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Atti II Conv. Int. Pallini pp. 273-289 Chronospecies in Ammonites F.E.A. Pergola, 87 et alii cur. 3 taw., 6 figg.

The concept of chronospecies in ammonites

JERZY DZIK

Zakfad Paleobiologii PAN, PL-02-089 Warszawa, Poland

RIASSUNTO specific population in samples of the same geologic age (objective in principle), Le sole unita tassonomiche direttamente identificabili in paleonto- (5) reconstruction of evolutionary lineages by assembling logia sono i paleofena o gruppi di esemplari in un campione che pre- series of population from samples of different age, senta una distribuzione continua e unimodale della frequenza di tutti which are identified as being in close ancestor- i caratteri. Paleofena coevi possono essere sistemati in biospecie men- descendant relationships, tre serie temporali di questo formano delle linee filetiche. I risultati di entrambi i procedimenti sono empiricamente comprovabili. (6) delimitation of chronospecies within the lineage (sub- La ricostruzione delle linee e una condizione necessaria a priori per jective in principle), una definizione coscienziosa delle cronspecie, cioe segmenti arbitraria- (7) naming the chronospecies. mente designati di una linea. Un esempio empirico dimostra che pud Chronospecies is an evolutionary concept. Before any non essere possibile riconoscere i paleofena senza la biometria. chronospecies can be precisely defined, the of Vengono discusse Ie prove per la presenza di due biospecie dimorfi- its lineage has to be determined. Even though the mean- che nel Calloviano di -Lukow (Polonia) e per la natura dimorfica dei ing of in is so frequently vague, the generi valanginiani Saynoceras e Valanginites. gradualistic evolutionary nature of chronospecies is gener- ally assumed as self-evident. Explicit presentation of a ABSTRACT phylogenetic hypothesis of ancestor-successor relationship appears thus to be a necessary precondition of sound in- The only directly identifiable taxonomic units in paleontology are terpretation of a chronospecies, and the reliability of its paleophena, or groups of specimens within a sample that show con- identification depends on reliability of a time correlation tinuous and unimodal frequency distribution of all characters. Coeval of different paleontological data sets. paleophena can be arranged into biospecies while time series of them may form lineages, the results of both these procedures being empiri- I shall discuss the methodological limitations at each cally testable. Lineage reconstruction is a necessary precondition for of these steps and confront theoretical aspects of evolu- conscientious definition of chronospecies, i.e. arbitrarily designated seg- tionary studies with taxonomic practice by presenting the ments of a lineage. An empirical example demonstrates that it may not ways of identification of chronospecies in ammonites. To be possible to recognize paleophena without biometrics. The evidence this end, four limestone concretions from the for presence of two dimorphic biospecies in the Callovian of ijukow, Lukow clays are chosen to show how paleontological sam- Poland, and for dimorphic nature of the Valanginian 'genera' Say- ples can be processesed. The Lukow material can fulfill noceras and Valanginites is discussed. even the most severe requirements for paleobiologically meaningful sampling, concerning especially the geograph- KEY WORDS ic, lithologic and biostratigraphic homogeneity. To illus- trate the method of identification of paleophena, the Ammonites - Evolution Chronospecies - Dimorphism Callovian - specimens of Quenstedticeras extracted from these sam- Poland. ples are measured and the results are presented on plots showing ontogenetic changes in various features of their INTRODUCTION shell geometry and ribbing. A series of populations occurring in stratigraphic ord- Although it is not necessary to introduce any kind of er in the section of Volgian clays in Brzostowka, worked taxonomic units of species rank for the sake of paleonto- out by Kutek & Zeiss (1974), well exemplifies an empir- logic evolutionary studies alone, this may be useful for ically reconstructed evolutionary lineage. The effects of scientific communication. One does not necessarily need application of different modes of defining boundaries be- species names to study evolution but one surely needs tween chronospecies (vertical or horizontal) will be dis- them to talk about the course of evolution. Since the in- cussed in this context. It will be shown why the applica- troduction by Simpson (1944, 1953) of the basic concepts tion of the vertical (typologic) approach to chronospecies of population biology to theoretical paleontology, the results in false presentation of evolutionary processes and methodology of empirical studies on evolution at the spe- makes the ranges of biostratigraphic units sensitive to cies level has been elaborated. The resulting standard tax- differences in sample size. onomic procedure can be summarized step by step in the Another example represents the opposite extreme of following way: reliability of sampling and availability of materi- (1) collecting samples of , als. It concerns the olcostephanids from the Valanginian (2) recognition of (paleo)phena in samples, clays cropping out in W^wat. In spite of the good preser- (3) assembling paleophena into biological populations wi- vation of fossils, this section is more typical of the usual thin particular samples. working conditions in ammonite taxonomy and biostratig- (4) delimitation of biospecies by identification of con- raphy in that the ammonite specimens are dispersed in 274 FOSSILI, EVOLUZIONE, AMBIENTE - J. DZIK

unbedded clay lacking any marker horizons. Neither large drift blocks in eastern Poland (Kosmulska 1973). population variability nor evolutionary transformations Ammonites in these concretions show excellently can therefore be reliably identified on the basis of the preserved aragonitic conchs (Kulicki 1979) with usually available collections. empty phragmocone chambers (PI. 1: 1). They could not Finally, the results of the search of names appropriate be redeposited or even exposed for a longer time before for the chronospecies represented in Brzostowka, Lukow, burial. and W^wal will be discussed. The valid specific name for For the purpose of the present presentation I extract- populationally understood chronospecies is the oldest one ed all ammonite specimens from 4 concretions. Careful proposed for any member (holotype) of any population preparation with simultaneous glueing and measuring of included in the chronospecies, irrespective of its morphol- specimens allowed to assemble a collection of 875 speci- ogy. This introduces some uncertainty when usual liter- mens. Ammonites occur in concretions in concentration ature data, lacking any references to population variabil- of about one hundred specimens per kilogram of the rock. ity, are to be used. Suggestions, how to proceed in such It is difficult to estimate number of specimens which were situations will be explained. destroyed during preparation. It probably represented from 10 to 20 per cent of particular samples. No appar- SAMPLING ent correlation between the size of specimens and losses in preparation have been found, therefore I consider the There is no significant difference in requirements to samples statistically representative of original fossil as- be met by good sampling between different areas of semblages. Almost all specimens have preserved at least paleontological studies. Whether samples are taken for a part of the living chamber and most of them have the paleobiological or evolutionary studies, they have to be chamber virtually complete. Ammonite shell fragments homogenous in respect to the geographic space and en- are practically absent in the rock matrix, which suggests vironment (Iithology). The time dimension represented that shell apertures were already broken before burial. in the part of the rock section from which the sample In a few cases, there is evidence of breakage with some was collected should be as short as possible. Sampling for displacement of produced parts, which took part after bu- evolutionary purposes may require especially rigorous ap- rial but before diagenesis. plication of the criterion of geological time homogenei- Particular samples significantly differ in the frequen- ty. It would be unrealistic to expect that the time dimen- cy distribution of specimen sizes. It is not clear whether sion of all samples used in such studies is insignificant. this is a result of differences in population dynamics or They must meet the precondition, however, that the time rather of preferential segregation at the time of burial. span involved in their accumulation was short enough to These differences are of much importance when trans- ensure that evolutionary transformations of the popula- formations of the morphology of shells are to be studied tion do not interfere with its variability. When this con- quantitatively. Changes in frequency distribution of mor- dition is not fulfilled, the range of the morphologic popu- phologies from sample to sample may result from differ- lation variability is artificially widened in effect of lump- ences in quantitative contribution to the samples of the ing together populations different in their evolutionary ontogenetic stage at which a particular character is best advancement (Westoll, 1956). The evolutionary transfor- developed. Transformations of the population dynamics mation itself cannot be adequately recognized from data may then simulate evolutionary processes (Dzik & Tram- distorted in such a way. mer 1980). It is widely assumed that only fossils collected from a single rock layer or even bedding plane can fullfill the PALEOPHENA basic requirements for properly performed sampling for evolutionary purposes. This unnecessarily severe precon- The only directly and objectively recognizable taxo- dition has appeared to be the most serious obstacle in ap- nomic unit in paleontology is a group of specimens wi- plication of macrofossils to studies on evolution. It is thin sample which shows continuous and unimodal fre- usually impossible to collect from a single bedding plane quency distribution of all characters. Mayr (1969) pro- a sufficient number of specimens to allow for reliable bi- posed for similar taxonomic units in neontology the term ometrical study of sampled populations. One should not 'phenon'. Insofar as samples in paleontology usually have hesitate to blend together fossils from different beds, a significant time dimension, such a time extension is the however, as long as there is no reason to believe that immanent feature of phena recognized by paleontologists. resulting sample is not homogenous in respect to space Therefore it seems reasonable to accentuate their special and geologic time. nature by calling them 'paleophena'. Paleophena may It has to be noted that such procedure, as long as done represent sexual dimorphs, particular groups of discon- consciously, does not necessarily diminish the scientific tinuously polymorphic populations, seasonal morphs, or value of obtained results. Any estimate of the population even separately fossilized organs belonging to the same variability is potentially falsifiable and can be tested by species (for instance, ammonite conchs and aptychi). In more extensive collecting of the same species from nar- some cases, paleophena may quite well represent groups rower parts of the section or by studying comparative of sympatric species, which are undistinguishable on the materials from another section. basis of their skeletal structures which are used to define The most reliable material for population studies in them (this is clearly the case when paleophena of aptychi paleontology can be obtained from single calcareous or are considered). sideritic concretions. Unless there is a special evidence Although discreteness of particular paleophena in a of reworking one may safely assume that the whole con- sample usually is apparent even for a naked eye, it is not tent of a single concretion was assembled instantaneous- always so easy to decide how many paleophena one is deal- ly in respect to the rate of evolution. It is well exempli- ing with. TheLukow samples well exemplify this problem. fied by concretions from the Lukow clays occurring as The morphologic variability of ammonites of the genus CHRONOSPECIES IN AMMONITES 275

Quenstedticeras, which occur there in abundance, followed of the ontogenetic advancement of specimens (Fig. 1,3). by Kosmoceras which does not contribute more than 10% Concerning whorl compression, most measurements form in number, presents a puzzle that used to be solved in two clusters separated by an area of rather unnumerous quite different ways by different workers (see Makowski records (Fig. 1) around whorl compression index close to 1952, 1962; Reyment 1971; Matyja 1986). These Quen- 1.1. Conchs larger than 10 mm in diameter are virtually stedticeras conchs can be arranged into a completely con- lacking in this field. The difference between the clusters tinuous series of morphologies, from very flat to very wide seems to disappear when smaller and smaller specimens in the cross section (PI. 2). Callomon (1985), who noted are considered, which may be partially due to an increase the same phenomenon in other samples of ammonites of of the error when minute specimens are measured with the family , concluded that it is practi- the same calliper. Moreover, two distinct groups within cally always represented by a single, very variable spe- each cluster can be recognized among larger specimens cies in any time horizon. which show the morphologic features of microconchs; To test the idea of Callomon (1985), I measured speci- they are separated and bordered by fields occupied by mens of Quenstedticeras from tukow. Although it is gener- juvenile macroconchs. This pattern strongly suggests the ally believed (Makowski 1962; Kulicki 1979) that the presence of four paleophena among representatives of number of shell whorls is the best measure of ontogenet- Quenstedtieeras in spite of continuous morphologic tran- ic advancement of any ammonite, it appeared impracti- sitions between them. cal to use this character when a great number of various- Apart from apertural modifications the differences be- ly preserved specimens had to be studied. The mumber tween macro- and microconchs in the cardioceratids are of whorls may differ between specimens of the same size expressed mostly in involuteness. Microconchs are usually due to their different whorl expansion rates. Then, more evolute and show a decreasing whorl expansion rate however, also other dimensions of the shell would differ at later stages of their ontogeny. To discriminate more (relative whorl height and umbonal diameter, for in- precisely between the presumed paleophena in the-fcukow stance). It does not seem that application of the shell di- samples, the indices of involuteness are plotted against ameter, instead of counting whorls (requiring time con- whorl compression for the measured specimens (Fig. 2). suming preparation of umbones), drastically biased results For the clarity of presentation, the ontogenetic stage of of the study. particular individuals is indicated by dot size, with the The measured characters of shell geometry and ribbing juveniles below 15 mm in diameter being represented by are here plotted against shell diameter, as an indicator smaller points and the adult specimens by larger ones.

Fig. 1. Ontogenetic changes of whorl compression in samples of ammonites of the genus Quenstedticeras from the Callovian of fcukow, Poland. The largest dots represent adult males. Circles = sample 210-D, black points = 210-C, black squares = 210-B, and white squares = 210-A. Note that the male conchs form two separate clusters with juvenile females inbetween. 276 FOSSILI, EVOLUZIONE, AMBIENTE - J. DZIK

Fig. 2. Recognition of paleophena in samples of the ammonite Quenstedticeras from fcukow, Poland. A plot of conch involuteness against the whorl compression reveals four groups among specimens with determinable sex. The smallest dots represent specimens with size ranging from 10 to 15 mm in diameter (smaller specimens were not considered because of too high measurement error). The largest dots represent adult males. Circles = sample 210-D, black points = 210-C, black squares = 210-B, and white squares = 210-A. Note the juvenile specimens concentrated in vertical belts in the center of the plot; with increasing size they tend to occur closer to the area of respective paleophena recognizable only at the adult stage of development.

The fields of identifiable macro- and microconchs are move to the second point of the standard of paleonto- delimited by lines connecting extreme records. The result- logical taxonomic studies: recognition of populations in ing picture (Fig. 2) shows two vertically elongated clusters samples. of juveniles in the center, with rare records inbetween, and four clearly separated fields of micro- and macro- FOSSIL POPULATIONS conchs. The presence of four separate paleophena in each of the Lukow samples appears evident. One may thus Since the classic reviews of the problem of sexual CHRONOSPECIES IN AMMONITES 277 dimorphism in ammonites by Makowski (1962) and Cal- identified dimorphic pairs. In the population of flat lomon (1963, 1969) appeared it has been generally ac- conchs, the males show more profound ontogenetic trans- cepted that the dimorphism is profoundly expressed in formations in the conch morphology, while females seem Mesozoic ammonites in the size and morphology of their to preserve more juvenile features of the shell geometry conchs (i.a. Westermann 1966; Lehmann 1966; Palfra- at the subadult stage. The whorl compression had in- man 1966, 1969; Riccardi et al. 1971; Ziegler 1974; creased strongly during growth of a subadult male conch Riegraf et al. 1984). Particular macro- and microconchs and finally reduced to the value typical for females, which in the -Lukow materials were already identified as sexual corresponds to the reduction of the whorl expansion rate dimorphs in species of Quenstedticeras and Kosmoeeras by at the attainment of adult stage. Among robust wide Makowski (1962). In the case of the investigated sam- conchs it was the male, which preserved more of juvenile ples, it seems reasonable to consider flat macro- and features in its shell geometry. Its female mate underwent microconch phena as representing one species and the a profound change in parameters of the whorl expansion wide ones as belonging to another. No evidence for the rate. Almost suddenly, at some stage of ontogeny, it be- presence of the third phenon within the range of Quen- gan to increase much more in the lateral diameter, giv- stedticeras species proposed by Matyja (1986) has been ing in effect a -like adult shell morphology quite found. dissimilar to that of the juveniles and males (PI. 2: 5). The above interpretation is supported by the observed Interestingly, this change may occur at quite variable stage course of ontogeny (Fig. 1). Specimens representing each of the ontogeny. In a few specimens it has been recog- of the two major clusters are initially undistinguishable nized at the conch diameter of only 2 mm; usually it oc- but diverge later on in their development, thus produc- curs at the diameter of about 25 mm, but there are also ing two pairs of subordinate clusters representing the par- specimens in which this change in the shell geometry evi- ticular paleophena. It is remarkable that the pattern of dently did not took place at all (Pi. 2: 2-3; Makowski ontogenetic differentiation of sexes differed in the two 1962, Pl. 13: 1). They are rather subordinate in the

Fig. 3. Ontogenetic change in development of intercalatory ribs in different samples of Quenstedticeras carinatum and Q. henrici from the Callovian of iukow, Poland. Rib index refers to the number of secondary ribs corresponding to 5 primary ribs (RI = 10 means thus that divisions are strictly dichotomous). Note that idealized regression line is U-shaped in every case but in both species specimens from the sample 2IO-B (black squares) tent to arrange along much deeper curve than specimens of other samples. Circles = sample 210-D, black points = 210-C, and white squares = 210-A. 278 FOSSILI, EVOLUZIONE, AMBIENTE - J. DZIK

-Lukow material, and because of the relative rarity they The most surprising feature of the data is that the dis- cannot be collected from any single concretion in a num- parity between the two pairs of samples concerns both ber sufficient to establish their relation to the other forms the biological populations recognized in each of them. of Quenstedticeras. A use of museum specimens might Although, as noted above, the patterns of ontogeny are result in lumping together specimens from quite differ- different, the higher ratio of secondary to primary ribs ent time horizons. Therefore the biological significance at the middle of the ontogeny characterized both flat and of these relatively rare occurrences of robust shells lack- wide conchs of Quenstedtieeras in the samples 210-A and ing abrupt changes in shell geometry remains unclear. B. This apparently synchroneous expression of the same Makowski (1962) considered them to represent separate trend in populations of different sympatric species is sug- population (biospecies) of Quenstedticeras. They might, gestive of a transfer of genetic information between them. however, originate by imprecise physiologic controls on It can hardly be explained by a residual transfer across ontogenetic development in these ammonites. the genetic barrier between species, however, and an in- A similar feature was already reported regarding shell fluence of non-Mendelian ways of exchange of the genetic ornamentation of Volgian virgatitds (Dzik 1986), where information (Jeppson 1985) are even less likely. Another an abrupt change in rib division took place, from dense- possible explanation, that the synchroneous change in rib- ly packed bifid ribs to polygyrate ones. This change is bing reflects a response to environmental factors, would correlated with a corresponding change in shell involute- undermine the usefulness of the ribbing features for time ness (Dzik 1986). The exact ontogenetic timing of the correlation. The interpretation most appealing to me is change in ribbing, although subject to directional evolu- that this superficially sophisticated parallelism in evolu- tion which suggests influence of directional selective pres- tion of the ribbing pattern is a by-product of a much sim- sure, was a matter of profound intrapopulation variabili- pler trend shared by the whole branch of early cardiocer- ty (Fig. 3). In effect of this unprecise control of the course atids. The pattern of ribbing depends on the involute- of ontogeny, the changes, which are apparent when par- ness of ammonite conchs (Dzik 1986), which is known ticular specimens are studied, appear almost unrecogniz- to change in the evolution of Quenstedticeras. It may thus able on plots of large samples. Asynchrony in develop- be that Q. earinatum followed the same as Q. henriei trend ment makes its record virtually unreadable at the popu- in increase of involuteness, even if it is not apparent from lation level. the analysis of available data. Some ontogenetic and evolutionary transformations of Whatever is the reason of the differences, it has to be the ribbing patterns occur also in the cardioceratids. This concluded that the same two biospecies are represented is of much importance regarding the methods of recog- in the samples 210-A and B, while the populations from nition which populations from different samples may be- the samples 2IO-C and D represent another pair of bi- long to the same biospecies. ospecies, probably from a different time horizon. What remains to be elucidated is the relationship between these FOSSIL BIOSPECIES pairs of conspecific populations.

The most important evolutionary transformation LINEAGES known to occur in the Quenstedticeras lineage is the change from basically dichotomous ribbing to ribbing with secon- The four biospecies recognized in the studied Lukow dary ribs many times more numerous than the primary samples almost certainly represent two evolutionary line- ones (see Callomon 1985). The pattern of ribbing under- ages. This is indicated by the significant overlap in charac- went deep modifications also in ontogeny. Generally, just ter frequency distribution between equivalent populations after the ribbing appears in juvenile conchs, the ratio of and by the obvious time proximity of these samples. If secondary to primary ribs is significantly higher than in later stages prior to development of the dimorphic fea- one assumes that the evolution of ribbing in Quensted- tures. At the stage of recognizable sexual dimorphism the tieeras was strictly directional, then the populations from ratio increases again (this feature does not seem to be a the samples 210-A and B are slightly younger. Reliabili- subject of otherwise profound dimorphism) to be lowered ty of such an assumption, however, is far from obvious, in adult male conchs just before the development of ter- and the differences may just be caused by temporal os- minal aperture. In female conchs the ribbing gradually cillations in direction of the evolution or may even dissapears (PI. 1 & 2). represent environmentally controlled features, insignifi- In the case of the Lukow material it appears that sam- cant from the evolutionary point of view. For a more lu- ples from different concretions differ significantly in con- cid presentation of the ways of reasoning in lineage recon- spicuousness of the change in ribbing at the middle stages struction it seems therefore better to refer to another ex- of the ontogeny (Fig. 4). In specimens from the sample 2IO-A and perhaps also 2IO-B the change is apparently ample, namely to the series of samples from the Volgian much deeper and the rib ratio is much higher than in the of Brzostowka near Tomaszow Mazowiecki, Poland. remaining samples 2IO-C and 210-D. Particular samples Several populations, representing both macro- and of ammonites differ too much in the population dynam- microconchs of early virgatitids, were identified in sam- ics to allow recognition of the pattern with adequate pre- ples collected by Professor Jan Kutek in an abandoned cision (for instance, juveniles dominate in the sample clay pit at this locality (Dzik 1986). Although, due to poor 210-A, while they are much less numerous in the sample preservation of the specimens, some uncertainty remains 210-B). This precludes also a precise quantitative whether they all are strictly monospecific or not, this does between-sample comparison of variability at particular stages of ontogeny. not seem to undermine validity of the general evolution- ary trends recognized there by Kutek & Zeiss (1974). Ac- cording to their evolutionary interpretation the stage of polygyrate ribbing, initially restricted to a narrow zone CHRONOSPECIES IN AMMONITES 279

Fig. 4. Distinction between typologic and population concepts of chronospecies as exemplified by the problem of demarcation of the boundary between Quenstedticeras henrici and Q. lamberti. According to the typologic approach all specimens having more than three secondary ribs per a primary rib (rib index 30) belong to Q. lamberti, if the population approach is followed all populations showing modal values of the rib index exceed- ing 30 at the size class 50-60 mm are to be included in this species. in some juvenile conchs, expands both back and forth in reaching, although the particular morphologies show the ontogeny being expressed in more and more juvenile rather long durations due to very wide population varia- stages (Fig. 5). At the same time, ontogenetic changes con- bility. cerning the ribbing pattern and the conch involuteness The picture of ammonite evolution in the Brzostowka became dumped, producing in effect a uniformly poly- section was obtained by a simple ordination of data on gyrate ribbing of microconchs and juvenile macroconchs, particular populations according to their position in the which slowly disappears in larger macroconchs. Complete- rock column. This populational methodology, introduced ly gradual evolutionary transformations were quite far- for the first time in perfect form to evolutionary paleon- 280 FOSSILI, EVOLUZIONE, AMBIENTE - J. DZIK

Fig. 5. Transformations of the ontogeny in the Zarajskites scythicus (Michalski 1884) lineage, the Volgian of Brzostowka near Tomaszow Mazowiecki, Poland. CHRONOSPECIES IN AMMONITES 281 tology by Brinkmann (1929), was named the stratophenet- and densely ribbed, with prominent ventral tubercles (PI. ie approach by Gingerich (1979). It has turned out to be 3: 9.), while the large specimens occurring in higher stra- a powerful tool of phylogenetics in every group of fos- ta are more or less smooth throughout their ontogeny. sils, to which it was applied. There is a complete gradation between forms having As long as samples consist of numerous specimens col- prominent lateral tubercles associated with low ventral lected from single beds or concretions, there is no seri- ribbing (Dobrodgeiceras morphotype) and those which are ous objection to reliability of results achieved by completely smooth (Valanginites morphotype). Whether stratophenetic approach. The paleontological practice is this is expression of evolutionary transformations or rarely so appealing. Usually one has to base his evolution- rather intrapopulation variability cannot be determined ary conclusions on a few poorly preserved specimens hav- with the available data. ing bad or inadequate stratigraphic control, just opposite The Dobrodgeiceras and Valanginites type conchs are to the situation in Brzostowka. This is well exemplified the only ones which can be matched as macroconchs with by problems connected with studies on Valanginian am- the microconch Saynoeeras in the W^wah section. Such monites at the locality W^iwaJ, not far from Brzostowka. a relationship was already proposed by Thieuloy (1965: Ammonites are preserved in W^wah with aragonitic 841), although he subsequently changed his mind (Kem- shell walls, pyritized phragmocones, and living chambers per et al. 1981: 277). It can be supported by the similari- filled with phosphorite. Any lithologic marker horizons ty in patterns of lateral tuberculation and the identity of are practically lacking in the main portion of the section, the early stages of ontogeny between the proposed di- and the frequency of well preserved ammonites is insuffi- morphs. All these ammonites have smooth conchs at the ciently high to allow for collection of representative sam- early stages of ontogeny (Fig. 6A), and their ornamenta- ples just by digging the clay bed by bed. Several large tion, however different between macro- and microconchs, collections, however, were assembled during many years developed at similar conch diameter. The hypothesis is of work by many students of the locality. therefore proposed that a lineage represented by Of special interest are the 'olcostephanids', which are paleophena of micro- (Saynoceras) and macroconchs represented in collections by numerous specimens with (Dobrodgeiceras and/or Valanginites) occurred in the sec- complete terminal aperture. Some of them represent typi- tion of Wgwah and evolved toward larger size and smooth cal minute microconchs with long lappets [Saynoceras), appearance of female conchs. Additional collections from while other, somewhat larger specimens have their aper- W^waf as well as from other localities of similar age can ture armed with a thick roll and may represent macro- be used to test this hypothesis. conchs Valanginites and Dobrodgeiceras) (PI. 3). Both the All the preceding steps of the discussed taxonomic macro-and microconchs collected in the lower part of the procedure were oriented toward recognition of objective section are generally smaller than those from higher lev- biological processes. The two remaining steps, definition els. A single phosphorite concretion in the middle of the of chronospecies and its naming, are just matters of con- section provides a small sample in which moderately venience in scientific communication. It does not mean ribbed proposed macroconchs are associated with rather that there is complete freedom of decision in paleonto- small microconchs. Although few in number, these speci- logical taxonomy. To be useful, even subjective taxonomic mens show a range of variability which clearly indicates definitions must observe strict and generally accepted that, except for the mean size change, no evolutionary rules. transformation of the microconch morphology occurs in the W^wahmaterial (Fig. 6). This is not so clear with CHRONOSPECIES respect to the morphology of the macroconchs. The small macroconchs occurring in lower strata are prominently The first problem to be invoked is how the time dimen-

Fig. 6. Ammonite shells extracted from a sigle dark phosphorite concretion collected in the middle of the section at Wawal and proposed to represent the single species Saynoceras verrucosum (d'Orbigny 1841). A. Juvenile ZPAL Am IX/22. B. Juvenile female ZPAL Am IX/11. C. Somewhat older juvenile female ZPAL Am IX/20. D. Juvenile male ZPAL Am UX/15. E-F. Adult males ZPAL Am IX/17, 16. All. x 2. 282 FOSSILI, EVOLUZIONE, AMBIENTE - J. DZIK

sion of a species should be defined to allow for its iden- of singular standard deviations from the mean, 68% of tification solely on the basis of its morphological features. specimens are represented, therefore taxa which do not Any lineage can be subdivided into chronospecies in two show overlap at one standard deviation from the mean basically different ways. Either it is assumed that partic- are easily distinguishable even without biometric popu- ular morphologic features of a specimen determine its at- lation studies. tribution to a species, or the taxonomic decision is based The usage of temporal subspecies (chronosubspecies) on features of a whole population. The first approach in paleontology was criticized by Gingerich (1986), be- results in that the line cutting the lineage into species cause they are usually confused with true geographic separates different morphologies. It is thus vertical ac- subspecies. Subspecies are recognizable only within ge- cording to the standard presentation of evolutionary ographically widespread biospecies; thus they can occur processes with the time scale on the abscissa. When the only in geographically distant localities. With increasing second approach is in use, the line separates different geographic distance, the reliability of time correlation populations (biospecies); it is thus horizontal. decreases, however, which makes the recognition of bi- The vertical (typologic) definition results in a false osubspecies in paleontology rather vague methodological- representation of the process of evolution. The ranges of ly. It is hard to disagree with this opinion, especially since chronospecies defined in such a way always overlap. The proposals to introduce nomenclatorial distinctions be- range of the overlap as well as the range of the whole tween chronosubspecies and biosubspecies (i.a. Dzik & chronospecies defined typologically strongly depend on Trammer 1980 proposed to put a dash between sub- and size of the samples used in studies, especially in the mar- specific names) have not found support among paleon- ginal parts of the stratigraphical range of chronospecies. tologists. Because of this feature of the method, I proposed to aban- It has thus to be concluded that the basic taxonomic don it, in spite of its still wide usage (Dzik 1986). unit in paleontology should be chronospecies defined in The populational definition of chronospecies, like any such a way that the distance between mean values of di- other known definition, allows for identification of the agnostic characters for its extreme populations should not time limits of a chronospecies in sections other than lo- be smaller than one standard deviation mesured in the cus typicus only with a limited resolution. A zone of un- center of the time range of the chronospecies. certainty always remains, within which it is impossible In paleontological practice it is rarely possible to quan- to decide objectively to which of two successive tify the variability of diagnostic characters so precisely chronospecies populations are to be classified. These limi- as to allow for calculation of standard deviation, particu- tations result from natural oscillations in direction of the larly because many definitions of chronospecies are poly- evolution and they cannot be removed by additional sam- thetic. Therefore, the procedure of defining chronospe- pling or by increasing the sample size. cies is usually qualitative rather than quantitative. With Decision regarding the time range of a chronospecies a statistically significant sample in hand one is easily able should be taken in a specified section, possibly the type to estimate the real range of variability even without bi- section of the nominal biospecies, that is the section, in ometrics. Such procedure had also to be followed in the which the holotype was found. There are already cases case of subdividing the lineage of Zarajskites from of such formal indication of the boundaries of a proposed Brzostowka into chronospecies (Dzik 1986). Measure- chronospecies in a rock section (i.a. Bergstrom 1971). ments are necessary only if there is a suspicion that the Although decisions concerning the ranges of proposed variability is not unimodal. chronospecies are always subjective, it is convenient to follow some standards of morphologic ranges for species TAXONOMIC NOMENCLATURE (Culver etal. 1987). This prevents creation of either too OF CHRONOSPECIES narrowly (which would hamper identification in other sec- tions) ot too widely defined chronospecies (which would To serve properly as a means of scientific communica- leave unexploited correlation potentials of the species). tion, a defined chronospecies has to be named according Standardized morphologic dimensions of chronospecies to precise and unequivocal rules. The rules which are used may also potentially appear a useful unit of evolution, al- to name chronospecies should also be consistent with the lowing for quantification of its progress and rate. regulations proposed for biospecies by the International While discussing possible measures of the rate of evo- Code of Zoological Nomenclature. lution, Haldane (1949) suggested to employ the lenght To identify the proper name which should be used for of time necessary to change the population mean by one a chronospecies, among existing names of fossils, it is not standard deviation. Commenting upon this proposal, enough to find a holotype which fits the mean morpho- Simpson (1953: 6-7) explained that, "the variation in a logic features of the chronospecies. The holotype may turn population constitutes the raw material for evolution (in- out to be an endmember of variability range of a biospe- sofar as this is controlled by natural selection) and that cies, which is actually distantly related to the studied seg- the standard deviation measures the (absolute, not rela- ment of the lineage or not related to it at all. The holo- tive) variation, so that a rate in terms of standard devia- type morphology has thus little to do with both identifi- tion measures utilization of the raw material. Rate of cation of chronospecies and its naming. Rather, it is im- change in standard deviations also tends to measure rate portant whether a particular holotype (irrespective of its of reduction of overlap in two related populations". Fol- morphology) is representative of a population included lowing this line a reasoning Dzik & Trammer (1980: 76) in the chronospecies or not. It may happen that a speci- proposed that succeeding chrono(sub)species should not men designated to be the type of a species is an aberrant have overlapping ranges of one standard deviations from form quite dissimilar to typical representatives of its own the mean in any diagnostic character. Within the range population. In fact, this is quite common a case because CHRONOSPECIES IN AMMONITES 283 curiosity in form always attracts the attention of students, French locality, interpreted by Douville (1912) to be con- while the most common morphology may remain for long specific with Russian Q. carinatum (Eichwald 1865). Ma- unnoticed, being regarded as too trivial. To make proper kowski (1962) chose for the Lukow population the name choice of the name for the chronospecies, it is thus ne- Q. vertumnum (Leckenby 1859), which is based on a mi- cessary to complete the list of all, ever proposed holoty- croconch holotype from England. Microconchs of this pes which are believed to belong to populations of the morphology occur in strata of different age, associated considered chronospecies. The first introduced holotype with significantly different robust macroconchs. It is not is the holotype of the chronospecies, irrespective of the clear what was the exact age of the type horizon of Q. relationship of its morphology to the morphology propo- vertumnum. Its association with Q. lamberti (J. Sowerby sed to be typical of the chronospecies. It gives the name 1819) sensu Douville 1912, a chronospecies more advan- for the whole chronospecies. ced than Q. henrici, is not unlikely. Therefore, the name Practical application of these simple nomenclatorial ru- Q. carinatum (Eichwald 1865) is used here, following its les may be quite complicated. In many cases it is not pos- interpretation by Douville (1912). Unfortunately, the sible to identify with certainty the population to which exact type horizon is unknown also in the case of the ho- a particular holotype belonged. Sometimes the problem lotype of Q. carinatum. In central Russia, the species is may be solved by additional collecting at the type locali- reported to occur throughout the ranges of both Q. hen- ty, but there are many examples where the locality is unk- rici and Q. lamberti (Sazonova & Sazonov 1967), though nown or no longer existing (i.a. Dzik 1979). In other ca- the latter species is understood by Russian studens in a ses identification of population is impossible. This con- different way than by Douville (1912) and it may be con- cerns especially species established in reworked materials specific with advanced Q. henrici. from condensed sections. When such situation is met, it In the case of the W^wahmaterial, nomenclatorial pro- is recommended, for the sake of nomenclatorial stabili- blems are even more complicated, in accordance with the ty, to base decisions on the assumption that holotypes uncertain status of the recognized chronospecies. The pro- represent the modal values of variability of their popula- posed dimorphism of Saynoceras is quite unlike that tions unless there is any evidence to the contrary (Dzik known in the Olcostephanidae (see Riccardi et al. 1974; 1986). also here PI. 3: 16-17), and it seems doubtful if Saynoce- Complications of similar kind arise when a single spe- ras is really a member of this family. Anyway, chrono- cimen is to be identified taxonomically and named. It is species within the lineage represented in WgwaI are best then tempting to assume that the specimen is represen- defined on the basis of macroconch morphology. Most tative of the mode of its population. It has to be kept unfortunately, the oldest species name used for these am- in mind, however, that, although very likely, this is no- monites is Ammonites verrucosus d'Orbigny 1841, based thing more than an assumption. With an increasing num- on a microconch holotype. I was not able to identify from ber of morphologically similar specimens collected, the the literature data which macroconch paleophenon was likelihood that the assumption was correct increases, but associated with the holotype in the type locality. Also the it may well be that the first specimen was an endmem- kind of relationships between the macroconch morpho- ber of variability range of its own population. The same types of Dobrodgeiceras and Valanginites is hard to deter- morphology which is typical of a population can be found mine because of the scarcity of materials from the type much above and below its horizon as an extreme repre- locality. The valid generic name for all these forms is un- sentative of different populations. The probability that doubtedly Saynoceras, and it is highly probable that the such an extreme morphotype is found depends on sam- species names A. verrucosus and A. nucleus Roemer 1840 ple size. When only a few specimens are collected it is non Phillips 1829 can also be synonimized within the pro- most likely that they are representative of the modal mor- posed range of the chronospecies. The latter species na- phology (though there is always a chance to collect the me, as understood now, is a younger homonym (see Kem- most unusual specimen as the first one). This is why the per 1981: 274), it seems therefore reasonable to name the ranges of typologically defined chronospecies are so sen- considered chronospecies Saynoceras verrucosum. sitive to sample size. Identification of chronospecies in small samples is thus probabilistic by its very nature, and ACKNOWLEDGEMENTS this fact should find its expression in nomenclature. Tra- ditional ways of indicating uncertainty, like cf., aff., or I am deeply grateful to Antoni Hoffman (Zalcfad Pa- sp. should be in common use. Regardless of whether or leobiologii PAN, Warsaw) who carefully read the manu- not this indicated formally, one has always to remember script and made many very helpful comments concerning that not every fossil specimen of ammonite or any other both the merits and language. Remarks of Ryszard Mar- group of organisms is specifically identifiable even if it cionowski (Uniwersytet Warszawski) are also gratefully is very well preserved. This is a trivial truism for any bio- appreciated. Photographs were taken by Mrs. Grazyna logically oriented taxonomist but there are many geolo- Podbielska. gists using fossils to determine the age of rocks who seem to be unaware of it.

Coming back to the discussed empirical data, it may REFERENCES be suggested that the Lukow population characterized by flat conchs represent the same biospecies as the popula- BERGSTROM, S.M., 1971 - Conodont biostratigraphy of the Mid- tion from the beds H. 1-3 of Villers-sur-Mer, France, from dle and Upper of Europe and Eastern North Ame- where the type specimen of Quenstedticeras henrici was rica. Mem. Geol. Soc. Amer. 127: 83-160. selected by Douville (1912). A population of robust Quen- BRINKMANN, R., 1929 - Statistisch-biostratigraphische Untersu- stedticeras close to that of -Lukow is also present in this chungen an mitteljurassischen Ammoniten viber Artbegriff 284 FOSSILI, EVOLUZIONE, AMBIENTE - J. DZIK

und Stammenentwicklung. - Abh. Ges. Wiss. Gottingen, Math- MAKOWSKI, K., 1962 - Problem of sexual dimorphism in ammo- Naturwiss, Kl. N.F. 13 (3): 1-249. nites. Palaeontologia Polonica 12: 1-92. CALLOMON,J.H., 1963 - Sexual dimorphism in ammo- MATYJA, B.A., 1986 - Developmental polymorphism in Oxfordian nites. Trans. Leicester Lit. Philos. Soc. 57: 1-36. ammonites. Acta Geol. Polonica 36. 1-3, 37-68. CALLOMON, J.H., 1969 - Dimorphism in Jurassic ammonites. In MAYR, E., 1969 - Principles of Systematic Zoology. 428 pp. G.E.G. Westermann (ed.) Dimorphism in Fossil Metazoa and McGraw-Hill Book Co., New York. Taxonomic Implications: 111-125. PALFRAMAN, D.F.B., 1966 - Ammonites: Taramelliceras (de Lo- CALLOMON, J.H., 1985 - The evolution of the Jurassic ammoni- riol) and Creniceras renggeri (Oppel) from Woodham, Buckin- te family Cardioceratidae - Special Papers in Palaeontology 33: ghamshire. Palaeontology 9: 290-311. 49-90. PALFRAMAN, D.F.B., 1969 - Taxonomy of sexual dimorphism in CULVER, S.J., BUZAS M.A. & COLLINS, L.S., 1987 - On the 1 ammonites: Morphogenetic evidence in Hecticoceras brighti value of taxonomic standardization in evolutionary studies - (Pratt). In G.E.G. Westermann (ed.) Dimorphism in Fossil Me- Paleobiology 13 (2): 169-176. tazoa and Taxonomic Implications: 126-154. DOUVILLE, R., 1912 - Etudes sur Ies Cardioceratides de Dives, REYMENT, R.A., 1971 - Callovian ammonites [Lamberti-Zone) Villers-sur-Mer et quelques autres gisements. Memoires Soc. found in an erratic concretion near Svedala, Scania. Bull. geol. Geol. France 45: 1-77. DZIK, J., 1979 - Some terebratulid populations from the Lower Instn. Uppsala, N.S. 3 (2): 19-25. of Poland and their relation to the biotic envi- RlCCARDl, A.C., WESTERMANN, G.E.G. & LEWY1 R., 1971 ronment. Acta Palaeont. Polonica 24 (4): 473-492. - The Lower Olcostephanus, Leopol- DZIK, J., 1986 - Typologic versus population concepts of chrono- dina and Vavrella from west-central Argentina. Palaeontogra- species: implications for ammonite biostratigraphy. Acta Pa- phica 136A: 1-6. laeont. Polonica 30 (1-2): 71-92. RIEGRAF, W., WERNER, G. & LORCHER, F., 1984 - Der Posi- DZIK, J. & TRAMMER, J., 1980 - Gradual evolution of conodon- donienschiefer. Biostratigraphie, Fauna und Fazies des siidwe- tophorids in the Polish . Acta Palaeont. Polonica 25 stdeutschen Untertoarciums (Lias a). 195 pp. F. Enke Verlag, (1): 55-89. Stuttgart. GlNGERICH, P.D., 1979 - The stratophenetic approach to phylo- SIMPSON, J.G., 1953 - The Major Features of Evolution. 434 pp. geny reconstruction in vertebrate paleontology. In J. CRA- Columbia Univerity Press, New York. CRAFT & N. ELDREDGE (eds.) Phylogenetic Analysis and Pa- SIMPSON, J.G., 1961 - Principles of Taxonomy. 247 pp. leontology: 41-76. Columbia University Press, Irvington. Columbia University Press, New York. GlNGERICH, P.D., 1986 - Species in the fossil record: concepts, SAZONOVA, I.G. & SAZONOV, I.T., 1967 - Paleogeografia Rus- trends, and transitions. Paleobiology 11 (1): 27-41. skoj platformy v jurskoje i ranniemielovyje vriemia. Trudy Uni- HALDANE, J.B.S., 1949 - Suggestions as to quantitative measu- gni 62: 1-260. rements of rates of evolution. Evolution 3: 51-56. THIEULOY1J-P., 1965 - Morphologie de l'ouverture buccale d'un JEPPSON, L., 1986 - A possible mechanism for convergent evolu- cephalopode microconche: Saynoceras verrueosum (d'Orb.). Bull. tion. Paleobiology 12 (1): 80-88. Soc. geol. France 7 (7): 839-842. KEMPER, E., RAWSON, P.F. & THIEULOY, J.P., 1981 - Ammo- THIEULOY, J.P., 1977 - Les Ammonites boreales des formations nites of Tethyan ancestry in the early Lower Cretaceous of Neocomiennes du sud-est Francais (Province Submediterraneen- north-west Europe. Palaeontology 24 (2): 251-311. ne). Geobios 10 (3): 395-461. KOSMULSKA, R., 1973 - Nowe dane o polozeniu kry jurajskiej w Lukowie. Przeglad geol. 21 (8-9): 440-441. WESTERMANN, G.E.G., 1964 - Sexual Dimorphismus bei Am- KULICKI, C., 1979 - The ammonite shell: its structure, develop- monoideen und seine Bedeutung fiir die Taxonomie der Otoi- ment, and biological significance. Palaeontologia Polonica 39: tidae (einschl. Sphaeroceratinae; Ammonitina, M. Jura). Pa- 97-142. laeontographica 124A: 33-73. WESTOLL, T.S., 1956 - The nature of fossil species. In P.C. KUTEK1J. & ZEISS, A., 1974 - Tithonian-Volgian ammonites from Brzostowka near Tomaszow Mazowiecki, central Poland. Ac- Sylvester-Bradley (ed.) The Species Concept in Palaeontolo- ta Geol. Polonica 24 (3): 505-542. gy: 53-62. The Systematic Association, London. LEHMANN, U., 1966 - Dimorphismus bei Ammoniten der Ahren- ZIEGLER, B., 1974 - Uber Dimorphismus und Verwandschaftsbe- sburger Lias-Geschiebe. Palaont. Z. 40: 26-55. ziehungen bei 'Oppelien' des oberen Juras (: Ha- MAKOWSKI, H., 1952 - La faune Callovienne de Kokow en Polo- ploceratacea). Stuttgarter Beitr. Naturkunde Bl 1: 1-42. gne. Palaeontologia Polonica 4: 1-64.

Plate 1

1. Limestone concretion from the Callovian Lukow clays with assemblage of juvenile Quenstedticeras conchs. 8h = Q. henriei Douville 1912. Qc = Q. earinatum (Eichwald 1965). 2-3. Adult macroconch of Q. henriei Douville 1912 from the Callovian of Lukow; ZPAL Am VIII/1. All natural size. CHRONOSPECIES IN AMMONITES 285 286 FOSSILI, EVOLUZIONE, AMBIENTE - J. DZIK

Plate 2

Quenstedticeras henriei Douville 1912 from the Callovian of Lukow, Poland. 1. Juvenile female ZPAL Am VIII/2. 6-8. Adult males ZPAL Am VIII/9, 4, 8, respectively.

Quenstedtieeras earinatum (Eichwald 1865) from the Callovian of Lukow, Poland. 2-5. Juvenile females ZPAL Am VIII/3, 5, 10, 11. 9-10. Adult males ZPAL AM VIII/6, 7. All natural size. CHRONOSPECIES IN AMMONITES 287 288 FOSSILI, EVOLUZIONE, AMBIENTE - J. DZIK

Plate 3

Saynoceras verrucosum (d'Orbigny 1841) from the Valanginian of W^wal'near Tomaszow Mazowiecki, Poland. 1. Complete adult male ZPAL Am IX/13; preserved in soft yellow phosphorite concretion collected in the upper part of the section. 2-6. Incomplete pyritized shells of adult males from the middle of the section; ZPAL Am IX/10, 5, 14, 19, 18, respectively. 7. Adult male ZPAL AM IX/12; in dark phosphorite collected in lower part of the section. 8. Crushed male shell in clay with complete lappets; ZPAL Am IX/9. 9. Unusually small adult female ZPAL Am IX/2 with ventral tuberculation; in dark phosphorite collected in lower part of the section. 10-11. Pyritized phragmocones of females ZPAL Am IX/4, 3. 12-15. Adult female shells from the upper part of the section; ZPAL Am IX/6, 1, 8, 7.

Spitieeras sp. from the Berriasian of Rogoznik, Pieniny Klippen Belt, Polish Carpathians. 16. Adult male with lappets ZPAL Am X/2. 17. Juvenile female ZPAL Am X/l; both specimens from a single block of light micritic limestone.

Olcostephanus sp. from the Valanginian of W^wah 18. Fragment of pyritized phragmocone ZPAL Am IX/21. Natural size except for Figs. 1-9 which are x 2. CHRONOSPECIES IN AMMONITES 289