Unexpected bird-like features and high intraspecific variation in the braincase of the Triassic relative of dinosaurs

Abstract

Silesaurus opolensis Dzik, 2003 from the Late Triassic (late Carnian) of Poland is a key taxon for understanding the evolution of early dinosaurs. High intraspecific variation observed in the S. opolensis braincase brings caution in taxonomic and diversity studies of early dinosauromorphs. The external and internal osteology of three almost complete braincases of S. opolensis show that this taxon shares several similarities with other early dinosauriforms, which supports a close relationship among these forms. However, the paroccipital processes of S. opolensis are directed ventrally like in birds, reaching the level of the ventral margin of the basioccipital condyle. In dinosauromorphs, these processes usually have an almost horizontal orientation (presumed to be the plesiomorphic condition). Modifications observed in birds and S. opolensis have resulted in the dorsoventral expansion of M. complexus and M. depressor mandibulae, which occupy the dorsolateral part of the posterior side of the skull. In adult birds, these muscles act strongly on the initial upstroke of the head during drinking. Therefore, the inferred condition of these muscles in S. opolensis may imply that Silesauridae evolved toward bird-like feeding behaviour.

Keywords:

• Silesaurus
• dinosauriforms
• archosauria
• neuroanatomy
• Carnian
• Late Triassic

Acknowledgements

We thank Jerzy Dzik (Institute of Paleobiology PAS and University of Warsaw, Poland) and Tomasz Sulej (Institute of Paleobiology PAS, Poland) for access to and/or loan of specimens in their care. The authors are also grateful to Jerzy Dzik and Jennifer Ast (Ast Clear Text, Uppsala) whose comments largely improved the final version of the manuscript. We thank Marian Dziewiński (Institute of Paleobiology PAS, Poland) for technical support during preparation of photographs.