Cranial anatomy and neuroanatomical features of a new specimen of Geosaurini (Crocodylomorpha: Metriorhynchidae) from west-central Argentina

Abstract

Fossil records of Geosaurini taxa in South American margins of the Eastern Pacific are scarce. We describe a new specimen of Geosaurini found in sedimentites of the Vaca Muerta Formation (Substeueroceras koeneni Biozone, uppermost Tithonian–lowermost Berriasian) and referred it as Dakosaurus cf. D. andiniensis. The close stratigraphic association of this specimen with late Tithonian to early Berriasian ammonoids allows confirming with biostratigraphic accuracy that, up to date, the first known occurrence of the Geosaurini taxa in South America is in late Tithonian. The specimen consists of a partially preserved skull and a partially exposed natural endocast of the brain cavity. This work is the first detailed description of a natural endocast of a metriorhynchid. The conspicuous blood vessel fillings covering almost all the dorsal surface of the cerebral hemispheres suggest that the dural envelope of this region was thin and that the cerebral hemispheres filled most of the cavity. A large dorsal venous sinus that overlays the dorsal region of the hindbrain, a tube-like cavity connecting the cranioquadrate passage and an enlarged internal carotid canal, previously described for ‘Metriorhynchus’ cf. ‘M.’ westermanni and present in Dakosaurus cf. D. andiniensis, suggest that these features were more widely distributed within metriorhynchids.

Keywords::

• Dakosaurus
• Thalattosuchia
• Neuquén Basin
• Tithonian
• Berriasian

Acknowledgements

The authors thank MS Fernández (MLP), Z Gasparini (MLP), B Aguirre-Urreta (FCEN-UBA) and D. Pol (MEF) for their comments and helpful discussions on a draft of this paper, and for provided specimen photographs. We are most grateful to C De Micco and B Aguirre-Urreta for field assistance. The authors also thank A Garrido (MOZ) and S Devincenzi (MJCM) for providing assistance and access to the collections under their care, and ML Hiriart (MLP) for taking the photographs. The authors are grateful to anonymous reviewers and editor who helped to enhance the work. This study is the contribution R-105 of the Instituto de Estudios Andinos ‘Don Pablo Groeber’ (UBA-CONICET).

Additional information

Funding

Agencia Nacional de Promoción Científica y Tecnológica [grant numbers PICT 2012-748 and PICT 2010-0464], Consejo Nacional de Investigaciones Científicas y Técnicas [grant number PIP 0433] and Programa de Incentivos Universidad Nacional de La Plata (Argentina) [grant number N607] supported this research.