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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.