from the one to the other.

C) DIFFERENTIAL CHARACTERS A second kind of differential criterion is illustrated in Fig. 8b. It rests in the sudden, almost explosive, appearance of what seems to be a novel character because of the speed of its emergence. Many groups, from genera to classes, give the impression of originating in such an explosive way, even to an encouragement of their being regarded as genetically macromutant. The speed of change and the magnitude of difference between species m and species n along bioseries IV[ in the figure, and between speciesp and species q along bioseries Q, provide the means of allowing the novel character, or at least its advanced grade, to be used as a differential, especially if the character, once it is established, sinks back into a conserva- tive slowness of change in later stages of structural adjustment to neighbouring bioseries. In a sustained attitude towards the appearance of novelty or of large leaps in evolutionary series, Schindewolf has spoken for a number of palaeontologists in contending that taxa are best defined at boundaries placed in the bioserial diastems or at the bioserial leaps (and in this phyletic way he has given yet another interpretation of a 'natural' system). Taxonomic weight given to a single differentia may make it the dominant character to which all other characters are subservient. Shell structure in brachiopods, for instance, has sometimes been elevated to a prime taxonomic place, notably by G. A. Cooper, at other times relegated to a generic or even specific

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incidental when it has seemed not to be accordant with other characters in its morphic context. Thus Williams (1965, p. 4 I2; in Williams & Others 1968 , pp. 47, 55), returning to Cooper's principles, revised the Triassic genus Thecospira in its ordinal allocation, transferring it from the strophomenids to the spiriferids. The genus has an anomalous association of characters including spiralia on the one hand, and a pronged cardinal process, pseudopuncta, and an umbonally cemented

BRACHIOPOD CLASSIFICATION THECOSPIRA

morpholocjical 'affinities' : strophomenidan (protreme) in / /,.~--~,, ..-s~.x\" shape / //" \, ,/ "\\x attachment entire interarea cardinal process I t\"-" / il \ "'-'!i I pseudopunctate shell \-,/..... ",. / \. / spiriferidan (telotreme)in brachidial spirQlia ultramicroscopic shell structure

weiqhted classification : homologous characters homoeomorphous characters IqbS strophomenidan entire interarea spiralia cardinal process pseudopuncta 1968 spiriferidan spiralia inferarea shell structure cardinal process pseudop~ncto FIo. IO. The differentia. The allocation of a species (or any other taxon) to a higher category commonly depends on a touchstone reference to a differential character. The transformation that follows a major change in the differentia selected is illustrated by Thecospira,whose determinate and whose homoeomorphous subsidiary characters, and whose phylogenetic affiliations, are in consequence inverted. (After Williams.)

habit on the other. In 1965 the spiralia were regarded as subordinate, and its strophomenid affiliation was preferred (see also Rudwick I968 , p. 349). But on the later evidence of its possessing a shell structure with secondary layer like that of some spiriferids it was in 1968 removed from the strophomenids, Williams thinking it to be 'inconceivable that strophomenide homoeomorphy [stropho- menoid shells being without a secondary layer] should evolve the growth of a spiriferoid-like shell as well as calcareous spires' (see Fig. IO). In a comparable example, Rollins & Batten (1968) demonstrated, with radical effects on classification, the occurrence of a monoplacophoran muscle system in the Devonian mollusc Sinuitopsis, previously regarded as a bellerophontacean gastro- pod allied to the pleurotomarians (Knight & Others 1965, pp. 173 , 177). The silt, formerly a major taxobase taken as an indirect sign of ancestral torsion and therefore used as a differentia, was thus shown, on the criterion of the musculature, to

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 T. N. George be of iterative origin and to have arisen in some lineages before torsion, in others after. Invoking a third criterion, Rollins & Batten suggested that in forms whose musculature could not readily be identified, parietal deposits and a posterior train were to be correlated with post-torsional evolution and used as linked differentiae in complementary evidence. The contrasted views of Durham and Melville (1957; in Durham & Others I966, p. 27 o) on the one hand and Philip (1965) on the other on the taxonomics of the sea-urchins provide a further example of the selective use of differential characters (linked with and partly justified by inferred phyletic affiliations) in classification. At various hierarchical levels symmetry and migrant periproct, smooth and crenulate tubercles, grooved and keeled teeth, peristomal structure, brachial slits, and imbricate test-plates are used as differentiae. Philip, in empha- sizing the over-riding importance of the position of the periproct in a change from radial to bilateral symmetry, retained the long-established distinction between regular and irregular sea-urchins; Durham and Melville stressed what they regarded as the more 'fundamental' features of jaw and peristomal elements to reach the conclusion that the irregular pygasteroids, the irregular holectypoids, and the irregular atelostomes were all independently evolved from regular ancestors. (See Fig. I I.) In each illustrative example, of the brachiopods, of the univalve molluscs, of the echinoids, the implications of the preferred classification, following the use of one or other character as a dominant differentia, are not merely limited to the trans- ference of Thecospira or Sinuitopsis or Pygaster from one taxon to another, but carry the pointers to gross homoeomorphy (see George 1962 , p. 53). The evidence for the promotion of an absolute differentia, with a subordination of other characters, is no doubt in each case strong on the author's premises; but the weighting of the single character, and the recognition of the character's 'fundamental' nature, remains a matter of subjective judgement in a method that carries an overprint of Linnaean-type stasis: unless the phyletic sequence is known, and the character priorities are established in at least chronoclinal order with cladal forking at the genesis of the differentia and with the criteria of homoeomorphy sufficiently analysed, there is no way of justifying the taxonomics except by cyclical argument. 4. Phylogenesis and taxonomy

The differentiae, when lineages are adequately preserved, are likely to be the product either of rapid bioserial change in one or more characters (see Fig. 8b), or of cladal divergence. It may be supposed, on present-day evidence, that full cladogenic branching in fossil series was always or almost always in conditions of allopatric isolation; and the association of individuals of two congeneric species of common ancestry on a single bedding plane, if it is not an accidental thanatocoenous association, must be taken usually to imply recolonization of a common environment after earlier geographical segregation. The genetic independence of such fossil species (if they are to be regarded as the analogues of living biospecies)

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ECHINOID CLASSIFICATION / ,ygo tero, ,, h 0c ypo,d l o he riro# O'0r

{echinaceons~

Durhom ond Melville

p,/qosteroids holect,/poids other irregular I . cjroups I

echinothur.oJds [diadematojds pedinoidsI ./.-~

Philip Nlperischoechinoidsl/ FIG. I I. Contrasted views on differentiae. The subjective selection of differential characters when phylogenetic details are imperfectly known is illustrated by the polyphyletic origin of the irregular echinoids proposed by Durham and Melville-- polyphyletic in their descent from different groups of regularsmand the monophyletic origin proposed by Philip. In both schemes the authors collate the use of the morphological differentiae with complementary hypotheses of descent; but there is manifestly some degree of circularity in the proposals that implies a reciprocal relationship between phyletics and taxonomics. The various taxa proposed, and the inferred evolutionary routes, are slightly simplified and generalized.

is indicated by their correlated biometrical differences. Their evolutionary divergence is explicated on the bioserial model: the clustering of variants in bimodal distribution is then an unambiguous sign of the relative allometry of character development, expressing genie contrasts, in the different bioseries. In simplest model, the independently modified rates of change--an almost universal aspect of sustained evolution--may be looked on as a reflection of biased selection pressure: what is environmentally preferred at one locality is inhibited at

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another. If there is no counter-effect through reciprocal gene-flow--if the demes are sufficiently isolated by disjunction or by distance--the preferred characters display a differential anagenesis that in due course produces variants few or none of which at one locality match any at another in at least one bioserial variant- range. With intensified differential selection the clades become increasingly divergent as evolution continues. Nevertheless, the divergence is not in the sudden

clade P I clade 0 .:.~.~.:.~ species f ..,..,-~...... :/:.: :C'):) / !iiii!!i:~ • / ,...... • genus Z grade N , i t r-...... iii-lj.ili:ii!.i ~ v~4 ~ ~., :, ,~ ., . ¢!.1 :.1 :i :I : :~,'! ! p .-~ " d

g <:222" !!,Y "'-L._I I a grade N 0 f j "~'"~' -~....~ F/i / /\\\\ -" ~-.]___--- ~ ,species b l-- / / 7 genus X grade L / / \ //'~..J~_1- ,species a

.. ~ylocjen,/: 2 3 4 5 " 6 7 8 f d biocharacters I I e ,b/ c GRADES AND CLADES " I (I RATES OF CHANGE IN BIOCHARACTERS F I G. z 2. Evolutionary divergence. Differential selection producing bioserial change has both lateral and chronological components. Cladal branching is a derivative of the environmental encouragement of different variants in different localities, expressed in a relative acceleration ofbioserial change in one locality as against another. Sequential species (a, b, ¢, d; a, b, e, f) of a single lineage may be allocated to a single genus; cladogenic species offer a basis for generic distinction (X, Y, Z) of a different kind.

appearance of contrasted characters, not in static differentiae, but in rates of bioserial change in chronoclinal continua; and the several bioserial chronoclines are repeated, except in grade, amongst all the clades. The divergence is recognized in a combinatorial association, a mosaic, of character-grades displaying an alternation of evolutionary 'progress' in the clades, a biocharacter of the plexus being relatively 'advanced' in one clade, relatively 'primitive' in another (see Fig. I2). 'Proof' of cladogenesis in usual taxonomic terms lies in the existence of several congeneric species at any one moment (including the neontological present-day moment), if the species have, or are believed to have, a common phylogenetic ancestry to justify their allocation to one genus. But while the indirect evidence of cladogenesis is to be seen in almost all groups, the taxonomic manipulation of cladogenic categories is not easy. In the classical example of Darwin's finches, 226

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disjunct clines, the isolation of island faunas, differential selection, endemic evolution, and other (analytical) factors offer the grounds for interpreting or applying a neo-Linnaean nomenclature, of 'non-dimensional' species in a comprehensive genus, that cyclically is both 'explanatory' and orthodox. The precise ancestry of the birds, and the details of their evolutionary divergence (including the details of their biospecific isolation), remain unknown: the 'explanation' for the existence of the several species of the one genus on cladistic grounds justifies, in an incongruous fusion of Linnaean taxa with evolutionary theory, the recognition of the several species and the one genus. (See Fig. x3. )

aisjuncttopoctines: Darwin's finches ..L"~'~

"-" f~,,,,~oo, ,~-~--~ "'. ,-, i rnagn,rostr,s/'• . . ..' { ~"^,,NCDONB ~_~__~__ -~ k.~¢HA~LES, CLADOGENES /" ~debilir°st~ !i / / \ \.;I septen

Fzo. 13. Island speciation. Darwin's finches provide an example that can be widely generalized of the present-day terminals of cladal lineages inferentially of common ancestry. Insular remoteness and differential selection have, it is supposed, been the main factors in the evolution of the different species or subspecies, magnirostris and scandens having a wider migrant range in a tolerable (and tolerant) environment than septentrionalis, dijficilis, and debilirostris. The morphological differences justify the nomenclatural distinctions between the species; the morphological similarities con- verselyjustify the linking of the species in the one genus Geospiza. The inferences and the implications of the hypothesis of clinal disjunction are intended to be phylogenetic; but the taxonornics is Linnaean, and Geospiza is a 'horizontal' genus of the timeless moment--as are all neontological genera containing more than one neontological species. (After the work of Lack.)

The taxonomics of chronoclines must, in the shadow of a Linnaean heritage, be adapted at present to bioserial continuity in ways that, even in the simple un- branched lineage, impose arbitrary and subjectively conceived taxa--taxa sharply separating genera in transition, even species and subspecies in transition (as in Hallam's oysters: Ostrea, Gryphaea; Gryphaea arcuata, Gryphaea maccullochi; Gry- phaea arcuata obliquata, Gryphaea arcuata incurva). The nomenclatural problem of taxonyms applied to cladal descendants evolved from a common ancestor is intensified when it is accepted that a strictly monophyletic taxon includes only subordinate lineage-taxa without collaterals if it is to represent an evolutionary segment: the taxon does not then embrace cladogenes, the clades by definition falling into separate taxa. At the same time, the common ancestor of the cladal branches lies on all the divergent lineages. If a single phyletic series alone is known it is then covered by a

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single taxonym (however it may be artificially segmented into subsidiary taxa); but if collateral branches are later found, or reconstructed, or attached by the systematist--branches each one of which includes the ancestor--there is an embarrassment of incompatibles in the taxonomics, for the reason justifying the creation of new taxa at the cladal fork to express the evolutionary divergence is extraneous to the taxonymic unity of any one clade. There are in fact likely to be very few monophyletic lines persistent for any length of time--very few taxa

genus named to include species genera named to signify d- ~ descentfr°m t'lpe'speciesa 7-/.c~ d~,x,evolut~nar' divergence~ n % "a/c/;, /~m~ genusD c~x ~ /m genus N ~e.\ genus A . . ~\ /:-C >5`°, "u"

o a a (a) isolated simple lineages (b) divergent lineages

CLADOGENES AND GENERIC NOMENCLATURE Fxo. x4. Divergence and its naming. The isolated simple lineage may, but only conventionally and arbitrarily, be segmented into species, and the grounds are no better for putting a convenient number of successive species together in a lineage genus. When there is cladogenic branching, however, the branches offer a means of justifying a generic separation that expresses the branching: N is different from and independent of D. Ambivalence arises when species c and d fall into genus A until the sequence b--m-n is known; or conversely that m and n fall into A until b--c-d is known. The allocation of the species to their appropriate genus (to D or A, or N or A) is then determined on grounds extrinsic to either their form or their phylogeny.

contain only one subordinate taxon--and there is always the expectation that new clades are constantly to be discovered and the definitions of formal taxa amended. Further, if taxa are to be defined or delimited at cladal forks, not only is the continuity of the lineage-taxonym broken when forking is identified, but the taxa so distinguished are in part phyletic, containing lower taxa strung out along the lineage, in part 'horizontal', cutting across the lineage continuum (see Figs. z 2, 14). Moreover, differential rate-changes in bioserial anagenesis not only give rise to the evolutionary clades; they also display pulsed acceleration and retardation, and comparable end-grades of structure may be repeatedly attained by parallel but staggered clades. Thus in iterative evolution gryphaeoid shells were repeatedly evolved from ostreoid in Mesozoic and Cainozoic times as close homoeomorphs; the transition from reptilian to mammalian structure (on whatever basis the structural grade is assessed) was polyphyletic along a number of therapsid lines; Lang's repetitious 'trends' in corals evoked the genomorph as a name for like 228

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 Systematics in palaeontology descendants of cladal stocks (see Wilson I963) ; the amphibians may have had a triple source in the crossopterygian fishes (Jarvik I96o , pp. 37ff.; Szarski I962; Thomson 1964) ; monomyarian bivalves are the descendant forms of half-a-dozen major dimyarian groups. The evidence of such evolution is almost universal: phylogeny is riddled with parallel homoeomorphs displaying equivalent grades of structure (see George i962 , pp. 32ff.), whose systematics is notoriously a conflict between the continuity of the multiple evolutionary series and the sharply discontinuous boxes of Linnaean morphotypes.

B) PHYLOGENES AND CLADOGENES How far replicate trends in the evolution of the graptolites may reflect parallel mutation is not to be determined from fossils, but the similarities of parallel form are repeatedly so very close that the parallelism may well be homogenous and the trends then be morphogenetic signs of affinity in a genetical sense. But whatever may be the reason for the iterative homoeomorphy, the impressive changes in the form of the colony through reduction in the number ofstipes prompted the comparatively simple morphological classification expressed in names that immediately imply their meaning--Tetragraptus, Didymograptus, Diplograptus, Dimorphograptus, Monograptus--and that measure successive grades in parallel astogenesis. When thecal shape and sicular budding are used in an alternative classification, as morphological differentiae superseding stipe reduction, they immediately impose upon the realigned course ofgraptolite evolution a polyphyly arising from multiple cladal branching from common ancestral types: the monograptids, for instance, are divisible into six or eight major lineages, some at least of which appear to be independently descended from dimorphograptids and perhaps dicranograptids. Accepting phylogenesis as a principal factor in the taxonomics of the graptolites, Bo~cek, P~ibyl, and others have presented a challenge to traditional practice on purist grounds of consistency and system, in advocating the erection of taxa designed to express the lineage and not the grade (for a general discussion see Urbanek, I96o , pp. 2o 3 ft.). The challenge to graptolite classification may be extended to almost all phyla as they are now classified: its interest lies in the compromise suggestion put forward by Bulman (i963, p. 412) to accommodate clade and grade. While acknowledging the logic ofphylogenetics, he sought to retain the names of the grades as being of great convenience both in stratigraphy and in comparative morphology. He was therefore prepared to have two different kinds of names--and two different kinds of taxa--to embrace the same organisms, one kind 'vertical' (as a lineage taxon) in concession to phyletic sequence, and one 'horizontal' (as a grade or morphological taxon) to express iterative parallelism. A dual system is a latitudinarian device that makes taxonomics an empirical exercise without intrinsic principles (unless the clade has an inherent priority, provides a more 'fundamental' basis, then the grade): it is also a means of using a Linnaean-type nomenclature for two different and incommensurable purposes in a single group of organisms--with the implied relegation of a bionomial to the status of a label. (Compare Bather i927, p. eft.)

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 T. N. George Bulman was hesitant about applying phyletic rather than morphological criteria in a purist taxonomics partly because there is no ending to the lineage series of such forms as are now included in (for instance) Tetragraptus--forms that were descended from many-branched ancestors and that had twin-branched, diplo- graptid, and monograptid descendants. (It is noteworthy that Boficek himself was consistent in his revised taxonomics only for the lineage segments with which he was concerned: he did not expand his system to include ancestral species at points of forking, or minor forking which may happen a number of times along any one major lineage, and his major novelties were mainly at the level of genera and their derivative families.) In a determinable lineage a single taxon would thus sub- merge all subtaxa by incorporating every included graptolite from the initiation of the phylum to its final extinction. This is a view that some decades ago was expressed by Burma when he said that since the lineage is a continuum and in its cladal branches still a continuum, there was no natural justification for breaking the continuity if taxonomy is to reflect reality; and he gave the contention a logical universality by generalizing into one gigantic taxon all the hierarchy of taxa that Linnaeus founded (taxonomy of a Linnaean kind then being wholly frustrated). Similar or analogous effects on classification are the product of microcladal quanta of change in the ecophenes of what is usually regarded as a single species. Thus within a restricted but wide-ranging group of Devonian conodonts the permutations of a comparatively small number of anagenetic biocharacters produced a multitude of forms classified (whether on a phyletic or on a morphological basis) only with the greatest difficulty (see Shaw 1969). The clinal gradient is sometimes steep, sometimes gentle, but in a rapid alternation ofbioserial emphasis there was produced an association of individuals no two of which are likely to be closely similar. Nomenclature cannot then be based on static point-characters, the micro- grades and clades in systematic cross-paths forming a complex reticulum ofmorphs that are as varied as the ecological stations they occupied (and no doubt as the genepool into which they dipped). In a minutely diversified mosaic there is no place for Linnaean-type species, the whole concept of which is thus, in Shaw's analysis, quite inapplicable to real organisms; it is 'non-objective and synthetic', and should be abandoned as 'misleading, inappropriate, and useless'. Derivatively, if there are no species there are no higher taxa either. The dilemma is a real one. Burma and P~ibyl in their criticism of morphotaxa assume or imply that there is or should be a natural foundation for taxonomics in evolutionary affinity, so that the taxa (however they are designated) fall into an evolutionary frame. The assumption or the implication can, however, be denied if on the contrary taxonomy is regarded solely as an art, having no commitment to predetermined pattern built into the organisms to be classified: taxa are then sets of convenience, in principle neither Linnaean nor phylogenetic, to be manipulated without reference to the 'natural' affinities of the organisms systematized, consistency demanding only that lineages as such need have no place in the taxonomics when an arbitrary morphology, as in a key, is the sole touchstone. (See Simpson i96i , p. I25.)

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(C) PHYLOGENETIC RECONSTRUCTION When lineage and clade are regarded as more comprehensive than grade in the organization of an acceptable systematics, the bioseries are a major element as organization instruments. They are, nevertheless, rarely so well represented in fossil assemblages as to provide a continuity that is not significantly broken by gaps--a continuity that, even if it is not completely represented by a bed-by-bed sequence ofchronoclinal palaeodemes, is sufficiently coherent to indicate with high probability the direction of evolution. In the vast majority of groups probability declines into imprecision, often into surmise. Prejudice then enters into the construction of the evolutionary sequence, and taxonomics comes to express the anticipations of the systematist in hypotheses prompted by his experience and his ingenuity. The systematic construction then has a degree of apriorism built into it that is not wholly to be distinguished in its circularity from the apriorism of a Linnaean-type system, and that is acceptable only in so far as the antecedent hypotheses are persuasive. Thus the Devonian clymenioid cephalopods have usually been referred to the ammonoids notably on the basis of the protoconch, a structure that throughout cephalopod evolution has been elevated as a predeterminate differentia between nautiloids (s.l.) and ammonoids, and which in the clymenioids is goniatite-like. They are, however, aberrant in their dorsal siphuncle and their non-goniatitic septa and suture lines, characters that widely remove them from 'normal' ammonoids. If it is supposed that the siphuncle migrated in annectant nautiloids from a subcentral position ventrally in the ammonoids, dorsally in the clymenioids (as in some other nautiloids), and that in sutural patterns the clymenioids were evolved from simpler nautiloids independently of the very different goniatites, the inferred phylogeny contradicts the assumption that the protoconch can safely be accepted as a dominant differentia. The taxonomic reference is a derivation of theory and not (yet) of proved phyletic sequence. (See Donovan 1964, pp. 266, 27 I; Teichert I967, pp. I87, I98. ) (See Fig. 15. ) In a wider field, the ancestry of the ammonoids has been a source of major disagreement in the selection of annectants from amongst the variety of Devonian cephalopods. On the one hand, the bactritoid nautiloids, in their progressive coiling from orthoconic or cyrtoconic forms of the morphogenetic sequence Bactrites to Mimagoniatites and Anarcestes, in their slightly inflated protoconch, and in their ventral siphuncle, may well be near the ancestry (see for instance Erben i964, pp. I26ff.; Erben in Teichert & Others I964, pp. 494ff.)- On the other hand, the bactritoids have also been regarded as specialized nautiloids (s.l.), some with cameral deposits not to be seen in ammonoids, some with divergent non-goniatitic types of suture, and an origin of the goniatites in the coiled tarphyceratids or the closely linked barrandeoceratids has been preferred (see Donovan 1964, p. 267). (See Fig. i5. ) All the alternative phyletic affiliations read into the cephalopod series are reflected in the taxonyms used to classify them; and there is a repetitive under- standing that when the clymenioids, for instance, are said not to be ammonoids

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or are referred to the ammonoids, it is already known what defines a clymenoid, what an ammonoid: for if the clymenioids are ammonoids, then the ammonoid umbrella must be defined sufficiently broadly to cover them. There is therefore some ambiguity in the manner in which the names are applied; and it may be no more than a verbal excercise to build a taxonomic system of inclusive and exclusive taxa on the criteria selected or promoted. If some degree of objectivity is given by a phylogenetic system (in a 'natural' reflection of the course of evolution) the taxonomic demands become much more rigorous: the clymenioids are then

PHYLOGENY AND CLASSIFICATION IN PALAEOZOIC CEPHALOPODS

prolecanitids goniatitids ~ prolecanitids cjoniatitids -- / / 0 \ / \\ \ / / cl,/menioids z anarcesroids ~'-~ 0 \ / ..- , ~ anarcestoids ... , ~ ,/ -bactnto~ds ~--, \ ', nautilidans / noutilidans clTmenioids \ ', / ~ / \ / orthocerotids ",, oncoceratids -O, Ii \ \\ iI \ \ tarph,/cerahds" / <~~ tarphycer~atids oncoceratids / ---.. / \ / Z ---- ...... / \ / "--( ellesmerXocerarids ellesmeroceratids

TEICHERT 1967 DONOVAN tg64 ]FIG. 15. Phylogenesis and taxonymic range. The contrasted views on evolution of the cephalopods, based on the degree of subjective emphasis placed on differentiating characters by different systematists, are illustrated in the reference of the clymenioids to the ammonoids on the one hand, to the nautiloids (s.l.) on the other, whose respective ranges are expanded or contracted accordingly, and whose definition then expresses the hypothetical phylogeny on which affiliation is recognised, rather than the intrinsic nature of the organisms. Similarly the ammonoids may be regarded as in descent from either the bactritoids or the tarphyceratids-barrandesceratids.

excluded from the ammonoids if they form a divergent clade from other ammonoids; or are ammonoids if their divergence from the ammonoids took place after the ammonoids branched from the nautiloids. There being no sure 'objective' evidence either way, a weighted preference for one differentia rather than another is a matter of subjective choice. The salopinid brachiopods of the Silurian rocks illustrate other problems in adapting Linnaean taxa to evolutionary series. They typify many groups of organisms in which there is no main strand of evolutionary change that can readily be used as a taxonomic anchor, but display multiple morphogenetic replications revealing an intricacy of phyletic relationships both difficult to resolve and difficult to systematize. How they are classified then in part becomes empirical--the inclusiveness of the individual taxon, at whatever level in the hierarchy, grows as its members are discovered, and the hierarchy of sets is or appears

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 Systematics in palaeontology in some degree to be self-created as more and more of the morphological patterns running through the sets are discovered (see, for instance, Williams & Rowell in Williams & Others, 1965, p. 214ff. ). When the salopinids are analysed, in the description of Walmsley, Boucot, & Harper (I969) , they show graded changes as draboviine forms that can be sectionalized into successive genera, first in evolutionary advance out of Fascifera into Salopina, secondly in cladal changes within Salopina through species leading to the divergent Megasalopina, Sphenophragmus, and Muriferella. The genera are essentially phyletically established, as grades in the seriation of selected biocharacters. Descriptively, in 'diagnostic' terms, they are characterized by the 'possession' of this or that kind of morphological feature--Sphenophragmus differs from Salopina in the brachiophore base and the margin of the adductor field being disjunct, Megasalopina in its naviculate ventral valve, Muriferella in its prominent dorsal diaphragm--and fall into a pattern of cladal derivatives. Within Salopina there is a comparable recognition of successive species showing bioserial anagenesis and divergence as intermediates between a near-ancestral shelvensis and the descendant genera. The species appear as lineage segments, phyletically (and stratigraphically) annectant, that are distinguished on graded morphological features; and as phylogenes they invite a further subdivision of the genus into three or four 'monophyletic' taxa to identify the cladogenes. Thus of the salopinid species, submedia is nearer Megasalopina, hazardensis nearer Sphenophragmus, crassiformis nearer Muriferella, than they are to each other by either morphological or phylogenetic measure; and the difficulties on an internally consistent and sustained system of taxonomics or of taxonyms in a Linnaean-type frame are only partly masked by the conformity of an orthodox nomenclature. (See Fig. I6.) Classificatory and nomenclatural relations become particularly complex when the well-documented lineage is over-abundantly represented in fossil series. In the classic illustration of the Chalk heart-urchins (Micraster), the facts of the lineage sequence can be well represented by the graded migration of variation fields in time. It was possible for Kermack (I954) to distinguish an early clade leading from leskei towards corbovis and diverging from coranguinum---even if some doubt persists, because of the wide overlap in variant range between the two divergents, whether there was complete speciation before the corbovid variants were extinguished in the Holaster planus Zone. The more important later cladal divergence ofsenonensis from coranguinum is also not complete, the latest variation fields of the Marsupites testudinarium Zone still revealing a wide range of overlap that scarcely justifies an inferred biospecific distinction between the two 'species'. Moreover, to meet the postulated expectations of biospecific isolation Kermack suggested that early members of the senonensis stock migrated to regions outside Britain to allow the divergents to be established before their return to sympatry as quasi-independent species. The independence remains doubtful, however: if Nichols is correct in his inference that the morphological differences between the two species reflect differences in habitat, coranguinum being a burrower and senonensis a surface-dweller, then they might

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D RABOVIIN AE SCHIZOPHORIIDAE

MEGASALOPINA ISPHENOPHRACMUS I MURIFERELLA SCHIZOPHORIA I I I I hazardensis I I l h tchcocki I crassiformis I I I missendenensis I I submedia I , I ! I lunata . tub Iota I .,._ ...... _.._._ o conservatrix"- _IZ she~vensis-q S AL 0 PIN A I I FASCIFERA HIRNANTIA

I enteletoceon stock

( cladoqenes ,

PHYLOGENESIS IN ORTHID BRACHIOPODS F xo. I6. Phylogenes and classification. The salopinid brachiopods in their cladal branches are annectants between early and late genera of draboviine stocks, the various species and lineages being identified or created on minor pulses in rates of change in bioseries common to all the forms. In the derived genera, grades of evolution in selected characters are arbitrarily used as means of generic distinction; but there is consequent anomaly in the reference of the assemblage of salopinid species to the single genus Salopina--anomaly that is a product of the nomenclatural necessity to place the forms in a Linnaean binomial frame when the frame perforce is constructed on a basis of phylogenetic, cladal, and comparative-morphological relationships. (After Walmsley, Boucot, & Harper.)

reflect niche adaptation without complete isolation. The large degree of variant overlap then implies that the forms can be regarded as no more than morphological subspecies. Adding further complication to the diagnosis of cladal genesis, a subanal fasciole is present in senonensis, absent in coranguinum; and it is on the differentia of the fasciole that individuals are distinguishable between senonensis and coranguinum in Kermack's bulk samples. Only on the linkage of fasciole with high test can the independence of the variation fields between the two taxa be accepted despite the wide overlap--a circular manner of establishing the distinction, for the fasciole is not always present in forms considered to belong to senonensis because of their relative height. (see Fig. 17). If senonensis and coranguinum should indeed prove to belong to divergent clades, they may nevertheless both be referred to the genus Micraster because of their close relationship. The use by Mortensen of the fasciole as an absolute differentia

~'34

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 Systematics in palaeontology defining the (sub)genus Isomicraster is then quite independent of other bioserial criteria of the lineages, and is an accidental (if for him a justifiable) means of distinguishing the clades equated with the genera (compare Stenzel I963). The permutative use of characters exemplifies the incongruities of a multiple system of classification which includes phyletic sequence, cladogenic speciation, an overlay of aUopatric analogy with neontological biospecies, and a character-isolate as differentia. It also leaves unresolved the problem of the two alternatives---one, that senonensis as indicated by its fasciole is not a derived member of the corangui- hum series and can be happily referred to an independent genus Isomicraster; the other, that the fasciole is an incidental in the lineage-context, an unimportant character to be ignored when senonensis so widely overlaps coranguinum in variant range as to justify the recognition of not more than a single species, certainly not more than a single genus, to include all the forms.

(D) SPECIES AND GENUS Present-day discussion of the nature of taxa is mainly concentrated on the species, especially the biospecies. The species as an artefact exists by definition. Its functional definition gives the species as biospecies a validity that at least for the local deme is 'objective'. In their reproductive isolation and in their internal gene- pool mixis, demes of the biospecies may also provide parallels between functional and morphological criteria in taxonomic definition when the parallels can be identified. Except in extended topoclines, whose peripheral demes may or may not be genetically conspecific, the neontological separation of Linnaean-type morphospecies into well-defined taxa may then also be an acceptable extrapolation of the available evidence at least on negative grounds--Canis vulpes is not Canus lupus either genetically or morphologically (nor is it Felis leo). The present-day momentary time-transect, of an abundance of genetically, and mainly morphologically, distinct species, is an incident in a palaeontological continuum. It is also a sign of earlier time-transects; and there was the same kind of specific isolation, including topoclinal isolation, at every stratigraphical horizon of the fossil record. Speciation along graded topoclines excepted, there is thus justification, on analogy with living organisms, for classifying contemporaneous fossils of any age into unambiguous species. The time-transect reality of isolated species, genetically or morphologically defined, is however a feature only of the geological moment. In chronoclinal phyletic series that display the sequential merging of the demes of a lineage there is no support for the kind of neontological taxonomics that Linnaeus founded. Even in the neontological environment topoclinal gradations in wide range are not readily, or at all, to be accommodated in a Linnaean mode, unless, disjunct and fragmented, they give the taxonomist the gaps he seeks between most species. The palaeontologist may find an association of discrete Linnaean-type fossil species on the single bedding-plane, but his expectation must always be that successive bedding planes will present him with classificatory and nomenclatural problems essentially insoluble on a Linnaean basis and met only inconsistently by the pro- tective device of 'aft.' or 'cf.'. In concession to Linnaean nomenclatural form, and

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cjeneralised variation fields of specimens of coronquinum zone mm bS" (o) 45 bO- 2 40

)_. SS ..0 35 ~ N. corancjuinum ..... ii Zi~ ~i ii Zi ii i~>J"" SO' /~2~,// ...... J 30 -/'" ~;/ [TrT]] N. senonensis 45 5'0 5'5 bO b5 SO 5S 60 b'5 mm lencjth lenqth

U. Cretoceous zones Microster Isomicroster coroncjuinum ~ I 1/7"~.~senonensis \\ Mar.testudinarium / ../" 7"----~ ~-.

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-- relotive proportions of test

corbovis • ~ senonensls

...... (d)inferred mode of life

CLADOGENESIS,ECOLOGY, AND NOMENCLATUREIN MICRASTER

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in compromise between merging and discrete sets, Westoll's holomorphospecies retains the specific name but expands the range of variants it covers to include transients that lie within the conventionally defined variation field of the comprehensive specieswthe concept then embracing arbitrary definition of a lineage- segment that is distinguished from contiguous segments only when evolutionary change becomes, in subjective judgement, excessive. The genus has fallen into relative neglect as interest has concentrated on the species during recent years. This is contrary to its former primacy when the species was intrinsically subordinate to the genus, not merely in being contained within the genus but also in being no more than a particulate expression of the generic essence (whose elusive quality it is the purpose of a Linnaean systematics to identify). The present neglect is a measure of a fading of the traditional attitude that the genus is more 'real' than the species. When expectation a priori is abandoned, and system is organized rather than discovered, the genus is deposed as a 'natural' taxon of any inherent kind; it is now 'no longer an important taxonomic entity, but merely a collection of species which resemble each other rather closely' and its name contributes to a 'binominal nomenclature that corresponds, in part, to wholly discarded concepts' (Cain 1962, p. 1 i). The ambiguities of meaning given to generic names, when they subsume 'species which resemble each other rather closely' either because of lineage continuity or because of morphic equivalence in cladal branches (Figs. I2, I4) , carry into the 'higher' category similar taxonomic defects to those of the species (whose members, variants and transients, are ambi- valent affiliates) without there being 'objective' and definitive criteria of the genus analogous to the gamodeme or to the biometrically unified set of the species. While there may be some 'objective' evidence for the existence of the neontological species as a 'real' if timeless entity, there is none for the genus, either neontological or palaeontological, and its continued use is factitious. Attempts to rest generic distinction on phylogenetics mainly invoke cladal divergence. If senonensis is truly divergent from coranguinum (the divergence re- inforced by the differentia of the subanal fasciole), then there is subjective ground for taking it out of the genus Micraster and establishing for it the genus Isomicraster (Fig. 17), even if the generic differences are no greater than the specific. If in the general case (Fig. I4) the point of forking defines the generic sets, the anomalies of nomenclature (not merely of the names allocated, but of the significance of the names) are obtrusive when forking is first discovered, and even greater when

Fro. 17. Speciation in Micraster. The species senonensis is distinguished from coranguinum on a single morphological difference, the presence ofa subanal fasciole, presumably correlated with a near-surface as against a burrowing habit. The fasciole used as a differential sign, senonensis (migrant to avoid the difficulties ofsympatry) diverged from an ancestral Leskei-cortestudinarium stock to become significantly different from coranguinum in the Marsupites testudinarium Zone. The overlapping variation fields of the two 'species' (Figs. I7a , I7b), and the incomplete divergence illustrated in the generalized diagram of Fig. 17c, scarcely support even a specific distinction between the two forms: they certainly discourage any reference of the forms to two separate generama morphological practice solely dependent on the differentia of the fasciole. (After Kermack and Nichols.) 237

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 T. N. George contrasted postulates of forking are put forward by systematists not agreed on interpretations of the imperfect evidence. If the cladistic base for generic recognition is carried to a limit, every neontological species, being contemporary with and independent of its neighbours, falls into its own private genus. If the retention of a generic nomenclature is granted, the field of a genus is inevitably a product of the systematist's taxonomic concepts, his subjective choice of an order of magnitude in cladal divergence to justify the creation of a genus ranging (for instance) from Salopina (Fig. 16) to Isomicraster (Fig. 17). The lineage genus is incommensurable with the clade genus. It reflects the systematist's subjective opinion on the degree of difference allowable between successive species in evolutionary continuity. Although perhaps only a single mutation may have resulted in their different degrees of coiling, Hallam's flat oysters fall into the genus Liostrea, his curved into Gryphaea (p. 218) ; but seven stages of species sequence separate the orthid Fascifera from Sphenophragmus (Fig. I6). The constrictive discomforts ofLinnaean-type taxonomics, long and well enough known in fossil series, are illustrated in Fig. 18. Each genus is a lineage as shown; it is also a clade genus if a common ancestry can be postulated before time Tx. Within the genus, evolution of species (the species differences measured by variant range) is relatively very slow (bradytelic) in A so that only two species are established to include all the variants in the long time-interval TrT4. The genus D, in tachytelic evolution, includes six or eight species in half the time-span, with species 18 to 2o widely different from species 12 to I4--far more so than species 4 of genus C is from species 3 of genus B.

(E) THE TAXONOMIC HIERARCHY If systematics could be rationally planned afresh, without the constraints of a built-in taxonomic pattern, it would be difficult to suppose phylogenes to be organized on any basis remotely resembling the Linnaean, whose presuppositions and whose nomenclature are palaeotechnic in being pre-Darwinian, statically out of context in the fluidity of a multitude of clines and clades, self-contradictory and inconsistent when applied to fossils in taxonomic attempts to combine morphological and morphogenetic elements as definitive criteria. Taxonyms that by the Rules are inviolate are seen to be different in kind and to signify different and incom- mensurate properties of the organisms named: they are incongruous in sequential graded series, which they disrupt, and they are oppressive in their implied theory as the verbal relics of an outmoded Linnaean scholasticism. (For a general discussion of principles, see Henning 1967, pp. 7off.) Still 'higher' categories, including family, order, and class, scale the system into manipulable sets. To the palaeontologist they are predominantly cladal--the canids and the felids are divergent groups of early-Clainozoic carnivores--but although purified intention in the taxonomics may rest on divergence, the use of morphological differentiae, as in the example of the monograptids, may supervene to introduce the same kind of discrepancies as genera reveal. When neither evolutionary origins are known in detail nor spectacular differentiae define morphic sets, traditional system becomes even more difficult to apply. 238

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 T. N. George Thus a group like the trilobites is to a degree taxonomically adrift, at least in the application of the names of higher categories. Having lost the morphological safety of ecdysial differentiae by the intrusion of a phylogenetic and an ontogenetic critique, they are now in process of being classified, with much uncertainty and disagreement on a variety of grounds, upwards from species and genus. Migration and new environments (ecological factors) had a notable part in determining not only the physical frame of the evolving lineages but also the ways in which they tended to be grouped; and conversely palaeogeographical boundaries or barriers were the determinants in some measure ofcladogenic routes, so that different kinds of taxa came to characterize the different 'provinces' and 'realms' colonized (see for instance Whittington 1966). Similarly there is much uncertainty (the crudely heterogeneous trematous groups being suppressed) in organizing the brachiopods into satisfactory orders, differentiated on safe anchors of reference (see Williams & Rowell in Williams & Others I965, pp. 215ff.); and the current classification of the fossil bivalves, widely different in basis from the classification of living bivalves, is a synthetic compromise of functional, ecological, morphological, and evolutionary criteria through which not a single theme but a plexus of thematic strands runs (see Newell I965). It need scarcely be added that there is no correlation between the taxonomic hierarchy and major cladogenic advances: divergent species are not in descent from ancestral genera, genera from families, families from classes: in most phyla, already segregated in Precambrian evolution before they first appear as Palaeozoic fossils, forking clades arise not from generalized unspecific ancestors but always from particular species (however species are defined), all of which as species may in continuing process be expected sooner or later to become extinct either by exter- mination or by sequential replacement (see Henning I966 , pp. I54ff. ). In a rejection of the simile of the evolutionary tree--massive trunk forking into branches, branches into twigs, twigs into terminal shoots, through Phanerozoic times to the neontological present--the imposition of classificatory quanta on cladal phylogenesis is explicitly incongruous when the hierarchical taxa are regarded as artefacts. It is no less incongruous as method in the palaeontological continuum even when the neontological hierarchy is regarded on traditional Linnaean assumption as having some measure of 'objective' 'reality'.

5. Adansonian methods Phyletic series in their full detail are not easily found or proved. They are usually recognized or proposed only with varying degrees of probability; and when chronoclines are disjunct, as they almost always are in the actualities of fossil sequence, the lineages and the clades are identified by means that reveal the prejudices or the expectations of the systematist. Moreover, a well-substantiated fossil sequence is usually given phyletic coherence by allusion to a few selected characters--thecal form and thecal budding in graptolites, fine shell structure in brachiopods, fascioles in heart-urchins--on which a special significance as differentiae is bestowed by the systematist in a weighted discrimination sometimes

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/3/197/4896119/gsjgs.127.3.0197.pdf by guest on 28 September 2021 Systematics in palaeontology justified only by esoteric expertise. The contrasted evolutionary sequences that arise when the experts differ, and the taxonomic conflicts that follow, are well enough illustrated in every discussion of homeomorphy and tend to undermine the palaeontological foundations of a revised systematics. A critique of the phylogenetic pretensions of many proposed fossil series, and of the subjective means by which they are identified, is thus a proper theme of many systematists who wish to bring some detachment to method (see for instance Sokal 1962; Sokal & Camin 1965). The organization of fossil forms may, it is conceded, best be expressed in a phylogenetic frame; but since on present evidence it is in almost all groups, down to the level of genera or even species, impossible to construct a convincing frame, the construction, it is contended, must be deferred until much more evidence is forthcoming. Meanwhile, hypothesis and conjecture by the individual systematist, however 'competent' he may be, should not inform method, more especially in the subjective weighting of differentiae. Only when 'all' characters (that are measurable, not merely notionally assessed) are given their equal place as unweighted data in the definition of sets (taxa) can a self- justified system emerge. There is a strong appeal in such austerity; and a numerical taxonomy that is founded on cluster analysis and coefficients of phenetic relationship brings a rectitude to morphological comparison not to be found in phylogenetic specula- tion. Thus in a synthesis of chonetacean brachiopods (Fig. 19) Sokolskaya (196o) deployed the genera in three families, Muir-Wood (1955) in four; and they further disagreed in the allocation of the genera to subfamilies. In doing so they used the same rigid frame of a four-fold hierarchy (genus to superfamily), their disa- greements (within the common intention) in attributing genera to families arising from differences in judging the importance of characters in a scale of relative subordination. Rowell (I967) , on the other hand, accepting the same morphological elements for manipulation as Sokolskaya and Muir-Wood but using the whole spectrum of the biometric data and analysing their associative occurrence with discrimination in equations of Markov type, 'proved' the earlier authors to be 'wrong' (that is, subjectively biased) in their inferences on the taxonomic affiliations of a number of the genera, and offered a 'truer' dendrogram of relationships on the basis of the evidence invoked. Moreover, the degrees of similarity shown by the genera allowed him no opportunity to separate taxa--subfamily, family, superfamily--of equal status with those of Sokolskaya and Muir-Wood. A numerical taxonomy, in removing the prejudices of the obtrusive systematist from the circularity of argument that bases phylogenetic reconstruction on differentiae of the systematist's selection and then formulates a classification justified by its being phylogenetic, is a salutary corrective to ill-founded hypothesis and is to be welcomed for reviving the kind of criticism that Adanson, who was Linnaeus's contemporary, made two hundred years ago. But its shortcomings, however, are manifold (see for instance Meeuse I964; Hull i967). It is morphological in the narrowest sense, using characters indifferently as though adaptive superficialities (ribs on a bivalve shell) are as important as built-in structure (ribs in a mammal). It is static and timeless--on the one hand it takes no account of the interplay of 241

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FIo. x9. Adansonian critique. The weighting of characters, and the selection of differentia, are subjective in the taxonomics of Sokolskaya and Muir-Wood, who were in conflict on the allocation notably of Megachonetes and Gonostrophia. An application of the principle of equal weighting implies radical criticism of the subjective method in a shuffling of the genera into reorganized clusters. The dendrograms appear in general pattern to be similar; but Sokolskaya and Muir-Wood constructed phylogenetic trees representing inferred evolution, whereas Rowell constructed his dendrogram on a simple comparative basis ofself-seleetive degrees of likeness displayed by a great number of characters used without phyletic bias. The sets are thus incommensurable in their significance--except the genera, which in their morphological attributes are assumed to have the same range in each dendrogram. (After Rowell.)

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bioserial change in the fluctuating resilience of biocharacter anagenesis, and on the other it obscures a distinction between for instance Liassic and Cretaceous gryphaeiform oysters, or Devonian and Triassic oxycone ammonoids, or Dinantian and Permian spirifers. It cannot well accommodate topoclines any more than chronoclines; and it emphasizes grades at the expense of clades. Adanson must not be forgotten or ignored; but if Adanson continues to be cautionary in his message, he does not remove phylogeny from palaeontological systematics: he merely redirects attention to whole morphs, when a major analytical component of evolutionary process places morphs in a dynamic morphogeny, in morphic stages in sequence. The palaeontologist reads into the message the relevance of an impartial bioserial dissection in taxonomic method, and warmly accepts, what Rowell's dendrogram of Fig. 19 well reveals, the inadequacies of Linnaean taxa as vehicles to carry the multivariate subtleties of morphological expression in sets of evolving organisms.

6. References The number of appropriate references in the field of palaeontological systematics is legion. The highly selective list that follows is limited to references quoted in the text--which themselves are mostly to a few illustrative fossils arbitrarily chosen from a host of examples that equally would have suited the purpose of the address.

BATHER, F. A. I927. Biological classification past and future. Quart. J. geol. Sot., 83, Ixii-civ. BLACKWRI:DER, R. E. x964. Phyletic and phenetic versus omnispective classification. Publ. Syst. Assoc., 6, I7-28. BULUAN, O. M. I963. The evolution and classification of the Graptoloidea. Quart. d. geol. Sot., XI 9, 4OI-I8. CAm, A. J. x958. Logic and memory in Linnaeus's system of taxonomy. Proc. Linn. Sot., x~, 144-63. I959. Deductive and inductive methods in post-Linnaean taxonomy. Proc. Linn. Sot., XTO, x 85-2 x7. I962. The evolution of taxonomic principles. , ;,th Symp. Soc. gen. Microbiol., I-I3. & HARmSON, G. A. x958. An analysis of the taxonornist's judgement of affinity. Proc. zool. Soc. London, x3x , 85-98. CARROLL, R. L. I97O. The ancestry of the reptiles. Phil. Trans. roy. Soc., B257, 267-3oo. CARamR, C. S. I965. Phylogenetic relations of the major groups of animals. In J. E. Moore (editor) : Ideas in modern biology. Proc. x6th int. Congr. Zool., 6, 427-43- DONOVAN, D. T. I964. Cephalopod phylogeny and classification. Biol. Rev., 39, 259--87• DURHAM, J. W. & MELVILLE, R. V. I957. A classification of echinoids. Jl Paleontol., 3 x, 242-72. & Others. I966. Echinodermata 3. Treatise invert. Paleontol, U, I. EAOAR, R. M. C. I953. Variation with respect to petrological differences in a thin band of Upper Carboniferous non-marine lamellibranchs. Liv. & Manchester geol. Jl, x, 16 I-9o. I956. Naming Carboniferous non-marine lamellibranchs. Publ. Syst. Assoc., 2, x I x-6. 1960. A summary of the results of recent work on the palaeoecology of Carboniferous non- marine lamellibranchs. C.R. 4me Congr. Strat. Gdol. Garb. Heerlen x958, x 37-49. ERBEN, H. K. I964, 1965. Die Evolution der/iltesten Ammonoidea. Neu. Jahrb. Geol. Paldontol., x2o, Io7-212; x~,2, 275-312. GEORGE, T. N. I953. Fossils and the evolutionary process. Adv. Sci., xo, I32-44. I956. Biospecies, chronospecies, and morphospecies. Publ. Syst. Assoc., 2, 123-37. I959. Rates of change in evolution. Sci. Progr., 46, 4o9-26. 1962. The concept of homoeomorphy. Proc. Geol. Assoc., 73, 9--64•

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HALLAU, A. I968. Morphology, palaeoecology, and evolution of the genus Gryphaea in the British Lias. Phil. Trans. roy. Soc., B254 , 9i-i28. HENNmO, W. 1965. Phylogenetic systematics. Urbana, Illinois. HULL, D. L. I967. Certainty and circularity in evolutionary taxonomy. Evolution, 2z, I74-89. JARVIK, E. 196o. Ttdories de l'dvolution des vert~bres. Paris. KEmX~ACK, K. A. 1954- A biometrical study of Micraster coranguinum and M. (Isomicraster) senonensis. Phil. Trans. roy. Soc., B237 , 375-428. I967. The inter-relations of early mammals. J. Linn. Soc. (Zool.), 47, 24I--9 • K_mGHT, J. B. & Others. I965. Mollusca. Treatise invert. Paleontol., I, I. KOHNE, W. G. 1968. Kimeridge mammals and their bearing on the phylogeny of the Mammalia. In E. T. Drake (editor) • Evolution and environment, New Haven and London. Io9-23. MAYR, E. I968. Theory of biological classification. Nature, 220, 545-8. I969. Principles of systematic zoology. New York and London. MEEUSE, A. T. J. i964. A critique of numerical taxonomy. Publ. Syst. Assoc., 6, I I5-2 I. Mum-WooD, H. M. I955. A history of the classification of the phylum Brachiopoda. Brit. Mus. (Nat. Hist.). 1962. On the morphology and evolution of the brachiopod suborder Chonetoidea. Mon. Brit. Mus. (Nat. Hist.). NEW~LL, N. D. I965. Classification of the Bivalvia. Amer. Mus. Nov., 22o6. NICHOLS, D. I959. Mode of life and taxonomy in irregular sea-urchins. Publ. Syst. Assoc., 3, 6I-8o. -- I962. Differential selection in populations of a heart urchin. Publ. Syst. Assoc., 4, lO5-18. PASTmLS, A. I964. Les lamellibranches non-marins de la zone ~t communis (Westphalien A) de la Belgique. Publ. Centre nat. gdol. houillkre, 9. PHILIP, G. M. I965. Classification of echinoids. Jl Paleontol., 39, 45 -62" ROLLINS, H. B. & BA'I'rEN, R. L. I968. A sinus-bearing monoplacophoran and its role in the classification of primitive molluscs. Palaeontology, z x, 132-4 o. ROWELL, A. J. 1967. A numerical taxonomic study of the chonetacean brachiopods. In C. Teichert & E. L. Yochelson (editors) : Essays in paleontology and stratigraphy, 113-4 o. Lawrence, Kansas. RtroWlCK. M. J. S. I968. The feeding mechanisms and affinities of the Triassic brachiopods Thecospira Zugmeyer and Bactrynium Emmerich. Palaeontology, xz, 329-6o. SHAW, A. B. I969. Adam and Eve, paleontology and the non-objective arts. Jl Paleontol, 43, Io85-98. SIMVSON, G. G. I96I. Principles of animal taxonomy. New York. Sovo, L, R. R. I962. Typology and empiricism in taxonomy. Jl theor. Biol., 3, 23°-67 • & CAmN, J. H. I965. The two taxonomies: areas of agreement and conflict. Syst. Zool., x4, 176-95 . SOKOLSKAYA, A. N. I96O. Nadsemeistro Chonetacea. In Y. A. Orlov (editor) : Osnovypalaeontologii, Moscow. 221- 3. STENZEL, H. B. 1963. A generic character, can it be lacking in individuals of the species in a given genus? Syst. Zool., I2, I I8-2o. SZARSKI, H. I962. The origin of the Amphibia. Quart. Rev. Biol., 37, I89-24 I- TEmHERT, C. 1967. Major features of cephalopod evolution. In C. Teichert & E. L. Yochelson (editors): Essays in paleontology and stratigraphy, Lawrence, Kansas. 162-2 Io. & Others. 1964. Mollusca. Treatise invert, paleontol., K, 3. THOMSON, K. S. 1964. The ancestry of the tetrapods. Sci. Progr., 5, 45I-9 • URBANEK, A. I96O. An attempt at biological interpretation of evolutionary changes in graptolite colonies. Acta palaeont, polon., 5, 127-234. WALKER, T. R. 1969. The evolution of the Argopecten gibbus stock (Mollusca: Bivalvia), with emphasis on the Tertiary and O uarternary species of eastern North America. Jl Paleontol, iem. 3- WALMSLEY, V. G., BOOCOT, A.J., & HARPER, C. W. I969 . Silurian and Lower Devonian salopinid brachiopods. Jl Paleontol., 43, 493-5 I6. WEIR, J. 1968. The Carboniferous non-marine Lamellibranchia. mon. palaeontogr. Soc. WESTOLL, T. S. I956. The nature of fossil species. Publ. Syst. Assoc., 2, 53-62.

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WILLIAMS, A. I968. Evolution of the shell structure of articulate brachiopods. Spec. Papers Palaeontol. Assoc., 2. & Others. I965. Brachiopoda. Treatise invert. Paleontol., H. WILSON, E. C. I963. An evaluation of the genomorph concept in corals. Syst. Zool., x2, 83-9o. WHITa'INOTON, H. B. 1966. Phylogeny and distribution of Ordovician trilobites. Jl Paleontol., 40, 696-737 • Woot), A. E. 1965. Clades and grades amongst rodents. Evolution, x9, 1 I5-3 o, ZIEGL~R, A. M. I966. The Silurian brachiopod Eocoelia hemisphaerica (J. de C. Sowerby) and related species. Palaeontology, 9, 523-43.

Manuscript received 8th April 1970; address delivered 23rd April x969 .

[Professor] T. Neville George, D.sc., PH.D., SC.D., D.-I~.S-SC., LL.D., F.R.S., F.R.S.E., F.G.S. Department of Geology, University of Glasgow, Glasgow, w2.

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