Inner Ear Anatomy of Caenagnathidae (Theropoda: Oviraptorosauria) Emphasizes Mosaic Evolution of the Avian Neurosensory System.

The endocranial anatomy of oviraptorosaurian theropods, and their inferred sensory adaptations, has played a key role in understanding the origin of bird-like senses among avian ancestors. However, within oviraptorosaurs, sampling has thus far been limited, and a major gap exists in our understanding of the endocranial anatomy of an enigmatic subclade, Caenagnathidae. Here, we describe the inner ear morphology of three caenagnathid oviraptorosaurs, spanning a wide range of stratigraphy and body size. Our data show that the incipiently bird-like inner ears of caenagnathids were relatively similar both over the course of >10 million years and over multiple orders of magnitude of body mass. Overall, the caenagnathids sampled here show similarities to other maniraptoran theropods and particularly other oviraptorosaurs, but with some key differences that possibly speak to behavioral differences in these groups. In particular, a reduced cochlear duct in the most complete labyrinth suggests restricted hearing sensitivity in caenagnathids, which is at odds with previous suggestions that they were more predatory than oviraptorids. Our results highlight the independent acquisition of some bird-like features along the avian stem and show that late-diverging members of these clades may exhibit divergent features that do not characterize the diversity within the entire group.