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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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This content will become publicly available on December 20, 2025
Ontogenetic Scaling of the Primate Middle Ear
ABSTRACT The study of primate auditory morphology is a significant area of interest for comparative anatomists, given the phylogenetic relationships that link primate hearing and the morphology of these auditory structures. Extensive literature addresses the form‐to‐function relationship of the auditory system (outer, middle, and inner ear) in primates and, by extension, provides insight into the auditory system of extinct primates and even modern humans. We add to this literature by describing the ontogenetic trajectory of the middle ear cavity and ossicular chain (malleus, incus, and stapes) due to their critical role in relaying auditory stimuli for interpretation. We examined middle ear morphology in neonatal primates and adult primates using a taxonomically broad sample. We focused primarily on nocturnal primate taxa (Daubentonia,Loris,Galago,Aotus, andTarsier), which are underrepresented in the literature. However, we also included three diurnal taxa (Macaca,Lemur, andSaguinus). Using 3D Slicer, we visualized middle ear structures in three dimensions using conventional micro CT data informed by diffusible iodine‐based contrast‐enhanced CT (diceCT) data. We illustrated how spatial relationships between otic elements, such as the various epitympanic sinuses of the middle ear and the auditory ossicles, vary throughout ontogeny. Our major findings include that the central tympanic cavity scaled with negative allometry in all taxa and that the accessory cavities scaled with isometry or positive allometry in most taxa. Despite these changes in chamber size, the size of the ear ossicles remained relatively consistent through ontogeny in most taxa. We confirmed our expectation that anthropoids exhibit an increase in the complexity of accessory cavities throughout ontogeny, mirroring the exponential pneumatization of the face in anthropoids. These findings provide an ontogenetic perspective and reveal further functional complexities of the middle ear as a conduit for sound proliferation and as a pressure regulator.
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- PAR ID:
- 10562062
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- American Journal of Primatology
- Volume:
- 87
- Issue:
- 1
- ISSN:
- 0275-2565
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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