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The idea that ecological niches remain stable during periods of rapid climate change has long been central to methods used to assess extinction risk. However, evidence to test this assumption, particularly beyond recent timescales, remains scarce. Here we examine how a terrestrial mammal responded to rapid climate warming during the Latest Danian Event (LDE; ~62.3 Ma) in the early Paleocene. Tetraclaenodon puercensis is an archaic ungulate that exhibits a size reduction during the LDE in the San Juan Basin of New Mexico, USA. The drivers of this phenomenon – hyperthermal dwarfism – remain poorly resolved and are often linked to biogeographic range shifts rather than in situ ecological responses. Using a novel multi-comparator approach to dental microwear texture analysis, we show that T. puercensis shifted from frugivorous to folivorous diets during the LDE. Such a shift is often observed among extant forest mammals during times of food scarcity and moisture stress, which are likely during Palaeogene hyperthermals. Our results provide the first robust evidence for mammalian ecological responses and adaptation to lower quality resources during a Palaeogene hyperthermal. Dietary niche shifts therefore provide a means of dealing with rapid warming without requiring broad changes in biogeographic ranges. 

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. 

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. 

After the Cretaceous-Palaeogene (K-Pg) mass extinction mammals thrived in the Cenozoic. However, the phylogenetic affinities of early Palaeogene ‘archaic’ mammals that lived immediately after the extinction remain unresolved. Taeniodonta is a group of puzzling ‘archaic’ mammals that appeared in the early Palaeocene of North America. They are arranged into two subgroups; the Conoryctidae and Stylinodontidae and are characterised by their extreme degree of dental wear, indicating an abrasive diet, which led to hypsodonty in the most derived species. Due, in part, to their worn teeth and their rarity in the fossil record, the position of taeniondonts in the mammalian phylogenetic tree remains unresolved. New fossils from San Juan basin, New Mexico, USA, including unworn teeth of four genera and postcranial elements of an early taeniodont, Conoryctes, shed light on their dental and postcranial anatomy. Both in the forelimb and hind limp of Conoryctes, there are anatomical adaptations towards fossoriality. Using these specimens, we scored taeniodonts and other Palaeogene mammals into a phylogenetic data matrix (620 characters, 135 taxa). We then conducted a phylogenetic analysis using parsimony. Our results show that Taeniodonta is a monophyletic group within Eutheria. We also found that Onychodectes is basal to the two subgroups previously proposed. Based on the new postcranial fossils and revised phylogeny, we concluded that digging behaviours were likely ancestral for taeniodonts. Therefore, a more fossorial mode of life may have been beneficial for their surviving and thriving in the wake of the K-Pg extinction. 

An explanation for why some species, such as non-avian dinosaurs, became extinct, whereas others, including mammals, survived the Cretaceous/Paleogene (K/Pg) mass extinction, 66 million years ago (Ma) is still debated. What were the mechanisms behind community restructuring and the emergence of new ecological opportunities after the K/Pg event, selectively driving extinction and survivorship patterns? Using Markov networks, ecological niche partitioning and Earth System models, we reconstructed disruptions in continental food web dynamics, simulating long-term trajectories in ecospace occupancy through the latest Cretaceous (83.6–66.0 Ma) and early Paleogene (66.0–61.6 Ma). This method uses partial correlation networks to represent how different trophic groups interact in a food web and builds on empirical spatial co-variations to explore dependencies between trophic groups. Our analyses are based on a spatiotemporally and taxonomically standardized dataset, comprising more than 1,600 fossil occurrences representing more than 470 genera of fish, salamanders, frogs, albanerpetontids, lizards, snakes, champsosaurs, turtles, crocodylians, dinosaurs (including birds), and mammals across the best sampled region for this interval, the Western Interior of North America. We explicitly tested whether: 1) shifts in food web architecture underwent major restructuring before and after the K/Pg transition, including whether some trophic guilds were more prone to these shifts than others; and 2) any of these changes were associated with fluctuations in the realized niche space, helping to explain survivorship and extinction patterns at the boundary. We find a shift in latest Cretaceous dinosaur faunas, as medium-sized species counterbalanced a loss of large herbivores, but that dinosaur niches were otherwise resilient and static until the K/Pg boundary. Smaller terrestrial vertebrates, including mammals, followed a consistent trajectory of increasing trophic impact and relaxation of ecological niche limits that began in the Cretaceous and continued after the extinction. Patterns of mammalian ecological radiation and niche restructuring indicate that these taxa did not simply proliferate after the extinction; rather, their earlier ecological diversification might have helped them survive the K/Pg event, whereas the static niche of dinosaurs might have contributed to their demise. 

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