The evolution of extreme hypercarnivory in Metriorhynchidae (Mesoeucrocodylia: Thalattosuchia) based on evidence from microscopic denticle morphology

ABSTRACT

Metriorhynchids were a peculiar group of fully marine Mesozoic crocodylomorphs. The derived genera Dakosaurus and Geosaurus exhibit a macroevolutionary trend towards extreme hypercarnivory, underpinned by a diverse array of craniodental adaptations, including denticulate serrated (ziphodont) dentition. A comparative analysis of serrations in Metriorhynchidae shows that known Dakosaurus species had conspicuous denticles, in contrast to the microscopic denticles of Geosaurus. A new tooth from the Nusplingen Plattenkalk of Germany provides evidence for a previously unknown large species of Geosaurus. Metriorhynchid specimens from the upper Kimmeridgian–lower Tithonian of Southern Germany show that ziphodont species of Dakosaurus and Geosaurus co-occurred in the Nusplingen and Solnhofen Seas. Although these genera are similarly denticulate, they diverge in overall crown morphology. Therefore, resource/niche partitioning via craniodental differentiation is posited as maintaining two contemporaneous genera of highly predatory metriorhynchids. Additionally, the new generic name Torvoneustes is proposed for “Geosaurus” carpenteri, the only known metriorhynchid with false-ziphodont dentition. A cladistic analysis shows that ziphodont dentition may have evolved independently in Dakosaurus and Geosaurus, or been acquired earlier by their common ancestor and secondarily lost in Torvoneustes and related taxa.

ACKNOWLEDGMENTS

We thank R. Schoch (SMNS) for access to SMNS specimens, particularly SMNS 81834, and O. Rauhut (BSPG), S. Chapman and P. Barrett (NHM), and R. F. Vaughan and R. Barnett (BRSMG) for access to specimens under their care; S. Kearns for his guidance on SEM, and S. Powell (BRSUG) for advice on image treatment; M. Kölbl-Ebert (JME) and E. Robert (Université Joseph Fourier) for information regarding their collection; and D. Bogdanov for the life reconstructions in Figures 4 and 7. We thank B. Beatty, E. Rayfield (BRSUG), D. Schwarz-wings (Museum für Naturkunde Berlin), and an anonymous referee for their comments, and F. R. O’Keefe and M. Wilson for their editorial assistance, which greatly contributed to improve the original manuscript.

M.B.A. receives financial support from Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq, Proc. No. 200381/2006–10), Brazil. Direct examination of Stuttgart and Munich specimens was only possible with the support of the Sylvester Bradley Fund to M.B.A. and Alexander von Humboldt Foundation to J.B.D. M.T.Y. receives support from the Natural Environment Research Council (grant NER/S/A/2006/14058), UK, and the NHM. S.L.B. is supported by an NSF Graduate Research Fellowship (Columbia University), his tenure in the UK was supported by the Marshall Scholarship, and his specimen visits were funded by the Jurassic Foundation, Paleontological Society, SYNTHESYS (DE-TAF-4368, FR-TAF-3918), and the University of Bristol Bob Savage Memorial Fund. SYNTHESYS (http://www.synthesys.info/) is financed by the European Community Research Infrastructure Action under the FP6 ‘Structuring the European Research Area Programme.’

Notes

^aThere is a potential synonymy between Cricosaurus elegans and C. suevicus. Note that currently all specimens attributed to C. suevicus are restricted to Malm Zeta 1, whereas those of C. elegans are known from Malm Zeta 2–3.

^bThere is a potential synonymy between Geosaurus giganteus and G. grandis. Geosaurus grandis is known from only the Mörnsheim Formation, whereas G. giganteus is known from Malm Zeta 2–3. The Nusplingen species, which is a posterior maxillary tooth, is very similar in form (but bigger) to Geosaurus specimens in Malm Zeta 2–3.