Typologic Versus Population Concepts of Chronospecies: Implications for Ammonite Biostratigraphy
Total Page：16
File Type：pdf, Size：1020Kb
Acta P al aeontologica Polonica Vol. 30, No. 1-2 p p. 71-92; pis. 9-10 Warszawa, 1985 JERZY DZIK TYPOLOGIC VERSUS POPULATION CONCEPTS OF CHRONOSPECIES: IMPLICATIONS FOR AMMONITE BIOSTRATIGRAPHY DZIK, J . 1985. Typologic ve rsus population concepts of chronospecies : im pli cati ons for a mmon ite bi ostratigra phy. Acta P ala eont. P olonlca , 30, 1- 2. 71- 92, 1986 (1985). The effe ct s of t he a p pli ca ti on of horizo n tal (po pulation) as op po sed t o vertical (t y po logic) ta x onomy on p recision of a mmonite ch ron ozonation a re discussed a n d a n example of stratigraphic inferen ce that ca n be derived f ro m t he p opulation m ethod is presente d . An almost com p lete m a croconch f rom Swtetoszewo , P ome r ania, is compared w ith a stan da rd, biometrically st udie d se ries o f samples f rom the Ea rly Volg ian of Brzost6wka, Centra l P olan d. Specimens i dentical m orph o logic ally w ith t h a t fr om Swtetoszewo can be fou n d amon g extreme e n d-members of the popu lation vari a bility much a bove a n d below the hor izon of the best fi t w ith t he m ean for the sample . Time co r relation is thus a p ro babilistic k ind of inference as the p robab le identi ty of time decreases with increa sing distance from the h or iz on of the best f it. For t his reason v ertical (typologie) di ag noses of chron ospecies cannot resu lt in better a n d more rella ble co r rela tions than a horizontal (po pulation) one. Because t he typolog ie species conce pt leads to fa lse r epre senta tion in phylogenetics a nd bi ogeo g ra ph y it is proposed to aban don its use ,nbi os t ratigraphy. J erzy D zi k, Zakla d Paleobiol o gii Polska Akademia Nauk, A I.Z W i r k i i W i gury 93, 02-089 W a r sza w a , P o land. R ecei v ed : S e p t emb er 1984. I NTRODUCTION Our knowledge of evolutionary mechanisms has increased rapidly owing to recent, thorough studies of stratigraphically dense series of samples of fossil mammals (Gingerich 1979), conodonts (La. Dzik and Trammer 1980), foraminifers (La. Malgren and Kennett 1981), gastropods (Buttner 1982; Mensink et al, 1984), and radiolarians (Lazarus 1983). The ammonites have contributed little to this area of study despite th e fact that they were 72 JERZY DZIK the first objects of empirical studies on phylogeny and strict methodology of phylogeny reconstruction was developed for ammonites by Brinkmann (1929; see also Raup and Crick 1981). This method, christened "strato phenetic approach" by Gingerich (1979), has been widely applied since its origination to many groups of fossils, with the notable exception of the ammonites. Despite great progress in understanding of the phylogeny of the Ammonoidea at higher taxonomic levels (Donovan et al. 1980), ap propriate interpretation of detailed relationships of particular ammonite species seems to be hampered by ambiguity of the concept of chrono species. In consideration of ammonite phylogeny the problem is especially well recognizable owing to great population variability and the long history of ammonite taxonomy, invented mostly by stratigraphers for stratigraphers with little attention paid to consistency with biological meaning of particular taxa. The aim of the present paper is to show that there is no contradiction between biological and geological points of view on the basic problems of taxonomy at the species level. Moreover, I shall try to show that in ignoring the biological interpretation of fossils, stratigraphers lose much important information. The case is worthy of attention also because the ammonites are unique among fossils in that they provide a commonly well preserved and easily accessible record of evolutionary modifications of the ontogeny. The group of ammonites on which the present study is based seems to be especially interesting in this respect, having a very complex and variable ontogeny. THE PROBLEM Let us consider an ideal transition series of samples representing a continuum of populations arbitrarily separated into two successive chrono species, a and b (fig . 1). There are two basically different methods of delimination of the boundary between related chronospecies. According to the first m ethod, currently in general use in ammonite studies, the boundary is defined on the appearance of a particular charac ter or set of characters considered to be diagnostic for younger chrono species. At the beginning of the time range of a chronospecies defined in this way such characters occur among extreme morphotypes of popula tions. Subsequently they spread out over the whole population being typical of most specimens until gradual suppression by newly appearing characters, typical of another, still younger species. This method of delimi tation was named a vertical one (Sykes and Callomon 1979) as the line separating chronospecies is vertical (see fig. 1). In fact, it is an application of the typologie concept to chronospecies. The second method, still rather rarely used, bases boundaries on parti cular features of whole samples of fossil populations. An exact beginning CONCEPTSO F CHRONOSPECIES 73 typologie ws ..... m - B ct III m m c: u 0 m .. a. III m 0 -::J c: 0 a. L 0 s: a. u MORPHOLOGY Fig. 1. Schematic presentation of r elationship between vertical (typologie) and horizontal (population) concepts of chronospecies. Horizontal bars rep resent standar d dev iat ions from the mean of every sample ; shade d parts of time rang es indicate limits of uncertainty in recognition of chronospe cies (in case of population conc ept) or ranges of subor dinate occurrence of the morphotype in sa m ples (in ca se of typo logie concept). Note that r anges of typologically defined chronospecies always over- lap sign ifica ntly. of a chronospecies can be indicated only in its type section, where it was arbitrarily chosen. In any other locality samples can be identified as belonging to the chronospecies if they do not depar t too much from the type sam ple in their morphologic features. Lack of any overlap in ranges of standard deviations was proposed as a minimum difference allowing re cognition of chronospecies (Dzik and Trammer 1980); a maximum morpho logic range of the species can be arbitrarily defined . There is thus a zone 74 JERZY DZIK at the boundary between succeeding chronospecies, corresponding to the morphologic shift in range of the standard deviation of the diagnostic feature, within which samples can be attributed either to older or to younger chronospecies (fig. 1). This is the horizontal or rather population method of chronospecies definition. Superficially, the first method looks like the stricter and more con venient one. It enables definitions of chronospecies (and zones correspond ing to them) on appearances of particular, easily and unequivocally recognizable characters. This is perhaps why the majority of ammonite students prefer the use of typologie species over the population method. It can easily be noted that the vertical definition of chronospecies makes time ranges of succeeding chronospecies significantly overlapping even if they represent a single continuum of populations and that never more than a single biospecies of the group existed simultaneously (see Gingerich 1979). The use of the method results thus in presenting non -existing sympatric speciations in evolutionary trees, artifacts which are, in fact, indistinguishable from real speciation events included in the same tree. The method leads unavoidably to artificial splitting of objective bio species into subjective typologie units. When ammonite assemblages from different localities are compared, one is then unable to say how many biospecies occurred in each of them, whether species (not necessarily identical with morphologic) diversity was similar or not, and so on. Bio geography built on such a poor basis makes little sense. There is more than enough reason to reject the vertical method as a tool for paleobio logical studies. Is the vertical method so much superior over the horizontal one in bio stratigraphical applications to counterbalance its biological deficiences? Let us analyze an example of age determination based on a Volgian ammonite from Pomerania. REFERENCE STANDARD Material. - The best known and most complete section of fossiliferous Early Volgian is that at Brzost6wka near Tomasz6w Mazowiecki in central Poland. Ammonites from this classic section are relatively well known owing to works of Michalski (1890), Lewinski (1923), and Kutek and Zeiss (1974). Kutek and Zeiss (1974) described ammonites collected in an abandoned clay-pit at Brzost6wka, where about 15 meters of marly shales and marls were exposed. They subdivided it into six units, five of them containing rich ammonite assemblages. A few years later, on the occasion of con struction in the area, an additional sample of flattened ammonites was col lected in black shales situated about ten meters above the top of the clay-pit exposure. A few specimens have been collected also from the CONCE PTS O F CHRONOSPECIES 75 overlying limestone series during many years of investigations of the area by Professor Jan Kutek. Together with specimens illustrated by Lewinski (1923) they form the youngest sample. Modes of preservation of th ese materials are rather diverse. Sp ecimens from limestones and limy concretions are preserved three-dimensionally, most of the specimens from the clay-pit section are partially flattened, whereas specimens from the overlying black shales are completely flat.