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1. Life history of an archaic placental mammal, Pantolambda bathmodon (Placentalia, Pantodonta)

   Funston, G.; dePolo, P.; Shelley, S.; Wible, J.; Williamson, T.; Brusatte, S. (October 2021, Journal of Vertebrate Paleontology, Program and Abstracts)

   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. 

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2. Life history of an archaic placental mammal, Pantolambda bathmodon (Placentalia, Pantodonta)

   Funston, Gregory F.; dePolo, Paige E.; Shelley, Sarah L.; Wible, John R.; Williamson, Thomas E.; Brusatte, Stephen L. (May 2021, Vertebrate anatomy morphology palaeontology)

   null (Ed.)

   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 interrelationships between taxa means that little is known of the evolution, ecology, and biology of these animals. As a result, the life history of these organisms is completely unstudied, despite likely playing a key role in the ability of these clades to rapidly proliferate and increase in body size in recovering ecosystems. However, intensive collection efforts in the San Juan Basin of New Mexico in the last decade have drastically improved the record of many Paleocene mammals, and offer the first opportunity to address questions about the life history of these animals. Here, we present preliminary results of an in-depth paleohistological analysis of Pantolambda bathmodon, an early, possibly gregarious pantodont, using an ontogenetic series of individuals. 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. This intensive approach allows 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 were erupted prior to birth, similar to precocious, nidifugous mammals today. Daily incremental lines in the enamel and dentine suggest rapid crown formation times (~45–70 days) and a gestation period of at least 15 weeks. A stress line in the postcranial bones, recording an anomalous decrease in growth towards the end of this individual’s life, may represent the weaning event. In the absence of geochemical evidence, it is unclear which of two stress lines in the teeth corresponds to this event, but these lines occur roughly one and two months after birth, respectively. The weanling perished approximately 2.5 months after birth, weighing about 17 kg. An adult individual exhibiting severe wear on the dentition allows us to estimate maximum longevity in Pantolambda bathmodon at about 7 years. In comparison with life history data on living mammals from the PanTheria dataset, Pantolambda bathmodon had a gestation length and weaning duration 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 relatively 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. 

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3. The origin of placental mammal life histories

   Funston, Gregory F.; dePolo, Paige; Sliwinski, Jakub T.; Dumont, Matthew; Shelley, Sarah L.; Pichevin, Laetitia E.; Cayzer, Nicola J.; Wible, John R.; Williamson, Thomas E.; Brusatte, Stephen L. (August 2022, Nature)

   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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4. Skull of a new periptychid mammal from the lower Paleocene Denver Formation of Colorado (Corral Bluffs, El Paso County).

   Weaver, Lucas N; Crowell, Jordan W; Chester, Stephen GB; Lyson, Tyler R (April 2024, Journal of mammalian evolution)

   The Periptychidae, an extinct group of archaic ungulates (‘condylarths’), were the most speciose eutherian mammals in the earliest Paleocene of North America, epitomizing mammalian ascendency after the Cretaceous–Paleogene (K–Pg) mass extinction. Although periptychids are mostly known from fragmentary gnathic remains, the Corral Bluffs area within the Denver Basin, Colorado, has yielded numerous exceptionally well-preserved mammalian fossils, including periptychids, from the earliest Paleocene. Here we describe a partial cranium and articulated dentaries plus an additional unassociated dentary fragment of a small-bodied (~273–455 g) periptychid from ca. 610 thousand years after the K–Pg mass extinction (Puercan 2 North American Land Mammal ‘age’) at Corral Bluffs. Based on these new fossils we erect Militocodon lydae gen. et sp. nov. The dentition of M. lydae exhibits synapomorphies that diagnose the Conacodontinae, but it is plesiomorphic relative to Oxyacodon, resembling putatively basal periptychids like Mimatuta and Maiorana in several dental traits. As such, we interpret M. lydae as a basal conacodontine. Its skull anatomy does not reveal clear periptychid synapomorphies and instead resembles that of arctocyonids and other primitive eutherians. M. lydae falls along a dental morphocline from basal periptychids to derived conacodontines, which we hypothesize reflects a progressive, novel modification of the hypocone to enhance orthal shearing and crushing rather than grinding mastication. The discovery and thorough descriptions and comparisons of the partial M. lydae skull represent an important step toward unraveling the complex evolutionary history of periptychid mammals. 

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5. Size variation in populatinos of Tetraclaenodon (Mammalia, 'Condylarthra'), from the Torrejonian NALMA of the San Juan Basin, New Mexico, reveals new insights into their evolution and paleoenvironment

   Holpin, S.; Williamson, T. E.; Shelley, S. L.; Brusatte, S. L. (October 2019, Society of Vertebrate Paleontology, Abstracts and Programs)

   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) 

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