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Although several well-preserved crania are known for the Mesozoic Eutriconodonta, three-dimensional reconstructions of the character-rich inner ear and basicranial region based on high-resolution computed tomography scans have previously only been published for the Late Jurassic Priacodon. Here we present a description of the petrosal and inner ear morphology of a triconodontid eutriconodontan from the Lower Cretaceous Cloverly Formation, which we provisionally assign to Astroconodon. The bony labyrinth of Astroconodon is plesiomorphic for mammaliaforms in lacking a primary osseous lamina, cribriform plate, and osseous cochlear ganglion canal. However, as in Priacodon and the zhangheotheriid Origolestes, Astroconodon has a secondary osseous lamina base that extends nearly the complete length of the cochlear canal. The cochlear canal is straighter in Astroconodon and other eutriconodontans compared to several basal mammaliaform clades (e.g., morganucodontans, docodontans), that exhibit varying degrees of cochlear canal curvature. The pars cochlearis of the petrosal was well vascularized in Astroconodon, exhibiting a network of venous canals that crossed the cochlea transversely on its ventral and dorsal aspects. Of particular note are several canals that passed along the base of the secondary osseous lamina. As in Priacodon and Origolestes, those canals do not show the extensive connections to the cochlear labyrinth as seen in the basal mammaliaforms Morganucodon and Borealestes. The inner ear of Astroconodon thus highlights the complex history of the mammaliaform cochlear canal, in which different clades appear to follow independent evolutionary trajectories and various key morphological features (e.g., cochlear canal length, curvature, vascularization and osseous supports for the basilar membrane) exhibit considerable homoplasy.
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This content will become publicly available on May 1, 2026
Comparative morphology of the internal structures of the mammalian cochlea
Abstract Therian mammals possess numerous unique morphological features in their auditory system. Many of these have been well studied in attempts to understand their functional importance, and some aspects of cochlear morphology have been shown to correlate with hearing ability across taxa. Among the unique features of the therian inner ear is the presence of bony structures supporting the sensory epithelia. While these have been implicated in the development of extended high-frequency hearing, their functional role remains unclear. In this study, I use micro-computed tomography (μCT) imaging to take quantitative measurements of the bony spiral laminae and cochlear scalae through the length of the basilar membrane. I present an analysis of the variation showing how these change from the cochlear base to the apex, and how they differ between clades. I find considerable variation in the structures between different clades, particularly in the area of the cochlear scalae which show differences between terrestrial and aquatic species. These results provide a broad survey demonstrating a diversity that could be further exploited to explore functional significance in more detail. A cursory comparison shows correlation between various morphological measures and hearing ability, with the dimensions of the basilar membrane appearing the most useful to make predictions.
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- Award ID(s):
- 2142526
- PAR ID:
- 10654544
- Editor(s):
- na
- Publisher / Repository:
- Zoological Journal of the Linnean Society
- Date Published:
- Journal Name:
- Zoological Journal of the Linnean Society
- Volume:
- 204
- Issue:
- 1
- ISSN:
- 0024-4082
- Subject(s) / Keyword(s):
- cochlea, inner ear, hearing, mammals
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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