A major macroecological event happened long after the Cambrian Explosion and the Great Ordovician Biodiversification Event, the Devonian Nekton Revolution . It signifies the rapid occupation of the free water column with nektonic animals, an important component of all post-Silurian and Recent marine ecosystems. This event coincides with an explosive diversification of gnathostomes (jawed fishes), the initial radiation of ammonoids (extinct, externally shelled cephalopods), and the intensification of the radiation of mollusks with planktotrophic larvae or hatchlings (ammonoids, bivalves, gastropods; see also Frýda et al. 2008;Manda and Frýda 2010).Many externally shelled mollusk groups (ammonoids, dacryoconarids, gastropods, nautiloids, etc.) show a progressive coiling in their embryonic and/or post-embryonic conch in the Devonian

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A review of fossil evidence supports a pelagic mode of life (in the water column) of ammonoids, but they may have spent their life close to the seabottom (demersal), planktonically, or nektonically depending upon the ontogenetic stage and taxon. There are good indications for a planktonic mode of life of ammonoid hatchlings, but a broad range of reproductive strategies might have existed (egg‐laying, fecundity). Isotope and biogeographical studies indicate that some forms migrated or swam for considerable distances, whereas others may have been primarily transported by oceanic currents during early and/or late ontogeny. Diverse ammonoid habitats are also supported by evidence from predator–prey relationships derived from characteristic injuries and exceptional fossil finds, which indicate chiefly predatory or scavenging lifestyles. Sublethal injuries preserved in some ammonoid shells, as well as rare stomach and coprolite contents, provide evidence of predation by other cephalopods, arthropods and various jawed vertebrates. Various lines of evidence suggest that different groups of ammonoids had quite different ecologies, but shell shape alone can only give upper constraints on ammonoid capabilities, a matter that needs to be considered when interpreting their diversity and evolutionary history.

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 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.

BackgroundA 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, 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.ResultsHerein, 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.ConclusionsThe 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).