Cranial anatomy of Indohyus indirae (Raoellidae), an artiodactyl from the Eocene of India, and its implications for raoellid biology

Abstract

The raoellid artiodactyl Indohyus indirae is known from northern Pakistan and northwestern India, with substantial skeletal material found in the Sindkhatudi locality near Kalakot in Kashmir. Eight specimens from this locality, two of which exhibit deciduous dentitions, offer invaluable insights into cranial osteology of Indohyus indirae. We present a comprehensive examination of the cranial osteology of Indohyus, highlighting detailed unique features using this material. These specimens, though flattened dorsoventrally and mediolaterally, allow for anatomical observations. Previous literature has extensively employed these specimens for phylogenetic analyses indicating that Indohyus is a close relative of cetaceans. These findings are partly based on the material described here. Notably, the cranial morphology of Indohyus differs in several aspects from that of most early and middle Eocene artiodactyls. For instance, the rostrum of Indohyus is longer than that of most middle Eocene artiodactyls, resembling Eocene cetaceans. The positioning of the nasal opening above the incisors is similar to terrestrial artiodactyls. However, the ventral wall of the nasopharyngeal duct does not extend to the ear region, which distinguishes it from Eocene cetaceans. Moreover, the frontal bone is thick and has a concave profile, forming a dorsal shield for the laterally facing orbits. Furthermore, the tympanic bone has an involucrum, a feature that characterizes all cetaceans. These cranial features may be indicative of specific cranial functions and, given their similarity in some regards to Eocene cetaceans, could be related to the land-to-water transition. Further research, including explicit functional studies, is required to investigate these hypotheses.