The cranial endocast of tritylodontid Bienotheroides (Cynodontia, Mammaliamorpha) and its relevance to mammalian neurosensory evolution

Abstract Tritylodontidae are a cosmopolitan group of derived cynodonts that thrived during the Mesozoic Era. Among the tritylodontids, the genus Bienotheroides is distinguished by its short snout, reduced maxilla, and deep zygomatic arch. Here we describe the endocranial anatomy (brain endocast, inner ear, blood vessels and cranial nerves) of Bienotheroides sp. collected from the Middle Jurassic Lower Shaximiao Formation and Xintiangou Formation in the Yunyang area of Chongqing, China. The cranial endocast features enlarged olfactory bulbs and a slender hypophysis. It is long and horizontally oriented in the brain cavity. The unossified zone is moderately developed. The paraflocculus is less developed than in most known cynodonts. The brain cast of Bienotheroides is divided into two cerebral hemispheres by a median interhemispheric sulcus, which is present only in the anterior half of the forebrain. The posterior portion of the forebrain remains undifferentiated. This condition appears to be more derived than the completely undifferentiated forebrain found in basal cynodonts. However, it is primitive compared with the fully divided cerebral hemispheres observed in probainognathian cynodonts, as well as in mammaliaforms. The 11 pairs of cranial nerves (excluding CN VI) in Bienotheroides are arranged longitudinally along the brain endocast, and its inner ear labyrinth is characterized by an elongated, uncoiled cochlea. The estimated encephalization quotient of Bienotheroides is similar to that of basal probainognathians or traversodontids, but relatively lower compared with derived probainognathians and mammaliaforms.