Pandemonium of Pantodonts: the anatomy of Pantolambda with notes on body size and behaviour

Abstract

Pantodonts, a group of herbivorous Paleogene mammals traditionally united by the ‘double-v’ cusp and crest arrangement of their upper premolars, have a long history of study with the first fossils reported in the mid-1800s. Despite the existence of nearly complete skeletons known from multiple genera, little work has been done on this group outside of description with some speculation about their paleobiology. This narrow focus of previous work arises in part because the taxonomy and systematics of the group (particularly that of the large-bodied Coryphodon) has historically been quite difficult to resolve. A provisional phylogenetic definition for Pantodonta is proposed here. The best way to tackle this issue is targeted redescriptions of both historical and new specimens with an eye towards constructing a phylogenetic hypothesis for the clade.

To this end, I present descriptions of new specimens of Pantolambda, from the San Juan Basin, New Mexico, USA. In particular, the specimens NMMNH P-27844, a juvenile of P. bathmodon, preserving its deciduous dentition, and NMMNH P-57738, a partial skeleton of P. cavirictum preserving a nearly complete forelimb, provide an expanded understanding of the morphology of these animals. The deciduous premolars of Pantolambda bear some interesting similarities to the permanent dentition of the putative pantodonts, the cyriacotheriids, including the orientation of the preparacrista, the texture of the enamel, and the location of the metaconules. Additionally, a partial braincase and scapula of P. intermedium are described and a case is made for its validity as a species.

Body size is a foundational characteristic of living organisms that correlates with a variety of physiological, life history, and population-level features. In pantodonts, it is particularly interesting because of their rapid achievement of large body sizes after the Cretaceous-Paleogene mass extinction. A variety of existing body size proxies (stylopodial circumference, dental measurements, and occipital condyle widths) were evaluated for pantodonts. The results of this analysis evidenced that there is a disconnect between long bone and dental proxies within the pantolambdids and barylambdids, but that the discrepancy is less within the coryphodontids. This mismatch is hypothesized to arise from the exaggerated small skull to body proportions in the former two groups.

The behaviour and community structure of Pantolambda bathmodon was further evaluated by examining NMMNH L-1079, a collecting pan below a pantodont bone bed. The minimum number of individuals (MNI) of the quarry on the basis of the calcanea was four. Further insight into the assemblage arose from palaeohistological sampling which increased the MNI of the pantodont individuals to six and allowed us to understand aspects of the demographics of the pantodont group. The ages of the group ranged from 2 to 5 years old with one lower molar root potentially coming from an 11 year old individual. This spread in ages and the relatively low amount of fluvial transport of the bones hints that Pantolambda bathmodon engaged in grouping behaviour and was likely gregarious. This sort of gregarious behaviour has been observed in other pantodonts like Alcidedorbignya and Corpyhodon and thus, occurs across the entire size range of the clade.

The palaeohistological sections used in the taphonomy study also give insight into the life history of Pantolambda. It gave birth live well-developed young and experienced a rapid pace of life. The short generations entailed by this pace of life can be speculated to have contributed to the rapid rise of pantodonts to large body size in the post-extinction landscape of the Paleocene.