Craniofacial form and function in Metriorhynchidae (Crocodylomorpha: Thalattosuchia): modelling phenotypic evolution with maximum-likelihood methods

Abstract

Metriorhynchid crocodylomorphs were the only group of archosaurs to fully adapt to a pelagic lifestyle. During the Jurassic and Early Cretaceous, this group diversified into a variety of ecological and morphological types, from large super-predators with a broad short snout and serrated teeth to specialized piscivores/teuthophages with an elongate tubular snout and uncarinated teeth. Here, we use an integrated repertoire of geometric morphometric (form), biomechanical finite-element analysis (FEA; function) and phylogenetic data to examine the nature of craniofacial evolution in this clade. FEA stress values significantly correlate with morphometric values representing skull length and breadth, indicating that form and function are associated. Maximum-likelihood methods, which assess which of several models of evolution best explain the distribution of form and function data on a phylogenetic tree, show that the two major metriorhynchid subclades underwent different evolutionary modes. In geosaurines, both form and function are best explained as evolving under 'random' Brownian motion, whereas in metriorhynchines, the form metrics are best explained as evolving under stasis and the function metric as undergoing a directional change (towards most efficient low-stress piscivory). This suggests that the two subclades were under different selection pressures, and that metriorhynchines with similar skull shape were driven to become functionally divergent.

The First Metriorhynchid Crocodylomorph from the Middle Jurassic of Spain, with Implications for Evolution of the Subclade Rhacheosaurini

“…There is a temporal and phylogenetic trend towards increasingly specialised piscivory within this group, as more derived and younger taxa had skulls that were less optimized for enduring strong bite forces [14], [16]. The more derived members of Metriorhynchinae are within the subclade Rhacheosaurini (Fig.…”

“…One of the major metriorhynchid subclades, Metriorhynchinae, consists of numerous small-to-medium bodied species within the genera Metriorhynchus , Gracilineustes , Rhacheosaurus and Cricosaurus [14], [15], [16], [17]. There is a temporal and phylogenetic trend towards increasingly specialised piscivory within this group, as more derived and younger taxa had skulls that were less optimized for enduring strong bite forces [14], [16].…”

“…Rhacheosaurins are not only characterised by having skulls not suited for enduring strong bite forces, but have elongate and tubular snouts, high tooth counts, and uncarinated and unserrated teeth [14], [16], [18] (although three specimens do have carinae: [19], [20]–[21]). Based on this craniodental morphology it has been posited that they were well suited for feeding on small, fast-moving prey [14], [16]. Interestingly, rhacheosaurins also exhibit a temporal and phylogenetic trend towards increasing marine specialisation (e.g.…”

“…posterior retraction of the external nares, regression of the calcaneum tuber, increase in caudal vertebrae count in the tail fluke [14], [20]). Currently, all discovered species within Rhacheosaurini are known from the Late Jurassic and Early Cretaceous [1], [16], [20], [22], [23], [24].…”

The Cranial Osteology and Feeding Ecology of the Metriorhynchid Crocodylomorph Genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe

“…There seems to have been a temporal and phylogenetic trend towards increasing super-predatory behaviour within this group, as progressively more derived and younger taxa had skulls that were better optimized for enduring strong bite forces [1], [13], [16]. Furthermore, because of the high diversity of tooth crown and serration morphologies among geosaurines [11], Young et al .…”