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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. 

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. 

Mammals survived the Chicxulub impact sixty-six million years ago and diversified into a wide variety of new ecological niches left by non-avian dinosaurs. Pantodonts, an enigmatic group, quickly achieved hefty postextinction body sizes to occupy large herbivore niches. We describe the first juvenile specimen of the Paleocene pantodont Pantolambda bathmodon (NMMNH P-27844) consisting of a partial skeleton including parts of the skull, a deciduous upper premolar series, nearly complete forelimbs, and elements of the carpus and hind limb. P-27844 is from the Torrejonian (~62.3 Ma) Tsosie Member of the Nacimiento Formation. P-27844 has the first deciduous teeth known for Pantolambda. dP2 and dP4 are submolariform with a triangular cross-section and a less developed protocone than adults. dP5 is molariform with a large paracone and metacone connected by wing-like cristae to form the w-shaped ectoloph typical of this genus’ molars. dP5 also has more pronounced conules than the molars. This molarization style of the ultimate premolar is seen across Pantodonta including in Alcidedorbignya inopinata, Barylambda faberi, and Coryphodon sp. The postcranial morphology of P-27844 is generally concordant with that of adults. This correspondence manifests particularly clearly in the forelimbs. The distal humerus exhibits the base of a posterolaterally directed epicondylar crest which likely anchored the anconeus and the extensor carpi radialis muscles, a deep radial fossa, and an open entepicondylar foramen. The ulna shows a welldeveloped anconeal process, a pronounced biceps and brachialis fossa, and a shallower groove to accommodate the abductor pollicis longus. The radius possesses a shallow pronator crest that originates near its distal end and extends about two-thirds of the way along the shaft. Interestingly, in contrast to adults, the radial shaft is straight rather than having moderate sigmoidal curvature and has not undergone epiphyseal fusion. Altogether, these osteological features illustrate that, even at its early ontogenetic stage, P-27844 possessed robust forelimb musculature. Using Developmental Mass Extrapolation from long bone measurements, P-27844’s body mass is estimated to be ~17 kg at time of death (~40% of adult body mass). Paleohistological analyses demonstrate the animal experienced a rapid pace of life for its body size and died ~2.5 months after birth. This specimen gives unprecedented insight into the early life history of Pantolambda. 

The rise of mammals after the extinction of the dinosaurs remains one of the most enigmatic intervals in the evolution of mammals. A relatively sparse Paleocene fossil record and confusing relationships between taxa means that little is known of the evolution, ecology, or biology of these animals. Accordingly, the life history of these organisms remains unstudied, despite likely playing a key role in the rapid proliferation and body size increase of these clades in recovering ecosystems. Here, we present results of an in-depth paleohistological analysis of Pantolambda bathmodon, an early, possibly gregarious pantodont, using a new ontogenetic series of specimens. Pantodonts were bizarre, herbivorous eutherians of unknown phylogenetic affinity, and were among the first mammal lineages to reach large body sizes in the Paleocene. In examining both dental and skeletal records of growth from the same individuals, including a juvenile still bearing deciduous teeth, our study is among the most comprehensive paleohistological analyses of any fossil mammal, allowing for unprecedented insights into the life history of this species. Neonatal lines in the teeth indicate that the deciduous premolars and the first upper molar erupted prior to birth, similar to precocious, nidifugous mammals today. Daily incremental lines in the enamel and dentine suggest rapid crown formation times (~70–180 days) and a gestation period of at least 20 weeks. A stress line in the teeth and postcranial bones, recording an anomalous decrease in growth towards the end of this individual’s life, may represent weaning. The weanling perished approximately 2.5 months after birth, weighing about 17 kg. Adult individuals exhibiting severe wear on the dentition allow us to estimate maximum longevity in Pantolambda bathmodon at about 7 years. In comparison with living mammals, Pantolambda bathmodon had gestation and weaning periods below average for a placental of its adult body size (42 kg), but within the range of known variation. However, its lifespan was exceptionally short, falling outside the bounds of comparable living mammals. Together, these lines of evidence suggest a rapid pace of life in Pantolambda bathmodon, despite its relatively large body size. Ongoing sampling of more individuals and geochemical analyses should allow for estimation of time to sexual maturity and help to confirm the identity of the weaning line, completing our picture of the life history of this pioneering species. 

Taeniodonta is a group of North America Palaeogene mammals that lived after the end-Cretaceous mass extinction. Taeniodonts show an extreme degree of dental wear, indicative of an abrasive diet, leading to hypsodonty in the most derived species. The rarity of fossils and their highly worn teeth makes their dental morphology difficult to study. We examined five new partial mandibles from the San Juan Basin, New Mexico, USA, most of which preserve unworn molars. One of the specimens preserves a deciduous ultimate premolar and using 3D micro-CT we were able to segment and study the unworn permanent tooth embedded in the jaw. We then conducted multivariate analyses on dental measurements to compare the new specimens to known teeth of early taeniodonts. We assigned the new specimens to at least three genera of Conoryctidae, a taeniodont subclade. Our results suggest that there is a broader dental diversity of the studied genera than previously thought. Morphological observations also suggest that progressive loss of cingulids and the addition of cuspids started early in the evolution of taeniodonts. These distinctive dental specializations strengthen the hypothesis that early Palaeocene mammals were able to rapidly adapt to fill the vacant ecological niches after the end-Cretaceous extinction 

After the Cretaceous-Paleogene (K-Pg) mass extinction mammals, which originated during the Mesozoic, managed to survive and thrive. However, the tempo and mode of evolution for eutherians (placentals and close relatives) after the extinction are still unclear. An ideal group to investigate the post KPg evolution of mammals is the taeniodonts, as they are among the few taxa to purportedly cross the boundary. They then underwent a radiation in the early Paleogene and are defined primarily by their unusual dentition which is suited to chew an abrasive and tough diet. Ten genera of taeniodonts are currently recognized and are commonly arranged into two families. The Conoryctidae is usually considered to have a more generalized body plan while Stylinodontidae possess relatively extreme digging adaptations and more highly derived dentitions with enlarged canines. We conducted a phylogenetic analysis by applying parsimony and Bayesian techniques to a dataset of characters gathered from extensive observation of new specimens. We found limited support for the conoryctid-stylinodontid division and the genera Conoryctes and Onychodectes are placed as key basal taxa outside the clade of the more robust derived taxa (Wortmania, Ectoganus, Psittacotherium, Stylinodon). We then assessed postcranial bones to determine functional modes for taeniodonts and to test changes across phylogeny. Qualitatively, most taeniodonts, including Onychodectes, possess indicators of digging, i.e., a well-developed deltopectoral crest and broad distal end of the humerus for increasing flexion, pronation and supination, a long olecranon process of the ulna and enlarged manual unguals. Then we conducted quantitative multivariate analyses (linear discriminant analysis), using 9 forelimb linear measurements and 29 tarsal ones, comparing taeniodonts to a suite of extant mammals with known locomotor mode and other Paleogene taxa. Our results suggest Onychodectes to be terrestrial/semifossorial and comparable with the numbat (Myrmecobius fasciatus). Ectoganus and Stylinodon are semi-fossorial and fall out near the gopher, Pappogeomys merriami and the aardvark (Orcyteropus afer). Therefore, our study indicates that digging behaviors are ancestral for taeniodonts, and suggest that burrowing may have been integral to their survival across the KPg boundary and their subsequent radiation. Grant Information: European Research Council Starting Grant (ERC StG 2017, 756226, PalM), National Science Foundation (EAR- 1325544, 1654952, DEB-1654952, 1654949) 

After the end-Cretaceous mass extinction, approximately 75% of life on land and in the sea disappeared. The mammals of the early Cenozoic rapidly diversified and dispersed, rising to numerical and ecological dominance beyond their Mesozoic norms. Among those initial groups that ushered in the Age of Mammals, Paleocene and Eocene ‘condylarths’ are thought to include the ancestors of modern odd-toed ungulates (horses, tapirs, rhinos). Tetraclaenodon is the oldest genus of the ‘condylarth’ group Phenacodontidae and one of the most abundant fossils from the San Juan Basin (SJB) of New Mexico. Tetraclaenodon was a medium sized (mean body mass ca. 10kg), terrestrial mammal which was lightly built and had an omnivorous to herbivorous bunodont dentition. Here we use multivariate and statistical analyses to investigate body mass and dental variation in 110 teeth of Tetraclaenodon spanning the Torrejonian (Paleocene) interval of the SJB. The specimens were grouped into six time bins by their biostratigraphical reference, from Tj1 (~63.8 Ma) through Tj6 (~62.7 Ma). Measurements of the length, mesial and distal width of the lower first molars (m1) were subject to principal component analysis (PCA), and m1 area was used to predict body mass using a regression equation. The PCA morphospace ordinates specimens along a PC1 axis that accounts for 90.05% of total variance and is significantly correlated with body size. A PERMANOVA test finds a significant difference in morphospace occupation (non-overlap) between clusters of specimens from Tj1-3 and Tj4-6, but there are no significant differences between the individual time bins within each cluster. This trend is also seen in the body size estimates: Mann-Whitney tests recover significant differences between the two clusters. These results suggest that Torrejonian populations of Tetraclaenodon were relatively constant in size throughout Tj1-3, but between Tj3 and Tj4 underwent an increase in body mass and subsequently stabilized (at this resolution) for the remainder of the Torrejonian. A similar trend is seen in contemporary populations of the periptychid ‘condylarth’ Periptychus, suggesting that there were selective environmental pressures acting on these herbivorous species. These body size differences may reflect the emergence of a new, larger Tetraclaenodon species in Tj4, or may be associated to an environmental change, perhaps relating to climate or vegetation. In either case, this illustrates dynamic evolution of mammals during the few million years after the extinction. Grant Information: European Research Council Starting Grant (ERC StG 2017, 756226, PalM), National Science Foundation (EAR- 1654952) 

Mammals originated during the Mesozoic and survived the Cretaceous-Palaeogene (K-Pg) mass extinction. Their evolution from small, opportunistic animals to more specialised animals with diverse locomotor behaviours following the extinction is still unclear. An ideal group to address this question is the Taeniodonta which are among the few eutherians that purportedly crossed the K-Pg boundary and diversified in the early Palaeogene. They are known thus far from the Palaeogene of North America and are characterised by their unique dentition adapted for an abrasive diet and their robust skeleton. There are 10 genera of taeniodonts classified into two families. The Conoryctidae are smaller with more generalised body plan, whereas the Stylinodontidae reached large body size (up to 100kg) and evolved crown hypsodonty. We focused our study on the postcranial functional morphology of the two taeniodont subgroups. We conducted linear discriminant analysis using 9 linear measurements of the humerus, comparing Onychodectes (a conoryctid) and Stylinodon, Ectoganus, Psittacotherium (stylinodonts) with extant mammals of known locomotion. We also used 29 linear tarsal measurements to evaluate the locomotor behaviour of Onychodectes and Conoryctes (conoryctids) and Ectoganus alongside a sample of extant mammals and other Palaeogene taxa. Our results show that Onychodectes, which is one of the most basal taeniodonts, might have been terrestrial/semi-fossorial, similar to the numbat. Postcranial features of Onychodectes show it possessed digging adaptations i.e. a long olecranon process of the ulna, enlarged manual unguals and a well-developed deltopectoral crest and broad distal end of the humerus. We find stylinodontid taeniodonts to be distinctly more fossorial, comparable to the striped skunk, gopher and the aardvark. Our study suggests that digging is an ancestral behaviour for taeniodonts implying the importance of burrowing for surviving the K-Pg extinction. 

The end-Cretaceous mass extinction, 66 million years ago, profoundly reshaped the biodiversity of our planet. After likely originating in the Cretaceous, placental mammals (species giving live birth to well-developed young) survived the extinction and quickly diversified in the ensuing Paleocene. Compared to Mesozoic species, extant placentals have advanced neurosensory abilities, enabled by a proportionally large brain with an expanded neocortex. This brain construction was acquired by the Eocene, but its origins, and how its evolution relates to extinction survivorship and recovery, are unclear, because little is known about the neurosensory systems of Paleocene species. We used high-resolution computed tomography (CT) scanning to build digital brain models in 29 extinct placentals (including 23 from the Paleocene). We added these to data from the literature to construct a database of 98 taxa, from the Jurassic to the Eocene, which we assessed in a phylogenetic context. We find that the Phylogenetic Encephalization Quotient (PEQ), a measure of relative brain size, increased in the Cretaceous along branches leading to Placentalia, but then decreased in Paleocene clades (taeniodonts,phenacodontids, pantodonts, periptychids, and arctocyonids). Later, during the Eocene, the PEQ increased independently in all crown groups (e.g., euarchontoglirans and laurasiatherians). The Paleocene decline in PEQ was driven by body mass increasing much more rapidly after the extinction than brain volume. The neocortex remained small, relative to the rest of the brain, in Paleocene taxa and expanded independently in Eocene crown groups. The relative size of the olfactory bulbs, however, remained relatively stable over time, except for a major decrease in Euarchontoglires and some Eocene artiodactyls, while the petrosal lobules (associated with eye movement coordination) decreased in size in Laurasiatheria but increased in Euarchontoglires. Our results indicate that an enlarged, modern-style brain was not instrumental to the survival of placental mammal ancestors at the end-Cretaceous, nor to their radiation in the Paleocene. Instead, opening of new ecological niches post-extinction promoted the diversification of larger body sizes, while brain and neocortex sizes lagged behind. The independent increase in PEQ in Eocene crown groups is related to the expansion of the neocortex, possibly a response to ecological specialization as environments changed, long after the extinction.