Buckman's first law of covariation–a case of proportionality

Buckman's first law of covariation states that in ammonoids, coarseness of ribs and other ornamentation correlates negatively with degree of compression (whorl height/width ratio) and involution. This ‘law’ has escaped several attempts at functional or physiological explanation. However, Buckman's first law becomes self‐evident, and almost collapses to a tautology, when we reword it as follows: Size of lateral and ventral ornamentation correlates with size of the aperture (its width and height, respectively). In a shell that is strongly compressed laterally, the diameter of the aperture, and hence the soft parts within, is small in the lateral direction. For such a shell, coarse lateral ribbing would imply impossibly strong relative compression and expansion of the soft body through the formation of a rib. Conversely, if the shell is depressed (compressed dorsoventrally), the diameter of the soft parts is small in the dorsoventral direction, and the ventral ribbing correspondingly small. Buckman's first law is thus simply a statement of proportionality that needs no special explanation. On the contrary, any departure from this ‘law’ would imply strong deformations of the soft body as ornamentation was formed. Measurements on the Triassic ammonite Pseudodanubites halli show that ventral ribs get relatively stronger on compressed specimens, in accordance with this view of Buckman's law. ? Ammonoids, intraspecific variation, morphometry, ornamentation.     

Systematic revision and sexual dimorphism in Choffatia (Ammonoidea: Perisphinctoidea) from the Callovian of Kutch,India

The genus Choffatia (Siemiradzki) is an important Callovian ammonite in Kutch biostratigraphy. Several species of the genus were described by the early workers who did not recognize intraspecific variability and sexual dimorphism in terms of the modern concepts. We have described here three dimorphic species based on material reposited in the Geological Survey of India, Kolkata as well as systematically collected additional specimens with precise stratigraphic information. These species are: Choffatia recuperoi (Gemmellaro, 1873), Choffatia cobra (Waagen, 1875), and Choffatia perdagata (Waagen, 1875). The microconchs of the three species, described here for the first time, are lappeted whereas the macroconchs have a simple aperture. However, these species have similar shell morphologies and they intergrade in shell shape and sculpture; thus it is difficult to distinguish them qualitatively. Statistical analyses (both bivariate and multivariate) support that they are distinct species and there is a good correlation among the degree of involution, the degree of inflation, and the strength of ornamentation. C. cobra is the most evolute, depressed, and strongly ribbed whereas C. perdagata is the most involute, compressed, and weakly ribbed. C. recuperoi occupies the intermediate position. This interspecific diversity of Choffatia thus supports Buckman's Law of Covariation.     

Meta-analysis of character utility and phylogenetic information content in cladistic studies of ammonoids

While previous workers have argued persuasively that ammonoid workers should use cladistic approaches to reconstruct phylogeny, relatively few cladistic studies have been published to date. An essential yet challenging part of cladistic analysis is the selection of characters. Are certain types of characters more likely to show homoplasy? Are certain aspects of shell anatomy more likely to contain phylogenetically informative characters? Are datasets with more characters inherently better? To answer these questions, a meta-analysis of character data from published ammonoid phylogenies was performed. I compiled 14 datasets, published between 1989 and 2007, representing parsimony-based phylogenetic analyses of ammonoids. These studies defined a combined total of 323 characters, which were grouped into categories reflecting different aspects of anatomy: shell size and shape, ornament, suture, early ontogeny, body chamber and apertural modifications. Tree searches were re-run to determine overall tree statistics, parsimony permutation tail probability (PTP) tests were calculated to assess the phylogenetic information content of the matrices, and retention and rescaled consistency indices for each character were calculated. My analyses revealed that studies with higher character/taxon ratios did not necessarily produce trees with more information content and less homoplasy, as measured by retention or rescaled consistency indices, because additional characters were often parsimony-uninformative. Rather, studies with relatively few characters could produce high-quality trees if the characters were well-chosen and character states carefully defined. Characters related to the body chamber and adult aperture typically had retention indices of either 0 or 1, rarely in between, indicating that they either worked perfectly or not at all. Suture characters tended to have higher indices than shell shape or ornament characters, suggesting more phylogenetic information and less homoplasy in the suture line than in shell traits. These results should aid in the selection of characters for future cladistic studies of ammonoids.     

Large‐scale evolutionary trends of Acrochordiceratidae Arthaber, 1911 (Ammonoidea,Middle Triassic) and Cope’s rule

Abstract: Directed evolution of life through millions of years, such as increasing adult body size, is one of the most intriguing patterns displayed by fossil lineages. Processes and causes of such evolutionary trends are still poorly understood. Ammonoids (externally shelled marine cephalopods) are well known to have experienced repetitive morphological evolutionary trends of their adult size, shell geometry and ornamentation. This study analyses the evolutionary trends of the family Acrochordiceratidae Arthaber, 1911 from the Early to Middle Triassic (251–228 Ma). Exceptionally large and bed‐rock‐controlled collections of this ammonoid family were obtained from strata of Anisian age (Middle Triassic) in north‐west Nevada and north‐east British Columbia. They enable quantitative and statistical analyses of its morphological evolutionary trends. This study demonstrates that the monophyletic clade Acrochordiceratidae underwent the classical evolute to involute evolutionary trend (i.e. increasing coiling of the shell), an increase in its shell adult size (conch diameter) and an increase in the indentation of its shell suture shape. These evolutionary trends are statistically robust and seem more or less gradual. Furthermore, they are nonrandom with the sustained shift in the mean, the minimum and the maximum of studied shell characters. These results can be classically interpreted as being constrained by the persistence and common selection pressure on this mostly anagenetic lineage characterized by relatively moderate evolutionary rates. Increasing involution of ammonites is traditionally interpreted by increasing adaptation mostly in terms of improved hydrodynamics. However, this trend in ammonoid geometry can also be explained as a case of Cope’s rule (increasing adult body size) instead of functional explanation of coiling, because both shell diameter and shell involution are two possible paths for ammonoids to accommodate size increase.     

Untangling phylogenetic,geometric and ornamental imprints on Early Triassic ammonoid biogeography: a similarity‐distance decay study

Ammonoids are diverse and widespread fossil, externally shelled cephalopods that flourished for more than 300 Myr before their total extinction 65 Ma ago. In spite of two centuries of intensive scientific studies, their mode(s) of life and long‐distance dispersal abilities remain poorly known. Here, we address this by focusing on the latitudinal distribution of Early Triassic (approximately 250 Myr) ammonoids through similarity‐distance decay analyses. We examine and compare rates of similarity‐distance decay between various groups with respect to systematics, shell geometry and ornamentation to untangle phylogenetic, geometric and ornamental imprints on the observed biogeographical pattern. Our data do not support any phylogenetic and shell ornamentation influence, but rather demonstrate the significant effect of (sub‐)adult shell geometry on the similarity–distance decay: most evolute morphs tend to have been more endemic than most involute forms. This contrasts with the classic hypothesis that long‐distance ammonoid dispersal mainly occurred during the earliest planktonic stages, and thus that (sub‐)adult morphological characteristics should not constrain large‐scale biogeographical patterns of ammonoids. Although direct control by Sea Surface Temperature can be discarded, this result may indicate that at least some adult Triassic ammonoid morphs were skilled active swimmers capable of achieving long‐distance migration, as observed for some present‐day coleoid cephalopods. □Ammonoid, dispersal, similarity‐distance decay, morphology, phylogeny, biogeography, Triassic.     

Ontogeny and evolution within the Myophorellidae (Bivalvia): Paedomorphic trends

The role of heterochronic phenomena in molluscan evolution is insufficiently understood but potentially significant. The aim of this paper is to explore some paedomorphic trends in the evolution of the Myophorellidae (Bivalvia: Trigoniida). Early ontogeny of general shell shape and ornamentation of one species of Steinmanella was analyzed and compared to data obtained for three species of Myophorella: two belonging to the subgenus M. (Promyophorella) (one from the Jurassic and one from the Cretaceous) and one belonging to the Jurassic M. (Myophorella). For general shell shape, a geometric morphometric analysis was performed on lateral views of the shells. Regarding ornamentation, flank costal disposition on the marginal carina, tubercle separation and relative development of the sub-commarginal subset of flank costae were quantified. A qualitative analysis was also performed. A two-trend shell shape development is considered as primitive. The first trend is marked by a relative reduction of the posterior margin together with a relative elongation of the shell. A tangential opisthogyrate growth component characterizes the second trend. There is a transitional stage where both trends interact. Early flank ornamentation is characterized by two or three sub-commarginal costae, continuous through the area, after which oblique costae with fine tubercles start to form. The subgenus M. (Myophorella) evolved by paedomorphic retention of juvenile shell shape and ornamentation, resulting in a large shell with coarse tubercles. Shell morphology in Steinmanella evolved by paedomorphic suppression of the primitive second trend in the development of the shell, resulting in an orthogyrate shell shape, and the retention of juvenile ornamentation (coarse tubercles, more sub-commarginal costae, juvenile rates of costal disposition). The paedomorphic (most likely by deceleration) retention of juvenile shell morphology within the Myophorellidae seems to have been recurrent within the group, resulting in many cases of convergence, and obscuring the phylogenetic relationships among its species.     

<Emphasis Type="Italic">Goniatites sphaericus</Emphasis> (Sowerby, 1814), the archetype of Palaeozoic ammonoids: a case of decreasing phenotypic variation through ontogeny

The Late Viséan (Early Carboniferous) ammonoid species Goniatites sphaericus (Sowerby, 1814) is revised with the use of the type material. With respect to conch morphology, suture line and particularly shell ornament, it is regarded as a senior synonym of the species Goniatites fimbriatus (Foord and Crick, 1897) and must therefore be re-established as an important index fossil for the ammonoid stratigraphy of the Rhenohercynian and Subvariscan shelves. The Goniatites fimbriatus Zone has been re-named Goniatites sphaericus Zone on this basis. The study of ontogenetic changes in the conch parameters shows that intraspecific variation occurs within a wide range. In the whorl profile, variation is very high in small juveniles, while in the coiling rate, intermediate growth stages are more strongly affected.     

Complexity and integration in sexual ornamentation: an example with carotenoid and melanin plumage pigmentation

Sexual ornaments often consist of several components produced by distinct developmental processes. The complexity of sexual ornaments might be favoured by mate choice of individual components in different environments which ultimately results in weak interrelationships (integration) among the developmental processes that produce these components. At the same time, sexual selection for greater exaggeration of individual components favours their stronger co-dependence on organismal resources. This should ultimately produce stronger condition-mediated integration among ornaments' components in individuals with the most exaggerated ornamentation. Here we distinguish between these two sources of integration by examining the relationship between integration and elaboration of sexual ornamentation in three bird species: two with carotenoid-based sexual ornamentation (the house finch, Carpodacus mexicanus and common redpoll, Carduelis flammea) and a species with melanin-based sexual ornamentation (house sparrow, Passer domesticus). We found that integration of components varied with elaboration of carotenoid-based ornamentation but not of melanin ornamentation. In the house finches, integration was the highest in individuals with small ornaments and decreased with ornament elaboration whereas the pattern was the opposite in common redpolls. These results suggest that in these species integration and complexity of carotenoid-based ornamental components are due to shared condition-dependence of distinct developmental pathways, whereas integration and complexity of the melanin ornamentation is due to organismal integration of developmental pathways and is largely condition- and environment-invariant. Thus, functionally, ornamentation of the house sparrows can be considered a single trait, whereas complexity of the house finch and redpoll ornamentation varies with ornament elaboration and individual condition.     

Sutural amplitude of ammonite shells as a paleoenvironmental indicator

The Sutural Amplitude Index (SAI), obtained by measuring the maximum height of sutural elements and the length of the suture pattern on ammonoid shells, provides a useful indicator of relative habitat depth for ammonoids with similar shell morphotypes. A higher SAI indicates greater septal support for the shell wall against implosion under increased hydrostatic pressures in deeper waters. Relating the sutural amplitude indices of ammonites found in a well-studied depositional basin, such as the Cretaceous Western Interior Seaway, to morphotype distributions illustrates the utility of this index in bathymetric interpretations. □ Ammonoidea, sutures, Sutural Amplitude Index, Cretaceous, Greenhorn Cyclothem. Western Interior.     

Condition-dependent variation in the blue-ultraviolet coloration of a structurally based plumage ornament

After years of investigation into the function of sexually dimorphic ornamental traits, researchers are beginning to understand how bright plumage colour in birds acts as an intraspecific signal. This work has focused primarily on pigment-based ornaments because they are highly variable in patch size, hue and brightness for some species. In contrast, structurally based ornaments have been little studied, in part because they do not appear to be as variable as pigment-based ornaments. We investigated a structurally based plumage ornament in a wild population of blue grosbeaks (Guiraca caerulea), a sexually dimorphic passerine. We report plumage variation that extends into the ultraviolet region of the spectrum. The pattern of covariation between four out of five elements of plumage variation suggests that structurally based ornamentation is pushed towards extreme expression of the trait as predicted by the sexual selection theory. The ''bluest'' birds have the highest percentage of blue feathers on the body. These ornamental feathers reflect light maximally at the shortest wavelengths (ultraviolet), with the greatest intensity and the greatest contrast. Age may have some effect on expression of blueness. In addition, plumage variables are correlated with growth bars in tail feathers (a record of nutritional condition during moult in a non-ornamental trait). This suggests that the ornament is partially condition dependent. Thus, blue plumage in male grosbeaks may serve as an honest indicator of age and quality.     

Context-dependent sexual advertisement: plasticity in development of sexual ornamentation throughout the lifetime of a passerine bird

Male investment into sexual ornamentation is a reproductive decision that depends on the context of breeding and life history state. In turn, selection for state- and context-specific expression of sexual ornamentation should favour the evolution of developmental pathways that enable the flexible allocation of resources into sexual ornamentation. We studied lifelong variation in the expression and condition-dependence of a sexual ornament in relation to age and the context of breeding in male house finches (Carpodacus mexicanus)--a species that develops a new sexual ornament once a year after breeding. Throughout males' lifetime, the elaboration of ornamentation and the allocation of resources to the development of sexual ornamentation depended strongly on pairing status in the preceding breeding season--males that were single invested more resources into sexual ornamentation and changed ornamentation more than males that were paired. During the initial (post-juvenile) moult, the expression of ornamentation was closely dependent on individual condition, however the condition-dependence of ornamentation sharply decreased throughout a male's lifetime and in older males expression of sexual ornamentation was largely independent of condition during moult. Selection for early breeding favoured greater ornamentation in males that were single in the preceding seasons and the strength of this selection increased with age. On the contrary, the strength of selection on sexual ornamentation decreased with age in males that were paired in the preceding breeding season. Our results reveal strong context-dependency in investment into sexual ornamentation as well as a high flexibility in the development of sexual ornamentation throughout a male's life.     

Disintegrating the fly: A mutational perspective on phenotypic integration and covariation

The structure of environmentally induced phenotypic covariation can influence the effective strength and magnitude of natural selection. Yet our understanding of the factors that contribute to and influence the evolutionary lability of such covariation is poor. Most studies have either examined environmental variation without accounting for covariation, or examined phenotypic and genetic covariation without distinguishing the environmental component. In this study, we examined the effect of mutational perturbations on different properties of environmental covariation, as well as mean shape. We use strains of Drosophila melanogaster bearing well‐characterized mutations known to influence wing shape, as well as naturally derived strains, all reared under carefully controlled conditions and with the same genetic background. We find that mean shape changes more freely than the covariance structure, and that different properties of the covariance matrix change independently from each other. The perturbations affect matrix orientation more than they affect matrix eccentricity or total variance. Yet, mutational effects on matrix orientation do not cluster according to the developmental pathway that they target. These results suggest that it might be useful to consider a more general concept of “decanalization,” involving all aspects of variation and covariation.     

Intraspecific morphological variation in late Miocene Bolboforma, and implications for their classification, ecology, and biostratigraphic utility

A preliminary morphological study of large, well preserved bolboform populations from the late Miocene section of DSDP Site 593, in the southern Tasman Sea, reveals a relationship between test size and intraspecific morphological variation interpreted as a gradual transformation in test shape and surface ornamentation. The succession begins with small, smooth, thin-walled subspheroidal tests. As test size increases, rudimentary ornaments become more common and the shape more globose. With further size increase, the walls thicken and ornaments become more structured, but specimens still retain a rounded globose shape. In still larger tests, the ornament becomes more prominent and tests develop a squatter shape with flatter aboral surfaces. The transformation generally ends with large, thick-walled, highly ornate forms with squat globose tests, robust ornaments, and relatively flat aboral surfaces. Although the size of the test increases notably through the transformation, there appears to be no corresponding change in the internal chamber size from its initial value, and so the transformation is largely due to the addition of calcite to the outer surface of the test wall. This appears to be an inherent biological feature and does not involve calcite dissolution/reprecipitation or other retrograde processes. The detail of the transformation varies between species and is presented here for Bolboforma subfragoris s.l., B. gruetzmacheri n. sp., and B. metzmacheri s.s., which exhibit different types of ornament — spinose, irregularly ridged, and cancellate, respectively.When different morphological forms of a single species occur in the same sample, they have the potential to be interpreted as different species. Taxonomic confusion is further compounded by the morphological similarity of many small, smooth-walled, and weakly ornamented forms of different species. These features are common in bolboform populations and have contributed to the misinterpretation of some species. Differences in test shape and ornament are most notable in large ornate forms and taxonomic interpretations should be based wherever possible on such forms. Once the taxonomy is clear, the biostratigraphic utility of the late Miocene bolboforms from Site 593 has been in the recognition of single species (or at most two) in a given sample. This provides an excellent biostratigraphic framework for a period of time that lacks significant alternative microfossils for age control in New Zealand sediments.The distribution of bolboforms in (mainly) southern hemisphere marine sediments appears to track the palaeocirculation of Southern Component Intermediate Water (SCIW). Hence addition of calcite to the outer surface of the test wall probably occurred while the bolboforms were entrained in subsurface flows of SCIW. In this respect, the increased complexity of ornamentation may have been a morphological adaptation to counter the loss of buoyancy as calcite was added to the outer surface of the test wall. This would have presumably contributed to the entrainment and dispersal of bolboforms in the flow of subsurface water masses, and it suggests that the tests of some bolboforms at least may represent a dispersal phase in the life cycle of this enigmatic group of microfossils.     

Early evolutionary trends in ammonoid embryonic development

During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.     

Reappraisal of the latest Albian (Mortoniceras fallax Zone) cephalopod fauna from the classical Salazac locality (Gard,southeastern France)

The Salazac locality (Gard, southeastern France) is renowned for the richness of its cephalopod fauna (especially ammonites) from the Mortoniceras fallax Zone (uppermost Albian, Lower Cretaceous). However, most ammonite species have paradoxically been scarcely illustrated up to now. Furthermore, the rare assessments of ammonite taxonomic diversity are presumably inaccurate, as they either result from a drastic typological approach or could not benefit from the most recent advances in modern concepts of ammonoid intraspecific and ontogenetic variations. In this work, we document an original cephalopod assemblage from Salazac represented by abundant, well-preserved specimens (564 studied ammonites, including ca. 50% of heteromorphs) constituting a high diversity. The sample size and quality of this new material allowed some extensive morphological and biometric analyses whenever possible (i.e., for subdatasets in which a significant number of measurements is available) and to thoroughly assess intraspecific and ontogenetic variations for a few species. This led to determine the diagnostic value of various shell characters commonly used to characterize both well-documented and poorly-defined species. In particular, we show that a significant part of the variation in the number of ribs per whorl within the heteromorphic genera Mariella and Turrilitoides is directly related to shell size and that the strength of tubercles covaries negatively with the number of ribs in Mariella, therefore challenging the diagnostic value of these characters. We further suggest that rib density covaries with whorl shape in Mariella, following Buckman’s first rule of covariation. In total, one species of nautilid and 38 taxa (25 genera) of ammonites are described, among which five species are documented from Salazac for the first time. In turn, this study provides valuable data for future phylogenetic, biostratigraphical, biogeographical and broad-scale diversity studies.     

Effects of shell shape,size, and sculpture in burrowing and anchoring abilities in the freshwater mussel Potamilus alatus (Unionidae)

Because of the sedentary lifestyle of freshwater mussels, studies examining their movement capabilities are scarce. However, the ability to burrow into the substrate and the ability to remain stationary are likely crucial components of their behavioural repertoire. The performance of these different tasks is likely to be affected by the presence of the shell ornamentation characteristic of many mussel species. Previous studies have suggested that shell ornamentation results in a trade‐off between burrowing ability and remaining stationary when an extrinsic force attempts to dislodge it from the substrate once buried. We examined the effect of morphology and shell ornamentation on burrowing performance and anchoring ability by artificially creating shell ornamentation on a relatively smooth‐shelled species (Potamilus alatus). Burrowing behaviours and performance and the force required to dislodge mussels (anchoring ability) were quantified with and without ornamentation. Interestingly, we found that the artificial shell ornamentation had no significant effect on burrowing behaviours and performance or dislodgement force. Burrowing and dislodgement, however, were both highly influenced by shell size and shape. All of the available information suggests that shell size, shape, and sculpture influence burrowing and anchoring in complex ways that needs further examination. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 136–144.     

AMMONOIDS ACROSS THE PERMIAN/TRIASSIC BOUNDARY: A CLADISTIC PERSPECTIVE

Abstract:  The rapid diversification of ceratitid ammonoids during the earliest Mesozoic has been taken at face value as an example of explosive radiation following the Permian/Triassic mass extinction. However, the validity of this interpretation has never been tested within a phylogenetic framework. A total evidence cladistic analysis of Mid–Late Permian and Induan (earliest Triassic) ammonoids confirms the monophyly of the Ceratitida. Partitioned phylogenetic analysis of suture line characters vs. shell shape and ornament characters confirms the importance of suture-line characters for resolving the higher taxonomy of ammonoids. When the cladogram is compared with the observed fossil record, the resultant tree implies that the divergence of a number of early Triassic lineages actually occurred during the latest Permian. If these range extensions are taken into account the ammonoid per-genus extinction rate across the Permian/Triassic boundary drops from c. 85 per cent to c. 60 per cent.     

Evolutionary stasis,ecophenotypy and environmental controls on ammonite morphology in the Late Cretaceous (Maastrichtian) Western Interior Seaway,USA

We test for the presence of evolutionary stasis in a species of Late Cretaceous ammonoid cephalopod, Hoploscaphites nicolletii, from the North American Western Interior Seaway. A comprehensive dataset of morphological traits was compiled across the entire spatial and temporal range of this species. These were analysed in conjunction with sedimentologically and geochemically derived palaeoenvironmental conditions hypothesized to apply selective pressures. All changes in shell shape were observed to be ephemeral and reversable, that is, no unidirectional trend could be observed in any of the morphological traits analysed. Correlations between palaeoenvironmental conditions and morphological traits suggests ecophenotypic processes were at play; however, either environmental changes were too minor and/or provided no isolating mechanism to drive speciation. These data support mechanisms of stasis such as homogenizing gene flow or stabilizing selection under a fluctuating optimum (probably reflecting spatiotemporally heterogeneous palaeoenvironmental conditions). Finally, changes in shell size were not significantly associated with changes in shell-specific δ¹⁸O, despite a correlation between shell size and δ¹⁸O averaged across horizons. This suggests a mismatch in scales of geochemical sampling that supports caution when making broad interpretations based on averaged geochemical data.     

Extensive intraspecific variation in a Triassic ammonoid from Siberia

The ammonoid species Czekanowskites rieberin.sp . from the Lower Anisian of Arctic Siberia (River Olenek) shows an enormous range of intraspecific variability. The 700 investigated specimens are packed in one single concretion. They are all adult with preserved living chambers and apertural structures. Morphologically they range from keeled smooth suboxycones with narrow umbilicus, through feebly ribbed platycones with a little wider umbilicus, to subcadicones with relatively wide umbilicus and straight ribs with bullae. The morphological variation is continuous; all specimmh belong to a single genetically linked population, variants of one biospecies. This phenomenon, which is quite common in boreal Triassic ammonoid faunas, has in the past led to serious taxonomic oversplitting. It presents a serious challenge to current ideas about a close correlation between modeof life and shell morphology in arnmonoids.□ Amntonoidea, boreal Triassic, ecology. Siberia, variability .