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Here we describe the bony anatomy of the inner ear and surrounding structures seen in three of the most plesiomorphic crown mammalian petrosal specimens in the fossil record. Our 25 study sample includes the stem therian taxa Priacodon fruitaensis from the Upper Jurassic of North America, and two isolated petrosal specimens colloquially known as the Höövör petrosals, recovered from Aptian-Albian sediments in Mongolia. The second Höövör petrosal is here described at length for the first time. All three of these stem therian petrosals and a comparative sample of extant mammalian taxa have been imaged using micro-CT, allowing 30 for detailed anatomical descriptions of osteological correlates of functionally significant neurovascular features, especially along the abneural wall of the cochlear canal.The high resolution imaging provided here clarifies several hypotheses regarding the mosaic evolution of features of the cochlear endocast in early mammals. In particular, these images demonstrate that the membranous cochlear duct adhered to the bony cochlear canal 35 abneurally to a secondary bony lamina before the appearance of an opposing primary bony lamina or tractus foraminosus. Additionally, while corroborating the general trend of reduction of venous sinuses and plexuses within the pars cochlearis seen in crownward mammaliaformes generally, the Höövör petrosals show the localized enlargement of a portion of the intrapetrosal venous plexus. This new excavation is for the vein of cochlear aqueduct, a 40 structure that is solely or predominantly responsible for the venous drainage of the cochlear apparatus in extant therians. However, given that these stem therian inner ears appear to have very limited high-frequency capabilities, the development of these modern vascular features the cochlear endocast suggest that neither the initiation or enlargement of the stria Corti, high endolymphatic potential, and absence of the lagenar macula), as well as the plesiomorphic nature of the monotreme cochlea with respect to many modern and fossil therians [16,17]. The complete loss of the lagenar macula in particular has been posited as a 70 therian-lineage evolutionary breakthrough that allowed for the later elongation and sophistication of the cochlear apparatus for non-linear amplification of high-frequency stimuli [18]. Conversely, the retention of a functional lagenar macula, along with its accompanying otoconial and neurovascular structures, in the monotreme and sauropsid lineagesis hard to conciliate with the soft tissue adaptations seen in modern therians. These "therian" features 75 include: 1) the exclusive reliance on the stria vascularis as the major endolymph producing organ, 2) the well-developed electromotility of prestins and other molecular components of the "cochlear amplifier", and 3) the radical elongation and geometrical reorganization of the cochlear sensory epithelium. The distribution of these characteristics in extant animals, within the wider framework of cynodont evolution, points to a fundamental trans...
Here we describe the bony anatomy of the inner ear and surrounding structures seen in three of the most plesiomorphic crown mammalian petrosal specimens in the fossil record. Our 25 study sample includes the stem therian taxa Priacodon fruitaensis from the Upper Jurassic of North America, and two isolated petrosal specimens colloquially known as the Höövör petrosals, recovered from Aptian-Albian sediments in Mongolia. The second Höövör petrosal is here described at length for the first time. All three of these stem therian petrosals and a comparative sample of extant mammalian taxa have been imaged using micro-CT, allowing 30 for detailed anatomical descriptions of osteological correlates of functionally significant neurovascular features, especially along the abneural wall of the cochlear canal.The high resolution imaging provided here clarifies several hypotheses regarding the mosaic evolution of features of the cochlear endocast in early mammals. In particular, these images demonstrate that the membranous cochlear duct adhered to the bony cochlear canal 35 abneurally to a secondary bony lamina before the appearance of an opposing primary bony lamina or tractus foraminosus. Additionally, while corroborating the general trend of reduction of venous sinuses and plexuses within the pars cochlearis seen in crownward mammaliaformes generally, the Höövör petrosals show the localized enlargement of a portion of the intrapetrosal venous plexus. This new excavation is for the vein of cochlear aqueduct, a 40 structure that is solely or predominantly responsible for the venous drainage of the cochlear apparatus in extant therians. However, given that these stem therian inner ears appear to have very limited high-frequency capabilities, the development of these modern vascular features the cochlear endocast suggest that neither the initiation or enlargement of the stria Corti, high endolymphatic potential, and absence of the lagenar macula), as well as the plesiomorphic nature of the monotreme cochlea with respect to many modern and fossil therians [16,17]. The complete loss of the lagenar macula in particular has been posited as a 70 therian-lineage evolutionary breakthrough that allowed for the later elongation and sophistication of the cochlear apparatus for non-linear amplification of high-frequency stimuli [18]. Conversely, the retention of a functional lagenar macula, along with its accompanying otoconial and neurovascular structures, in the monotreme and sauropsid lineagesis hard to conciliate with the soft tissue adaptations seen in modern therians. These "therian" features 75 include: 1) the exclusive reliance on the stria vascularis as the major endolymph producing organ, 2) the well-developed electromotility of prestins and other molecular components of the "cochlear amplifier", and 3) the radical elongation and geometrical reorganization of the cochlear sensory epithelium. The distribution of these characteristics in extant animals, within the wider framework of cynodont evolution, points to a fundamental trans...
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