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  1. After successfully diversifying during the Paleocene, the descendants of the first wave of mammals that survived the end‐Cretaceous mass extinction waned throughout the Eocene. Competition with modern crown clades and intense climate fluctuations may have been part of the factors leading to the extinction of these archaic groups. Why these taxa went extinct has rarely been studied from the perspective of the nervous system. Here, we describe the first virtual endocasts for the archaic order Tillodontia. Three species from the middle Eocene of North America were analyzed: Trogosus hillsii, Trogosus grangeri, and Trogosus castoridens. We made morphological comparisons with the plaster endocast of another tillodont,Tillodon fodiens, as well as groups potentially related to Tillodontia: Pantodonta, Arctocyonidae, and Cimolesta. Trogosus shows very little inter‐specific variation with the only potential difference being related to the fusion of the optic canal and sphenorbital fissure. Many ancestral features are displayed by Trogosus, including an exposed midbrain, small neocortex, orbitotemporal canal ventral to rhinal fissure, and a broad circular fissure. Potential characteristics that could unite Tillodontia with Pantodonta, and Arctocyonidae are the posterior position of cranial nerve V3 exit in relation to the cerebrum and the low degree of development of the subarcuate fossa. The presence of large olfactory bulbs and a relatively small neocortex are consistent with a terrestrial lifestyle. A relatively small neocortex may have put Trogosus at risk when competing with artiodactyls for potentially similar resources and avoiding predation from archaic carnivorans, both of which are known to have had larger relative brain and neocortex sizes in the Eocene. These factors may have possibly exacerbated the extinction of Tillodontia, which showed highly specialized morphologies despite the increase in climate fluctuations throughout the Eocene, before disappearing during the middle Eocene. 
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    Free, publicly-accessible full text available January 1, 2025
  2. 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. 
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  3. 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) 
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  4. 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 
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  5. After the end-Cretaceous extinction, placental mammals quickly diversified, occupied key ecological niches and increased in size, but this last was not true of other therians. The uniquely extended gestation of placental young may have factored into their success and size increase, but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping9,10 by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30–75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals. 
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  6. Abstract We describe the tympanic anatomy of the petrosal of Deltatherium fundaminis, an enigmatic Paleocene mammal based on cranial specimens recovered from New Mexico, U.S.A. Although the ear region of Deltatherium has previously been described, there has not been a comprehensive, well-illustrated contribution using current anatomical terminology. The dental and cranial anatomy of Deltatherium is a chimera, with morphological similarities to both ‘condylarth’ and ‘cimolestan’ taxa. As such, the phylogenetic relationships of this taxon have remained elusive since its discovery, and it has variably been associated with Arctocyonidae, Pantodonta and Tillodontia. The petrosal of Deltatherium is anteriorly bordered by an open space comprising a contiguous carotid opening and pyriform fenestra. The promontorium features both a small rostral tympanic process and small epitympanic wing but lacks well-marked sulci. A large ventral facing external aperture of the canaliculus cochleae is present and bordered posteriorly by a well-developed caudal tympanic process. The hiatus Fallopii opens on the ventral surface of the petrosal. The tegmen tympani is mediolaterally broad and anteriorly expanded, and its anterior margin is perforated by a foramen for the ramus superior of the stapedial artery. The tympanohyal is small but approximates the caudal tympanic process to nearly enclose the stylomastoid notch. The mastoid is widely exposed on the basicranium and bears an enlarged mastoid process, separate from the paraoccipital process. These new observations provide novel anatomical data corroborating previous hypotheses regarding the plesiomorphic eutherian condition but also reveal subtle differences among Paleocene eutherians that have the potential to help inform the phylogeny of Deltatherium. 
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  7. null (Ed.)
    A small piece of cartilage or bone, the element of Paaw, occurs in the tendon of the stapedius muscle in some extant marsupial and placental mammals. It has been nearly a century since the last comprehensive treatment of the distribution of the element of Paaw in mammals. The current report updates knowledge on this structure by synthesizing the subsequent literature and providing new observations of extant marsupials from the collections of the Section of Mammals, Carnegie Museum of Natural History, and two online resources for CT scanned data: DigiMorph.org and MorphoSource.org. We found an element of Paaw in some representatives of all seven extant marsupial orders: Didelphimorphia, Microbiotheria, Notoryctemorphia, Peramelemorphia, Paucituberculata, Dasyuromorphia, and Diprotodontia. In the first four orders, the element is substantial, longer than the long axis of the fenestra vestiuli (oval window), which holds the stapedial footplate; it is smaller than the long axis of the fenestra vestibuli in Paucituberculata and we do not have measures to report for the last two orders. In most marsupials examined, the element of Paaw contacts the petrosal behind the oval window, suggesting it functions as a sesamoid bone, increasing the lever arm of the stapedius muscle. Although there is some variability in the presence of the bone both between and within individual museum specimens, we interpret this as the result of preparation techniques rather than true variation. To place the element of Paaw in its anatomical context, we describe in detail the ear region and middle-ear auditory apparatus of the gray four-eyed opossum, Philander opossum (Linnaeus, 1758), a didelphid from Central and South America, based on a CT scanned specimen from Carnegie Museum of Natural History. It has an ossified element of Paaw with a volume greater than the stapes. Comparisons are made with petrosals of Didelphis marsupialis Linnaeus, 1758, and Monodelphis domestica (Wagner, 1842), also based on CT scanned specimens. 
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  8. null (Ed.)
    Mammals exhibit vast ecological diversity, including a panoply of locomotor behaviours. The foundations of this diversity were established in the Mesozoic, but it was only after the end-Cretaceous mass extinction that mammals began to increase in body size, diversify into many new species and establish the extant orders. Little is known about the palaeobiology of the mammals that diversified immediately after the extinction during the Palaeocene, which are often perceived as ‘archaic’ precursors to extant orders. Here, we investigate the locomotor ecology of Palaeocene mammals using multivariate and disparity analyses. We show that tarsal measurements can be used to infer locomotor mode in extant mammals, and then demonstrate that Palaeocene mammals occupy distinctive regions of tarsal morphospace relative to Cretaceous and extant therian mammals, that is distinguished by their morphological robustness. We find that many Palaeocene species exhibit tarsal morphologies most comparable with morphologies of extant ground-dwelling mammals. Disparity analyses indicate that Palaeocene mammals attained similar morphospace diversity to the extant sample. Our results show that mammals underwent a post-extinction adaptive radiation in tarsal morphology relating to locomotor behaviour by combining a basic eutherian bauplan with anatomical specializations to attain considerable ecomorphological diversity. 
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  9. Placental mammals had a smaller brain-to-body-size ratio after the dinosaur extinction but later developed the largest vertebrate brains. 
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