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Abstract Cranial skeletal material of the Eocene palaeanodont Metacheiromys marshi was examined using high-resolution CT scans. The present study represents the first time that CT scans have been conducted on skulls of this extinct fossorial mammal. The bony osteology of the auditory region is described in detail, including the ectotympanic and entotympanic, the petrosal in both tympanic and endocranial views, and the middle ear ossicles. The results of this investigation confirm a number of derived resemblances between palaeanodonts and xenarthrans, including a large entotympanic element in the medial wall of the auditory bulla, the presence of an anteroventral process of the tegmen tympani, and a posttemporal canal. However, the present study also provides novel derived auditory features linking palaeanodonts and pangolins, consistent with current understanding of palaeanodont phylogenetic relationships, including the absence of an ectotympanic styliform process, a posterolaterally oriented aperture to the cochlear fossula, and a convex mallear head / concave incudal head. Several autapomorphic features characterizing the auditory osteology of Metacheiromys are also noted. The presence of a large, spherical mallear head, and of a capacious tympanic cavity extended into sinuses in surrounding bones, likely represent adaptations for fossoriality, consistent with palaeobiological inferences drawn from the postcranial anatomy of Metacheiromys .
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Cranial anatomy of Indohyus indirae (Raoellidae), an artiodactyl from the Eocene of India, and its implications for raoellid biology
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.
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- Award ID(s):
- 2142526
- PAR ID:
- 10654547
- Publisher / Repository:
- Palaeontologica Electronica
- Date Published:
- Journal Name:
- Palaeontologia Electronica
- ISSN:
- 1935-3952
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Journal Article:
- https://doi.org/10.26879/1307
