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The early Palaeocene (66–61.6 mya) witnessed the establishment of mammal-dominated terrestrial ecosystems after the extinction of the non-avian dinosaurs. Understanding the mammals that formed these communities is crucial not only for disentangling the origin of living mammal clades, but also the forces that structured these first precursors of modern ecosystems. The potential role of life history as a driving factor in the composition of early mammalian ecosystems has long been appreciated but has historically been difficult to evaluate. A central focus thus far has been on differences in reproductive strategy between three major North American mammal clades, specifically multituberculates, metatherians, and eutherians. However, virtually no work has considered whether reproductive strategy was uniform within these clades. Recent advances combining paleohistology with geochemistry have opened a new window into reproduction in extinct mammals, revealing a highly precocial lifestyle in the eutherian pantodont Pantolambda, but it is unclear whether this life history style characterized early eutherians more broadly. Results from another eutherian, the phenacodontid Tetraclaenodon, challenge this notion. Both cementochronology and osteohistology indicate a drastically slower life history in the slightly smaller Tetraclaenodon, at virtually the opposite end of the eutherian spectrum from Pantolambda. After a relatively short gestation period (~2 months), Tetraclaenodon retained slow-growing deciduous teeth for as long as four years. The oldest individual in our sample grew exceptionally slowly towards the end of its life, which spanned at least 8–9 years. The ratio of gestation period to body size (10–15 kg) in Tetraclaenodon is similar to small-bodied carnivorans like the coyote (Canis latrans), Caracal (Caracal caracal), and African civet (Civettictis civetta). However, these extant species vary significantly in the duration of suckling (1.5–4 months), and therefore total maternal investment. 

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. 

It has long been debated why groups such as non-avian dinosaurs became extinct whereas mammals and other lineages survived the Cretaceous/Paleogene mass extinction 66 million years ago. We used Markov networks, ecological niche partitioning, and Earth System models to reconstruct North American food webs and simulate ecospace occupancy before and after the extinction event. We find a shift in latest Cretaceous dinosaur faunas, as medium-sized species counterbalanced a loss of megaherbivores, but dinosaur niches were otherwise stable and static, potentially contributing to their demise. Smaller vertebrates, including mammals, followed a consistent trajectory of increasing trophic impact and relaxation of niche limits beginning in the latest Cretaceous and continuing after the mass extinction. Mammals did not simply proliferate after the extinction event; rather, their earlier ecological diversification might have helped them survive.

 

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. 

Resolving the phylogenetic relationships among Paleocene mammals has been a longstanding goal in paleontology. Constructing an accurate and comprehensive phylogeny for Paleocene mammals is a worthwhile objective in itself, but it also provides a framework on which we can better understand the origin of placental mammals and the evolutionary processes underlying the diversification of mammals before, during, and after the end-Cretaceous mass extinction. More recently, a robust Palaeocene mammal phylogeny has become a much-coveted tool for reconciling discrepancies between morphological and molecular evidence for the phylogeny and diversification of Placentalia. Here, we present a novel phylogenetic dataset to test hypotheses regarding Paleocene mammal phylogeny and the origin and diversification of Placentalia. To date, our matrix combines phenomic data for 36 extant mammal species and 107 fossil species scored for 2540 morphological characters alongside 26 genes sequenced for 47 species. We utilized a reductive morphological scoring strategy in order to minimize assumptions and test hypotheses on homology. Multiple sequence alignments were performed in MEGA-X for each gene. We then analysed the data using Bayesian methods and explored the effects of different approaches. Relaxed clock analyses using a molecular constraint and an FBD prior are congruent with the diversification of many extant orders prior to the K-Pg boundary. Relaxed clocked total-evidence analyses (morphology and molecules) using an FBD prior resulted in older ages of diversification than those estimated by our relaxed clock molecular constraint model and previous molecular studies. Within Placentalia, our phylogenies provide support for the divergence of Atlantogenata (Afrotheria and Xenarthra) from Boreoeutheria (Euarchontoglires and Laurasiatheria). Among the Paleocene taxa, ‘condylarths’ are distributed along the base of Laurasiatheria with members of ‘Arctocyonidae’ recovered as sister taxa to Artiodactyla; enigmatic groups such as Pantodonta and Taeniodonta are recovered as crown placentals whereas Leptictida is not. Our Paleocene mammal phylogeny is a critical step toward better understanding placental mammal evolution. Ultimately, this work will facilitate the investigation of fundamental questions previously encumbered by the lack of a well-resolved phylogeny. 

In the wake of the end-Cretaceous extinction, pantodonts were among the first mammals to achieve truly large body sizes. Paleocene pantodonts occupied large herbivore niches across North America, Asia, and Europe. In North America, the Torrejonian genus, Pantolambda, encompasses three species ranging from large dog- to small cow-sized. Of the three species, P. intermedium is the most poorly represented with known material consisting of a fragmentary dentary with m1-2 and isolated lower premolars. All the originally referred material was recovered from the Gidley Quarry, Montana. We describe cranial and postcranial fragments of the species from the Nacimiento Formation of the San Juan Basin (SJB), New Mexico. Interestingly, although it is intermediate in size between P. bathmodon and P. cavirictum, P. intermedium occurs lower in the stratigraphy (Tj2) than these other species and is the first appearance of pantodonts in the SJB. The presence of P. intermedium in the SJB is validated with a worn dentary (NMMMNH P-19774) containing m1-2. A pronounced entoconid on m1 and m2 distinguishes these teeth from those of P. cavirictum, whose entoconid is weakly developed, and from those of P. bathmodon, which lacks an entoconid on the anterolingually-sloping postcristid. An isolated m3 (NMMNH P-72117) shows a partial, narrow trigonid with a wide talonid basin that is shallower than in P. bathmodon. A concreted, partial braincase (NMMNH P- 21646) bears low sagittal and nuchal crests similar to P. bathmodon. A partial scapula (NMMNH P-21647) preserves the glenoid region and the distal portion of the scapular body. The glenoid cavity is an elongated oval that tapers anteriorly to a prominent, triangular supraglenoid tubercle. A coracoid process distinct from the tubercle is not present. A similar pattern is observed in P. bathmodon. The condition observed in Pantolambda contrasts with other pantodonts. Alcidedorbigna possesses a relatively small tubercle distinct from a rounded coracoid process and the larger-bodied pantodonts, Barylambda and Coryphodon, exhibit both a prominent tubercle and a well-developed coracoid process. A prior hypothesis posited that P. intermedium from Montana could simply represent larger morphs of P. bathmodon following Bergmann’s Rule. However, the presence of P. intermedium in New Mexico in a similar environment to and at the same latitude as P. bathmodon and P. cavirictum supports its distinction from the other two morphs as a unique species. Funding Sources European Research Council Starting Grant (ERC StG 2017, 756226, PalM); National Science Foundation (NSF; EAR 1654952, DEB 1654949) 

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. 

South American Ungulates (SANUs) exhibit astonishing morphological and ecological diversity due to their almost complete isolation during their early evolution. This unique diversity coupled with the limited fossil record of their earliest evolution makes it difficult to establish their phylogenetic position within the placental mammal tree. Litopterna is the second most diverse order of SANUs after only Notoungulata, with species ranging from the middle Paleocene (~63 Ma) to the late Pleistocene. Among SANUs, litopterns are characterized by having cursorial limbs similar to Holarctic groups like Perissodactyla. Currently there are 67 genera of litopterns grouped into nine families, and the affinities of the Paleogene families remain unclear. Furthermore, it is unclear how litopterns are related to older groups of “archaic” Paleogene ungulates of South America (Kollpaninae and Didolodontidae) and North America (e.g., Mioclaenidae), and other SANUs. To test the phylogenetic relationships of Litopterna, we assembled a new morphological matrix with ~1000 craniodental and postcranial characters for 79 taxa. The data were subjected to Bayesian and maximum parsimony analyses. We conducted tip-dated and undated Bayesian analyses using a Mk + G model of morphological evolution. Fifty percent majority rule consensus trees were obtained from the sampled trees from each analysis. The parsimony analysis resulted in ten most parsimonious trees and a strict consensus was computed. The consensus trees derived from the different analyses were largely congruent. A traditional monophyletic Litopterna failed to be recovered as Protolipternidae was closely related to Didolodontidae. Litopterna was found more closely related to Kollpaninae than to North American Mioclaenidae, and Kollpaninae did not form a monophyletic group with the latter. Adianthidae and Indaleciidae were found in a relatively basal position within Litopterna. Macraucheniidae was found as a sister group to Proterotheriidae, whereas Anisolambdidae was the sister group of Sparnotheriodontidae, these four families forming a monophyletic group. By utilizing a more comprehensive approach, these results alter previous conceptions of the intrafamilial affinities within Litopterna and their position among other Paleogene ungulates, shedding new light on how litopterns evolved and diversified during the Paleogene of South America. Funding Sources ANID-PFCHA-Doctorado en el extranjero Becas Chile-2018-72190003, ERC starting grant PalM 756226, NSF DEB 1654949 and 1654952