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1. LOCOMOTOR BEHAVIOR OF PALEOCENE MAMMALS: INSIGHTS FROM THE SEMICIRCULAR CANALS OF THE INNER EAR

   Bertrand, O.C.; Shelley, S.L.; Williamson, T.E.; Wible, J.R.; Chester, S. G.; Holbrook, L. T.; Lyson, T. R.; Meng, J.; Miller, ?I. M.; Smith, T.; et al (June 2023, European Association of Vertebrate Palaeontologists)

   Alba, D.M.; Marigó, J.; Nacarino-Meneses, C.; Villa, A. (Ed.)

   The end-Cretaceous mass extinction triggered the collapse of ecosystems and a drastic turnover in mammalian communities leading to the demise of many ecologically specialized species. While Mesozoic mammals were ecomorphologically diverse, recognizable ecological richness was only truly established in the Eocene. Questions remain about the ecology of the first wave of mammals radiating after the extinction. Here, we use the semicircular canals of the inner ear as a proxy for locomotor behavior. Thirty new inner ear virtual endocasts were generated using high-resolution computed tomography scanning. This sample was supplemented by data from the literature to construct a dataset of 79 fossils spanning the Jurassic to the Eocene alongside 262 extant mammals. Vestibular sensitivity was measured using the radius of curvature against body mass and the residuals of this relationship were analyzed. The petrosal lobule size relative to body mass were compared with the inner ear data as they have a role in maintaining gaze stabilization during motion. Paleocene mammals exhibited smaller canal radius of curvature, compared to Mesozoic, Eocene, and extant taxa. In the early Paleocene, canal radius and associated petrosal lobules were relatively smaller on average compared to other temporal groups, suggesting less ability for fast movements. 

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2. Vestibular sensitivity and locomotor behavior in early Paleocene mammals

   Bertrand, O. C.; Shelley, S. L.; Williamson, T. E.; Wible, J. R.; Chester, S. G.; Holbrook, Luke T.; Lyson, T. R.; Meng, Jin; Smith, Jin; Smith, T.; et al (June 2023, European Association of Vertebrate Palaeontologists)

   The end-Cretaceous extinction triggered the collapse of ecosystems and a drastic turnover of mammalian communities. During the Mesozoic, mammals were ecologically diverse, but less so than extant species. Modern ecological richness was established by the Eocene, but questions remain about the ecology of the first wave of mammals radiating after the extinction.Postcranial fossils are often used to determine locomotor behavior; however, the semicircular canals of theinner ear also represent a reliable proxy. These canals detect the angular acceleration of the head duringl ocomotion and transmit neuronal signals to the brain to allow stabilization of the eyes and head. Accordingly, vestibular sensitivity to rapid rotational head movements is higher in species with a larger canal radius of curvature and more orthogonal canals. We used high-resolution computed tomography scanning to obtain inner ear virtual endocasts for 30 specimens. We supplemented these with data from the literature to constructa database of 79 fossil from the Jurassic to the Eocene and 262 extant mammals. We compared data on canal morphology and another lifestyle proxy, the size of the petrosal lobules, which have a role in maintaining eyes’ movements and position. We find that Paleocene mammals exhibited a lower average and more constricted range of Agility Indices (AI), a new measure of canal radius size relative to body size, compared to Mesozoic, Eocene and extant taxa. Inthe early Paleocene, body mass and canal radius increased, but the former outpaced the latter leading to an AIdecline. Similarly, their petrosal lobules were relatively smaller on average compared to other temporal groups, which suggests less ability for fast movements. Additionally, Paleocene mammals had similar AIs to extant scansorial and terrestrial quadrupeds. In contrast, the lack of canal orthogonality change from the Mesozoic to the Paleocene indicates no trend toward lower vestibular sensitivity regardless of changes in body size. This result may reflect functional differences between canal orthogonality and radius size. Our results support previous work on tarsal morphology and locomotor behavior ancestral state reconstruction suggesting that ground dwelling mammals were more common than arboreal taxa during the Paleocene. Ultimately, this pattern may indicate that the collapse of forested environments immediately after extinction led to the preferential survivorship of more terrestrially adapted mammals. 

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3. Vestibular sensitivity and locomotor behavior in early Paleocene mammals

   Bertrand, O. C.; Shelley, S. L.; Williamson, T. E.; Wible, J. R.; Chester, S. G.; Holbrook, Luke T.; Lyson, T. R.; Meng, Jin; Smith, Jin; Smith, T.; et al (November 2022, Journal of Vertebrate Paleontology)

   The end-Cretaceous extinction triggered the collapse of ecosystems and a drastic turnover of mammalian communities. During the Mesozoic, mammals were ecologically diverse, but less so than extant species. Modern ecological richness was established by the Eocene, but questions remain about the ecology of the first wave of mammals radiating after the extinction.Postcranial fossils are often used to determine locomotor behavior; however, the semicircular canals of theinner ear also represent a reliable proxy. These canals detect the angular acceleration of the head duringl ocomotion and transmit neuronal signals to the brain to allow stabilization of the eyes and head. Accordingly, vestibular sensitivity to rapid rotational head movements is higher in species with a larger canal radius of curvature and more orthogonal canals. We used high-resolution computed tomography scanning to obtain inner ear virtual endocasts for 30 specimens. We supplemented these with data from the literature to constructa database of 79 fossil from the Jurassic to the Eocene and 262 extant mammals. We compared data on canal morphology and another lifestyle proxy, the size of the petrosal lobules, which have a role in maintaining eyes’ movements and position. We find that Paleocene mammals exhibited a lower average and more constricted range of Agility Indices (AI), a new measure of canal radius size relative to body size, compared to Mesozoic, Eocene and extant taxa. Inthe early Paleocene, body mass and canal radius increased, but the former outpaced the latter leading to an AIdecline. Similarly, their petrosal lobules were relatively smaller on average compared to other temporal groups, which suggests less ability for fast movements. Additionally, Paleocene mammals had similar AIs to extant scansorial and terrestrial quadrupeds. In contrast, the lack of canal orthogonality change from the Mesozoic to the Paleocene indicates no trend toward lower vestibular sensitivity regardless of changes in body size. This result may reflect functional differences between canal orthogonality and radius size. Our results support previous work on tarsal morphology and locomotor behavior ancestral state reconstruction suggesting that ground dwelling mammals were more common than arboreal taxa during the Paleocene. Ultimately, this pattern may indicate that the collapse of forested environments immediately after extinction led to the preferential survivorship of more terrestrially adapted mammals. 

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4. Virtual endocranial and inner ear endocasts of the Paleocene ‘condylarth’ Chriacus : new insight into the neurosensory system and evolution of early placental mammals

   https://doi.org/10.1111/joa.13084  

   Bertrand, Ornella C.; Shelley, Sarah L.; Wible, John R.; Williamson, Thomas E.; Holbrook, Luke T.; Chester, Stephen G. B.; Butler, Ian B.; Brusatte, Stephen L. (October 2019, Journal of Anatomy)

   Abstract The end‐Cretaceous mass extinction allowed placental mammals to diversify ecologically and taxonomically as they filled ecological niches once occupied by non‐avian dinosaurs and more basal mammals. Little is known, however, about how the neurosensory systems of mammals changed after the extinction, and what role these systems played in mammalian diversification. We here use high‐resolution computed tomography (CT) scanning to describe the endocranial and inner ear endocasts of two species,Chriacus pelvidensandChriacus baldwini, which belong to a cluster of ‘archaic’ placental mammals called ‘arctocyonid condylarths’ that thrived during theca. 10 million years after the extinction (the Paleocene Epoch), but whose relationships to extant placentals are poorly understood. The endocasts provide new insight into the paleobiology of the long‐mysterious ‘arctocyonids’, and suggest thatChriacuswas an animal with anencephalization quotient (EQ)range of 0.12–0.41, which probably relied more on its sense of smell than vision, because the olfactory bulbs are proportionally large but the neocortex and petrosal lobules are less developed. Agility scores, estimated from the dimensions of the semicircular canals of the inner ear, indicate thatChriacuswas slow to moderately agile, and its hearing capabilities, estimated from cochlear dimensions, suggest similarities with the extant aardvark.Chriacusshares many brain features with other Paleocene mammals, such as a small lissencephalic brain, large olfactory bulbs and small petrosal lobules, which are likely plesiomorphic for Placentalia. The inner ear ofChriacusalso shares derived characteristics of the elliptical and spherical recesses with extinct species that belong to Euungulata, the extant placental group that includes artiodactyls and perissodactyls. This lends key evidence to the hypothesized close relationship betweenChriacusand the extant ungulate groups, and demonstrates that neurosensory features can provide important insight into both the paleobiology and relationships of early placental mammals. 

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5. Comparative morphology of the internal structures of the mammalian cochlea

   https://doi.org/10.1093/zoolinnean/zlaf025  

   Peacock, John (May 2025, Zoological Journal of the Linnean Society)

   na (Ed.)

   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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