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1. Journal of Vertebrate Paleontology, Program and Abstracts

   Holpin, Sofia; Shelley, Sarah L.'; Williamson, Thomas E.; Brusatte, Stephen (October 2020, Society of Vertebrate Paleontology)

   With the dawn of the Paleogene, the mammalian survivors of the Cretaceous–Paleogene mass extinction, 66 million years ago, found themselves in an emptied landscape. Within a million years of the bolide impact, placental mammals reached a diversity and abundance never seen during the Age of Dinosaurs. The North American ‘condylarths’ were amongst the first mammals to diversify during the early Paleogene and are often considered the ancestral ‘stock’ from which other euungulate groups evolved. Amongst these, Phenacodontidae are often regarded to lie at the base of the perissodactyl family tree, but their phylogenetic position, and that of other ‘condylarths’, remain contentious. Tetraclaenodon, a medium-sized herbivorous phenacodontid from the Torrejonian (~64 to ~62 Ma) of North America is generally recognized as the oldest member of Phenacodontidae, and thus is instrumental for untangling the evolutionary relationships of ‘condylarths’ and perissodactyls. Here we present new information on Tetraclaenodon based on a description of new and previously known fossil material from the San Juan Basin of New Mexico, U.S.A., which we studied using high-resolution computed tomography (CT) scanning. From CT scans of the cranium, we segmented the brain endocast, which is relatively small and smooth (lissencephalic), similar to that of other Paleocene mammals. The petrosal lobules, which are involved in eye movement coordination, are small. The semi-circular canals associated with balance, provide an agility score of 3 indicating that Tetraclaenodon was probably moderately agile, similar to the extant raccoon dog or the aardwolf. A multivariate analysis of tarsal measurements for a sample of Paleocene and extant mammals, which informs locomotor style, indicates that Tetraclaenodon was most suited to terrestrial locomotion. This is in line with anatomical and myological features of the limbs of Tetraclaenodon and other phenacodontids, early perissodactyls and extant mammals. These findings contradict previous studies that designated Tetraclaenodon as a scansorial mammal, capable of habitually climbing trees. Our results shed light on the locomotory adaptations of Tetraclaenodon in comparison to more cursorial phenacodontids and perissodactyls, such as Phenacodus and Hyrachyus. The earliest member of the perissodactyl stem lineage apparently lacked the more cursorial adaptations of their relatives in the late Paleocene and onwards. 

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2. The rise of the Age of Mammals and the role of early Paleocene ‘condylarth’ Tetraclaenodon (Mammalia, Phenacodontidae) from the San Juan Basin of New Mexico

   Holpin, S.; Shelley, S.L.; Bertrand, O.C.; Williamson, T.E.; Brusatte, S.L. (October 2021, Italian Paleontological Society, Abstracts and Programs)

   With the dawn of the Paleogene, the mammalian survivors of the Cretaceous–Paleogene mass extinction, 66 million years ago, found themselves in an emptied landscape. Within a million years of the bolide impact, placental mammals reached a diversity and abundance never seen during the Age of Dinosaurs. The North American ‘condylarths’ were amongst the first mammals to diversify during the early Paleogene and are often considered the ancestral ‘stock’ from which other euungulate groups evolved. Amongst these, Phenacodontidae are often regarded to lie at the base of the perissodactyl family tree, but their phylogenetic position, and that of other ‘condylarths’, remain contentious. Tetraclaenodon, a medium-sized herbivorous phenacodontid from the Torrejonian (~64 to ~62 Ma) of North America is generally recognized as the oldest member of Phenacodontidae, and thus is instrumental for untangling the evolutionary relationships of ‘condylarths’ and perissodactyls. Here we present new information on Tetraclaenodon based on a description of new and previously known fossil material from the San Juan Basin of New Mexico, U.S.A., which we studied using high-resolution computed tomography (CT) scanning. From CT scans of the cranium, we segmented the brain endocast, which is relatively small and smooth (lissencephalic), similar to that of other Paleocene mammals. The petrosal lobules, which are involved in eye movement coordination, are small. The semi-circular canals associated with balance, provide an agility score of 3 indicating that Tetraclaenodon was probably moderately agile, similar to the extant raccoon dog or the aardwolf. A multivariate analysis of tarsal measurements for a sample of Paleocene and extant mammals, which informs locomotor style, indicates that Tetraclaenodon was most suited to terrestrial locomotion. This is in line with anatomical and myological features of the limbs of Tetraclaenodon and other phenacodontids, early perissodactyls and extant mammals. These findings contradict previous studies that designated Tetraclaenodon as a scansorial mammal, capable of habitually climbing trees. Our results shed light on the locomotory adaptations of Tetraclaenodon in comparison to more cursorial phenacodontids and perissodactyls, such as Phenacodus and Hyrachyus. The earliest member of the perissodactyl stem lineage apparently lacked the more cursorial adaptations of their relatives in the late Paleocene and onwards. 

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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 (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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4. 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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5. Testing the phylogeny of Periptychidae and "archaic" Paleocene mammals uner different optimality criteria

   Puschel, H. P.; Shelley, S. L.; Williamson, T. E.; Wible, J. R.; Brusatte, S. L. (October 2019, Journal of Vertebrate Paleontology, Abstracts and Program)

   It is now well established that the end-Cretaceous mass extinction had enormous repercussions for mammalian evolution. Following the extinction, during the Paleocene, mammals started to radiate, occupying new and diverse ecological niches. However, the phylogenetic relationships between the socalled “archaic” mammals of this time, and their position within Placentalia, remain contentious. The Periptychidae are a clade of distinctive “archaic” ungulates, composed of ~17 genera of small to large bodied, highly bunodont, terrestrial herbivores that were among the first placental mammals to appear after the end-Cretaceous mass extinction. Although the Periptychidae has been historically considered a distinctive “condylarth” subgroup, their higherlevel relationships have been rarely tested. Here, we present an inclusive cladistic analysis to determine and test the phylogenetic affinities of Periptychidae and other key Paleocene groups within Placentalia under different cladistic optimality criteria. We scored 140 taxa for 503 dental, cranial and postcranial characters, incorporating new morphological and taxonomic data. The data were then subject to parsimony and Bayesian tree of morphological evolution, running 5000000 generations with samples every 200 generations and discarding 25% of the samples as burn-in. Stationarity was achieved and a 50 percent majority rule consensus tree from the sampled trees was obtained. The parsimony analysis recovered 48 most parsimonious trees. The two consensus trees derived from the different analyses are largely congruent and recover a monophyletic Periptychidae, although the parsimony consensus tree is better resolved. These results are consistent with simulation studies showing that parsimony tends to be more precise (more nodes reconstructed) than Bayesian analyses, although less accurate. The main topological differences between the results relate to the position of poorly known Puercan (earliest Paleocene) species. Our results affirm the monophyly of Periptychidae and its nesting within a group of “condylarths” positioned at the base of Laurasiatheria and closely related to Artiodactyla. Within Periptychidae we found support for the three major subfamilial divisions in both analyses. These results highlight the importance of using different optimality criteria when resolving a phylogeny and provide a new insight into how placental mammals were evolving after the end-Cretaceous extinction. Grant Information: CONICYT PFCHA/DOCTORADO BECAS CHILE/2018, European Research Council Starting Grant (ERC StG 2017, 756226, PalM), National Science Foundation (NSF EAR 1654952, DEB 1654949) 

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