High evolutionary rates and the origin of the Rosso Ammonitico Veronese Formation (Middle-Upper Jurassic of Italy) reptiles

The fossil record of metriorhynchids and plesiosaurians from the Rosso Ammonitico Veronese Formation (RAVFm, Middle–Upper Jurassic, Italy) is represented by elements collected between the eighteenth and twentieth centuries. All the metriorhynchid material is referred to the genus Neptunidraco. The first RAVFm plesiosaurian material was collected in the nineteenth century and referred to Plesiosaurus: elements are here described and interpreted as a chimerical association of crocodylomorph and plesiosaurian bones, providing the first co-occurrence of these clades in the RAVFm. The second plesiosaurian is the associated skeleton that we refer to Anguanax zignoi gen. et sp. nov. Bayesian phylogenetic analysis confirms the basal geosaurine affinities of Neptunidraco resulted by parsimony analysis. Using both methods, Anguanax was recovered as a basal pliosaurid, sister group of the clade including Marmornectes and Thalassophonea. Bayesian inference methods indicate that both Italian lineages diverged from other known lineages between 176 and 171 Mya, also showing divergence rates significantly higher than any other representative of their respective clades. We suggest a phase of rapid evolutionary adaptation to deeper marine environments in the ancestors of the Rosso Ammonitico Veronese reptiles as a response to the latest Liassic regressive regime in Northern Tethys.     

Re-description of a putative Early Cretaceous “teleosaurid” from France,with implications for the survival of metriorhynchids and teleosaurids across the Jurassic-Cretaceous Boundary

Thalattosuchia was a diverse clade of marine crocodylomorphs known from the Early Jurassic to the Early Cretaceous. Recent studies have hypothesized that their extinction was two-phased: (1) habitat loss near/at the Jurassic-Cretaceous boundary heavily reduced their morphofunctional diversity, particularly in Europe, while (2) climate change and a shift in marine fauna during the Early Cretaceous (either at the Valanginian-Hauterivian boundary or during the early Hauterivian) finished off the already stressed clade. Unfortunately, the Cretaceous fossil record of thalattosuchians is poor, with only one putative “teleosaurid” specimen and approximately ten metriorhynchid specimens. Here we re-describe the youngest known teleosaurid from the Cretaceous (Valanginian of south-eastern France). Originally considered to be a teleosaurid (possibly Steneosaurus), we demonstrate that it belongs to Metriorhynchidae, and a newly discovered subclade, Plesiosuchina. It differs from Plesiosuchus in the pattern of tooth enamel ornamentation and the variation in dentary alveoli size. Referring this specimen to Metriorhynchidae means there are no definitive Cretaceous teleosaurid specimens. Furthermore, it suggests that both durophagous and piscivorous teleosaurids became extinct at the end of the Jurassic. Interestingly, this is the fourth metriorhynchid lineage known to cross the Jurassic-Cretaceous boundary. As such, it would appear that the two thalattosuchian families responded very differently to the lowering sea levels at the end of the Jurassic: teleosaurids possibly became extinct, while metriorhynchids were seemingly unaffected.     

Les crocodiliens fossiles du Jurassique moyen et supérieur de Bourgogne

The middle and upper Jurassic limestones of Côte-d'Or (France) contain some crocodilian remains referred to the genera Teleidosaurus, Steneosaurus, and Metriorhynchus. Their preservation allows a determination at a specific level only for Teleidosaurus gaudryi (Lower Bathonian), Steneosaurus larteti (Upper Bathonian), S. heberti and S. cf. intermedius (Middle Oxfordian). Sedimentological evidence shows that Steneosaurus and Teleidosaurus are found either in very pure limestones sedimented on an epicontinental platform, in a very calm and sheltered environment, in the intertidal or supratidal zone, often associated with subrecifal facies; or in the infratidal zone, more distinctly subjected to open-sea influences (deposits, currents, reworkings). Specialised Metriorhynchidae such as Metriorhynchus are completely absent in the former type of sediment but are present in the later. One may infer, considering anatomical evidence (no dermal armour, caudal fin…) that Metriorhynchus was better adapted to swimming and rather lived in the open sea. The Teleosauridae preferred very shallow or temporarily emerged places, where they could move around thanks to their limbs which were still capable of walking on land; however, this did not prevent them from going out to sea, where the habitats of both families probably largely overlapped.     

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

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.     

What is Geosaurus? Redescription of Geosaurus giganteus (Thalattosuchia: Metriorhynchidae) from the Upper Jurassic of Bayern,Germany

The holotype and referred specimens of Geosaurus giganteus, a metriorhynchid crocodile from the Tithonian (Upper Jurassic) of Germany, is redescribed, along with a historical overview of the genus and species. This taxon is unique among metriorhynchids as its serrated, strongly lateromedially compressed dentition is arranged as opposing blades, suggesting it was adapted to efficiently slice through fleshy prey. A new phylogenetic analysis of Crocodylomorpha is presented, which finds G. giganteus to be nested within what is currently considered Dakosaurus, whereas the other species currently assigned to Geosaurus form a clade with Enaliosuchus and the holotype of Cricosaurus. The phyletic relationship of G. giganteus with other metriorhynchids indicates that the current definition of the genus Geosaurus is polyphyletic, and that the inclusion of subsequent longirostrine species to this genus is in error. The re‐analysis presented herein demonstrates Geosaurus to be composed of three species sensu stricto. The appropriate taxonomic amendments to the Metriorhynchidae are also provided. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 157 , 551–585.     

The snout of Cricosaurus araucanensis: a case study in novel anatomy of the nasal region of metriorhynchids

Metriorhynchids are the only crocodyliforms adapted to pelagic marine life. Snout natural endocasts of the Tithonian (Late Jurassic) metriorhynchid Cricosaurus araucanensis indicated that skeletal changes defining the peculiar metriorhynchid body plan were coupled with changes of the soft cephalic anatomy such as the enlarged salt glands and restructuring of the paranasal sinus system. Seven new natural endocasts of the snout and a 3‐D reconstruction of C. araucanensis are described. Data from these casts and the reconstruction are congruent, and they are combined into an accurate reconstruction that improves our knowledge of the pre‐orbital anatomy. The olfactory tract, bulbs, olfactory nasal region and the anterior extension of the antorbital sinus within the maxilla are recognized. Osteological correlates of the salt gland body are also proposed. Palaeobiological inferences are erected based on the integration of natural endocasts and 3‐D reconstruction data. It is proposed that C. araucanensis nasal salt glands were highly vascularized with a blood supply comparable with those of extant marine birds. Reduced olfactory bulbs and olfactory nasal region indicate that the aerial olfaction, differing from extant crocodilians, was not well developed.     

Largest known specimen of the genus Dakosaurus (Metriorhynchidae: Geosaurini) from the Kimmeridge Clay Formation (Late Jurassic) of England,and an overview of Dakosaurus specimens discovered from this formation (including reworked specimens from the Woburn Sands Formation)

We report the largest known British specimen of the metriorhynchid crocodylomorph genus Dakosaurus discovered offshore from Chesil Beach, Dorset, England (Kimmeridge Clay Formation). This specimen is large toothed, which has extreme enamel spalling on the labial surface, enlarged carinae (‘carinal flanges’), carinal wear, macroziphodont denticles, and the crown retains much of its labiolingual width along most of its apicobasal length. This suite of morphologies is unique to Dakosaurus. All known Kimmeridge Clay Formation Dakosaurus specimens are isolated tooth crowns. A skull previously referred to Dakosaurus lacks all the cranial apomorphies of D. maximus and D. andiniensis, and cannot be referred to this genus. Furthermore, the vast majority of putative Dakosaurus tooth crowns from the ‘Potton Sands’ ( = Woburn Sands Formation) do indeed represent Dakosaurus (as well as two Plesiosuchus specimens), and they most likely originate from the Kimmeridge Clay Formation.