A new early diverging thalattosuchian (Crocodylomorpha) from the Early Jurassic (Pliensbachian) of Dorset, U.K. and implications for the origin and evolution of the group

ABSTRACT

Among archosaurs, thalattosuchian crocodylomorphs experienced the most extensive adaptations to the marine realm. Despite significant attention, the phylogenetic position of the group remains uncertain. Thalattosuchians are either the sister-group to Crocodyliformes, basal mesoeucrocodylians, or nest among longirostrine neosuchians. The earliest definite thalattosuchians are Toarcian, and already possess many synapomorphies of the group. All phylogenetic hypotheses imply a ghost lineage extending at least to the Sinemurian, and a lack of older or more plesiomorphic forms may contribute to the uncertain phylogenetic placement of the group. Here we describe a new species, Turnersuchus hingleyae, gen. et sp. nov., from the early Pliensbachian Belemnite Marl Member of the Charmouth Mudstone Formation (Dorset, U.K.). The specimen includes partially articulated cranial, mandibular, axial, and appendicular elements. It can be attributed to Thalattosuchia based on the following features: distinct fossa on the posterolateral corner of the squamosal; broad ventrolateral process of the otoccipital covering the dorsal surface of the quadrate; large supratemporal fenestrae lacking a flattened skull table; broadly exposed prootic; orbital process of quadrate lacking bony attachment with the braincase. This specimen represents the earliest thalattosuchian currently known from diagnostic material. Phylogenetic analyses of two published datasets recover Turnersuchus as the earliest diverging thalattosuchian, and sister to Teleosauroidea + Metriorhynchoidea. Bayesian tip-dating analyses suggest a Rhaetian or Sinemurian divergence of Thalattosuchia from other crocodylomorphs, depending on topology, with confidence intervals spanning from the Norian to the Pliensbachian. The new specimen extends the fossil record of Thalattosuchia, but the time-scaling analyses demonstrate that a significant ghost lineage remains.

ACKNOWLEDGMENTS

The authors would like to thank P. Turner and E. Hingley for discovering, excavating, and donating the main blocks of the specimen and K. Turner, E. Thompson, L. Pickering, and L. Fowler for discovering and donating other parts. We thank E. Hingley for preparing the specimen. P. Davidson and the Charmouth Heritage Coast Centre assisted with excavation and uniting the parts. We thank P. Davis (LYMPH) for providing information on the geologic setting and for specimen photographs. We thank T. Davies of the XTM Facility, Palaeobiology Research Group, University of Bristol, for assistance with CT scanning. We thank M. Rabi and C. Brochu for comments that improved the manuscript and D. Schwarz for handling the manuscript. TNT is made freely available through the Willi Hennig Society. EWW, PLG, and AHT were supported by NSF-DEB 1754596. PLG was also supported by Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior (CAPES grant number: 88887.583087/2020-00) and the São Paulo Research Foundation (FAPESP grant number: 2022/05697-9).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.