HIDDEN IN THE TROPICS: NEW TRENDS IN DIVERSITY AND BODY SIZE IN A CLADE OF SOUTH AMERICAN NATIVE UNGULATES (LITOPTERNA)

Litopterns were a group of South American native ungulates that partially comprised the continent’s largely endemic Cenozoic mammal fauna. Recently updated phylogenies of the two major litoptern clades, Proterotheriidae and Macraucheniidae, allow us to examine trends in diversity and body size in these groups in a phylogenetic framework. Our study also incorporates data from four new or recently-described litopterns from Quebrada Honda, Bolivia, which has emerged as an important fossil locality for understanding the middle Miocene and middle latitudes of South America, both of which are historically under-sampled.

Using the paleotree R package, we observe multiple trends in diversity and body size (using m1 length as a proxy) that are not apparent without phylogenetic context. Proterotheriids were more diverse in the Paleogene than their sparse fossil record indicates, and they gradually declined in diversity from the middle Miocene to late Pleistocene, rather than experiencing early and late Miocene diversity peaks as traditionally believed. Macraucheniids did experience early and late Miocene diversity peaks and fared better than proterotheriids in the Pliocene and Pleistocene. For both clades, our analysis shows many intervals in which the number of lineages required by the phylogeny greatly exceeds the number of taxa recorded. Given the traditional focus on high-latitude sites, litopterns’ presumed habits as closed-habitat browsers and mixed feeders, and their great diversity at the middle Miocene site of La Venta, Colombia much of this undiscovered diversity will likely be found at low-latitude tropical sites.

It has long been known that the earliest proterotheriids and macraucheniids were smaller than their later relatives, but our analysis suggests the connection between phylogeny and body size differed in these clades. At least five proterotheriid lineages became larger in parallel during the Paleogene, suggesting strong directional selective pressure, but the group's body size was roughly static throughout the Neogene. Macraucheniids increased in size throughout their history and can be grouped into three phylogenetically-conserved size classes, roughly representing Eocene, Oligocene–Pliocene, and Pleistocene species.