Thrinaxodon (Permian-Triassic cynodont) mandibular middle ear + mammalian olfactory bulb endocast
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Ben Creisler
Dec 8, 2025, 4:17:35 PM (10 days ago) Dec 8
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Ben Creisler
The middle ear of modern mammals is detached from the mandible and has a soft-tissue eardrum, which allows airborne sound to be heard across a wide range of frequencies. A rich fossil record shows that the middle ear bones of mammals evolved from the jaw bones of their synapsid predecessors, but how this transformation was associated with changes in hearing function is unknown. Our finite element analysis (FEA) of the harmonic response of the mandibular ear bones and soft-tissue eardrum of the synapsid Thrinaxodon suggests that this 250-Mya-old mammal precursor was already capable of tympanic hearing similar to extant mammals and provides evidence that this functional transition occurred very early in mammal evolutionary history.
Abstract
The middle ear of mammals is a major functional innovation, distinctive in that it is detached from the mandible and has a tympanic membrane supported by a ring-like ectotympanic. These novelties of the middle ear have enabled modern mammals to develop more sensitive hearing than all other tetrapods, especially at higher frequencies. Fossils from recent decades have clarified the evolution of the detached middle ear from the jaw bones of Paleozoic therapsids and Mesozoic cynodonts, and the evolution of the tympanum. These discoveries make it possible to answer important questions about the functional significance of these features. Here, we evaluate the relative hearing efficacy of a well-known cynodont precursor to mammals, Thrinaxodon liorhinus. Using finite element analysis (FEA), we calculated the harmonic response of the Thrinaxodon ear to bone-conducted and airborne sound and estimated the sound pressure level (SPL) at the stapedial footplate across a broad range of frequencies. We provide evidence that airborne sound received at the tympanum was the most effective mode of sound reception in Thrinaxodon. In contrast, bone conducted sound through the mandibular bones barely met our estimated hearing threshold. Our findings suggest that, like modern mammals, cynodonts were already reliant on a soft tissue tympanum to receive airborne sound, albeit with limited sensitivity to high frequencies. This is a detailed biomechanical evaluation of tympanum function in the cynodont predecessors of mammals and yields insight into the sequence of functional innovations during the evolution of mammal hearing.
Understanding how olfactory capabilities evolve is challenging, especially in extinct species where direct assessment of behavior is not possible. In mammals, the volume of the braincase reflects the volume of the brain. This study demonstrates that the relative volume of its anterior part, the olfactory bulb endocast, significantly correlates with the number of intact chemoreceptor genes, a genomic marker of olfactory function. Since the braincase is a bony structure often preserved in fossils, this finding validates the use of the olfactory bulb endocast as a proxy for estimating olfaction in extinct mammals. By bridging anatomy and genomics, these results provide a powerful tool to investigate sensory evolution and reconstruct behavioral ecology in deep time.
Abstract
Olfaction is a critical sense for tetrapods, playing a key role in survival and reproduction by aiding in food detection, predator avoidance, and social interactions. Olfactory performance has been experimentally tested in only a few taxa, so comparative analyses rely on anatomical and genomic proxies. Among anatomical proxies, the olfactory bulb endocast is widely used, particularly in extinct species, where it is often the only preserved proxy and can be reconstructed even in million-year-old fossils. While the functional significance of chemoreceptor genes has received attention, the extent to which the olfactory bulb endocast correlates with genomic proxies remains unclear. Using brain endocasts across all mammalian orders, we investigated the relationship between the absolute (absOB) and relative (relatOB) volumes of the olfactory bulb endocast and the number of intact chemoreceptor genes. While no clear correlations were found between absOB and the genomic proxies tested, we identified a significant correlation between relatOB and the total number of combined intact chemoreceptor genes (CombChemo), primarily driven by olfactory receptor genes (OR). Leveraging this correlation, and aiming to infer olfactory capabilities in taxa for which only the skull is available, we estimated OR numbers for three mammalian orders lacking genomic data, as well as for five extinct mammals. Building on studies that have established a link between intact OR and olfactory sensitivity and discrimination, we conclude that relatOB enables inference of olfactory capabilities in mammals. This provides a basis to investigate sensory evolution and opens perspectives for interpreting paleoecology and behavior of extinct mammals.
New papers:
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Alec T. Wilken, Chelsie C. G. Snipes, Callum F. Ross and Zhe-Xi Luo (2025)
Biomechanics of the mandibular middle ear of the cynodont Thrinaxodon and the evolution of mammal hearing
Proceedings of the National Academy of Sciences 122 (51) e2516082122
doi: https://doi.org/10.1073/pnas.2516082122
https://www.pnas.org/doi/10.1073/pnas.2516082122
Significance
Alec T. Wilken, Chelsie C. G. Snipes, Callum F. Ross and Zhe-Xi Luo (2025)
Biomechanics of the mandibular middle ear of the cynodont Thrinaxodon and the evolution of mammal hearing
Proceedings of the National Academy of Sciences 122 (51) e2516082122
doi: https://doi.org/10.1073/pnas.2516082122
https://www.pnas.org/doi/10.1073/pnas.2516082122
Significance
The middle ear of modern mammals is detached from the mandible and has a soft-tissue eardrum, which allows airborne sound to be heard across a wide range of frequencies. A rich fossil record shows that the middle ear bones of mammals evolved from the jaw bones of their synapsid predecessors, but how this transformation was associated with changes in hearing function is unknown. Our finite element analysis (FEA) of the harmonic response of the mandibular ear bones and soft-tissue eardrum of the synapsid Thrinaxodon suggests that this 250-Mya-old mammal precursor was already capable of tympanic hearing similar to extant mammals and provides evidence that this functional transition occurred very early in mammal evolutionary history.
Abstract
The middle ear of mammals is a major functional innovation, distinctive in that it is detached from the mandible and has a tympanic membrane supported by a ring-like ectotympanic. These novelties of the middle ear have enabled modern mammals to develop more sensitive hearing than all other tetrapods, especially at higher frequencies. Fossils from recent decades have clarified the evolution of the detached middle ear from the jaw bones of Paleozoic therapsids and Mesozoic cynodonts, and the evolution of the tympanum. These discoveries make it possible to answer important questions about the functional significance of these features. Here, we evaluate the relative hearing efficacy of a well-known cynodont precursor to mammals, Thrinaxodon liorhinus. Using finite element analysis (FEA), we calculated the harmonic response of the Thrinaxodon ear to bone-conducted and airborne sound and estimated the sound pressure level (SPL) at the stapedial footplate across a broad range of frequencies. We provide evidence that airborne sound received at the tympanum was the most effective mode of sound reception in Thrinaxodon. In contrast, bone conducted sound through the mandibular bones barely met our estimated hearing threshold. Our findings suggest that, like modern mammals, cynodonts were already reliant on a soft tissue tympanum to receive airborne sound, albeit with limited sensitivity to high frequencies. This is a detailed biomechanical evaluation of tympanum function in the cynodont predecessors of mammals and yields insight into the sequence of functional innovations during the evolution of mammal hearing.
***
News:
Free pdf:
Quentin Martinez, Cécile Molinier, Ilse K. Barraza-Soltero, Elena Berger, Kévin Le Verger, Anne-Claire Fabre, Guillaume Billet, Vincent Fernandez, Gabriel S. Ferreira, Thomas van de Kamp, Elias Hamann, Marcus Zuber, Roberto Portela Miguez, Lionel Hautier, and Eli Amson (2025)
The olfactory bulb endocast as a proxy for mammalian olfaction
Proceedings of the National Academy of Sciences 122(50): e2510575122
doi: https://doi.org/10.1073/pnas.2510575122
https://www.pnas.org/doi/10.1073/pnas.2510575122
Free pdf:
https://www.pnas.org/doi/epdf/10.1073/pnas.2510575122
Significance
Quentin Martinez, Cécile Molinier, Ilse K. Barraza-Soltero, Elena Berger, Kévin Le Verger, Anne-Claire Fabre, Guillaume Billet, Vincent Fernandez, Gabriel S. Ferreira, Thomas van de Kamp, Elias Hamann, Marcus Zuber, Roberto Portela Miguez, Lionel Hautier, and Eli Amson (2025)
The olfactory bulb endocast as a proxy for mammalian olfaction
Proceedings of the National Academy of Sciences 122(50): e2510575122
doi: https://doi.org/10.1073/pnas.2510575122
https://www.pnas.org/doi/10.1073/pnas.2510575122
Free pdf:
https://www.pnas.org/doi/epdf/10.1073/pnas.2510575122
Significance
Understanding how olfactory capabilities evolve is challenging, especially in extinct species where direct assessment of behavior is not possible. In mammals, the volume of the braincase reflects the volume of the brain. This study demonstrates that the relative volume of its anterior part, the olfactory bulb endocast, significantly correlates with the number of intact chemoreceptor genes, a genomic marker of olfactory function. Since the braincase is a bony structure often preserved in fossils, this finding validates the use of the olfactory bulb endocast as a proxy for estimating olfaction in extinct mammals. By bridging anatomy and genomics, these results provide a powerful tool to investigate sensory evolution and reconstruct behavioral ecology in deep time.
Abstract
Olfaction is a critical sense for tetrapods, playing a key role in survival and reproduction by aiding in food detection, predator avoidance, and social interactions. Olfactory performance has been experimentally tested in only a few taxa, so comparative analyses rely on anatomical and genomic proxies. Among anatomical proxies, the olfactory bulb endocast is widely used, particularly in extinct species, where it is often the only preserved proxy and can be reconstructed even in million-year-old fossils. While the functional significance of chemoreceptor genes has received attention, the extent to which the olfactory bulb endocast correlates with genomic proxies remains unclear. Using brain endocasts across all mammalian orders, we investigated the relationship between the absolute (absOB) and relative (relatOB) volumes of the olfactory bulb endocast and the number of intact chemoreceptor genes. While no clear correlations were found between absOB and the genomic proxies tested, we identified a significant correlation between relatOB and the total number of combined intact chemoreceptor genes (CombChemo), primarily driven by olfactory receptor genes (OR). Leveraging this correlation, and aiming to infer olfactory capabilities in taxa for which only the skull is available, we estimated OR numbers for three mammalian orders lacking genomic data, as well as for five extinct mammals. Building on studies that have established a link between intact OR and olfactory sensitivity and discrimination, we conclude that relatOB enables inference of olfactory capabilities in mammals. This provides a basis to investigate sensory evolution and opens perspectives for interpreting paleoecology and behavior of extinct mammals.
***
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