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Parallel Evolution controlled by adaptation and covariation in ammonoids (Cephalopoda)

August 2010

August 2010

Conference: Eighth International Symposium Cephalopods – Present and Past, Dijon

Authors:

University of Zurich

Kenneth De Baets

University of Warsaw

Université de Lille - Sciences et Technologies

We documented the parallel steps of evolution in two lineages of Early and Middle Devonian ammonoids.

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Ontogeny of highly variable ceratitid ammonoids from the Anisian (Middle Triassic)

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Mar 2021

Ammonoids reached their greatest diversity during the Triassic period. In the early Middle Triassic (Anisian) stage, ammonoid diversity was dominated by representatives of the family Ceratitidae. High taxonomic diversity can, however, be decoupled from their morphologic disparity. Due to its high phenotypic variability, the high diversity of ceratitids of the Anisian of Nevada was initially assumed to be caused by artificial over-splitting. This study aims to contribute data to settle this issue by applying geometric morphometrics methods, using landmarks and semi-landmarks, in the study of ontogenetic cross-sections of ammonoids for the first time. The results reveal that alterations in ontogenetic trajectories, linked to heterochronic processes, lead to the morphologic diversification of the species studied herein. Our knowledge, based on these ontogenetic changes, challenge the traditional treatment of species using solely adult characters for their distinction. This study furthermore demonstrates that the high diversity of the Anisian ammonoid assemblages of Nevada based on the traditional nomenclatoric approach is regarded to be reasonably accurate.

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Parallel evolution controlled by adaptation and covariation in ammonoid cephalopods

April 2011 · BMC Evolutionary Biology

A major goal in evolutionary biology is to understand the processes that shape the evolutionary trajectory of clades. The repeated and similar large-scale morphological evolutionary trends of distinct lineages suggest that adaptation by means of natural selection (functional constraints) is the major cause of parallel evolution, a very common phenomenon in extinct and extant lineages. However, ... [Show full abstract] parallel evolution can result from other processes, which are usually ignored or difficult to identify, such as developmental constraints. Hence, understanding the underlying processes of parallel evolution still requires further research. Herein, we present a possible case of parallel evolution between two ammonoid lineages (Auguritidae and Pinacitidae) of Early-Middle Devonian age (405-395 Ma), which are extinct cephalopods with an external, chambered shell. In time and through phylogenetic order of appearance, both lineages display a morphological shift toward more involute coiling (i.e. more tightly coiled whorls), larger adult body size, more complex suture line (the folded walls separating the gas-filled buoyancy-chambers), and the development of an umbilical lid (a very peculiar extension of the lateral shell wall covering the umbilicus) in the most derived taxa. Increased involution toward shells with closed umbilicus has been demonstrated to reflect improved hydrodynamic properties of the shell and thus likely results from similar natural selection pressures. The peculiar umbilical lid might have also added to the improvement of the hydrodynamic properties of the shell. Finally, increasing complexity of suture lines likely results from covariation induced by trends of increasing adult size and whorl overlap given the morphogenetic properties of the suture. The morphological evolution of these two Devonian ammonoid lineages follows a near parallel evolutionary path for some important shell characters during several million years and through their phylogeny. Evolution of some traits (involution, umbilical lid) appears to be mainly driven by adaptation to improve the hydrodynamic properties of the shell, whereas other characters (sutural complexity) evolved due to covariation with features that play a central role in the morphogenesis of mollusc shells. This example provides evidence that parallel evolution can be driven simultaneously by different factors such as covariation (constructional constraints) and adaptation (natural selection).

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Intraspecific variability through ontogeny in early ammonoids

December 2013 · Paleobiology

Mollusks in general and ammonoids in particular are known to display a sometimes profound morphological intraspecific variability of their shell. Although this phenomenon is of greatest importance, it has rarely been investigated and quantified. It is especially crucial for taxonomy and incidentally for biodiversity analyses to account for it, because otherwise, the number of described species ... [Show full abstract] might exceed that of actual species within any group. Early ammonoids (Early Devonian, Paleozoic) typically suffer from this bias. For instance, most specimens from the same layer and the same region (e.g., the Erbenoceras beds of the Moroccan eastern Anti-Atlas studied here) differ morphologically from each other. Depending on the importance given to certain morphological characters, therefore, one could create a new species for almost every specimen. In this study, we measured nearly 100 such specimens from a restricted stratigraphic interval and quantified their intraspecific variability. There is a variable but strong overlap of the quantified shell characters at most ontogenetic stages, and only two species are here separated rather than the four previously recognized in Morocco. When ontogenetic trajectories of the Moroccan specimens are compared with coeval faunas from other regions (assigned to other species), a strong overlap between the morphospaces occupied by these taxa becomes apparent. The justification of some of these latter species is thus questionable even if their mean values in some conch parameters differ considerably from the mean values of the Moroccan species. Hence, the number of currently valid species of these loosely coiled early ammonoids is probably much too high. Extreme caution must therefore be taken when examining the diversity of groups in which the intraspecific variability is poorly known.

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Evolutionary Patterns of Ammonoids: Phenotypic Trends, Convergence, and Parallel Evolution

August 2015

A major goal in evolutionary biology is to characterize and understand the patterns and processes that shape the evolutionary trajectory of clades through time and space. One common pattern in extinct and extant lineages is the pervasive occurrence of persistent phenotypic evolutionary trends. These trends of evolutionary lineages suggest that adaptation by means of natural selection is a major ... [Show full abstract] cause of homoplastic patterns such as convergence and parallel evolution. Such patterns can result from other processes, which are usually ignored or difficult to identify, such as chance, environmental changes, constructional or developmental constraints. How the evolution of organisms follows preferentially specific phenotypic trajectories remains poorly known. The ammonoids are well-known to display persistent phenotypic shifts through time. Commonly described trends in ammonoid lineages are increase in adult size, changes in coiling and increase in suture frilling. Some new methods for the study of ammonoid evolution are here exemplified by recent case studies.

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Buckman’s Rules of Covariation

August 2015

Most planispirally and regularly coiled ammonoid species show to varying degrees continuous morphological intraspecific variation of their shell. In many species, individuals range from slender, involute, compressed, and weakly ornamented forms to “robust” forms, which are more evolute, more depressed, and with coarser ornamentation. Additionally, more compressed shells tend to have more complex ... [Show full abstract] sutures. Within a species from a single sample, the frequency of these variants is represented by a continuous unimodal distribution (often normal). These covariation patterns of intraspecific variation have been abundantly documented and were coined “Buckman’s Laws of Covariation”. These rules help to delineate ammonoid morphospecies. This interdependent morphological variation suggests that shell morphogenesis may not be random but constrained either by adaptive constraints or simple constructional scaling rules. The cause(s) remain debated. Hence, although Buckman’s rules of covariation are now widely documented and acknowledged, several aspects of their scope and limits still remain to be investigated.