Raoellidae are extinct small-sized semiaquatic artiodactyls that are the closest relatives to crown clade Cetacea. They display morphological features showing the transition between terrestrial and aquatic lifestyles and therefore bring crucial information to understand the earliest steps of cetacean evolution. Raoellid cranial morphology, including the ear region and endocranial morphology, has been documented using cranial remains referred to Indohyus indirae from the Kalakot area, Jammu and Kashmir in India. The study of these specimens highlighted that several cetacean features are already present in raoellids. The previously available Indohyus material was very deformed, preventing access to quantitative data and leading to potential misinterpretations. We describe new undeformed cranial material from the Kalakot area, documenting another raoellid species, Khirtharia inflata. The new observations allow us to complete our knowledge of raoellid cranial morphology, including the original shape of the cranium and brain endocast and to confirm the specificities of raoellid morphology within Artiodactyla. We further provide the first quantitative data for the different brain components and show that Raoellidae had low encephalization and neocorticalization values, much lower than cetaceans and close to early diverging, primitive, dichobunoid artiodactyls. Reconstruction of the blood sinuses above the cerebellum supports the previous “intraosseous” hypothesis about the initial steps of the development of the caudal venous rete mirabile in cetaceans. The presence of several cetacean cranial features in Raoellidae, such as the peculiar shape of the frontal, the strong postorbital constriction, the periotic involucrum, or the elongation of the olfactory bulbs, questions the definition of the Cetacea clade.
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RBINS Staff Publications 2023 OA
New remains of the early Eocene hyaenodontid Indohyaenodon raoi are described from the Vastan Lignite Mine in Gujarat, western India, including the first known rostrum, upper dentition, and postcrania, substantially expanding our knowledge of the species and providing insights into its functional morphology and relationships. Craniodental morphology suggests that I. raoi had a broad diet, including non-vertebrate material as well as flesh of a diversity of prey species. Postcranial morphology is broadly similar to that of other early hyaenodontids and suggests a scansorial locomotor repertoire. Dental morphology indicates that I. raoi is closely related to other South Asian hyaenodontids, with shared features including strong cingula, narrow premolars, and a reduced P4 protocone. We present the most comprehensive phylogenetic analysis of Hyaenodontidae to date, which corroborates this relationship but finds South Asian hyaenodontids to be the stem of a group that includes most African hyaenodontids. This and other higher-level relationships within Hyaenodontidae are, however, weakly supported, and substantially different alternative hypotheses of relationships are not significantly less parsimonious, reflecting strong character conflict. Factors contributing to this conflict include the isolation of hyaenodontid faunas on different continents during much of the Eocene, canalization and simplification of carnivorous dentitions, and a lack of non-dental material for critical hyaenodontid groups. The new phylogeny is consistent with either an African or an Asian origin for the group.
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