Oreodonts and Trunks

Oreodonts were a group of even-toed, hooved mammals that lived from about 50 million years ago to about 8 million years ago. They are incredibly common, as their heads were very hardy and fossilized very well. I'm studying systematics and phylogenetics of this group, which is very complicated - oreodonts don't have any modern descendents, and though they were related to camels they don't really share much in common with them. Oreodonts were stocky, pig-shaped animals. Some had trunks, some had big cheekbones, and all of them had big fangs. Some were potentially aquatic, and some were maybe even arboreal. They were very diverse, and a lot of what I'm doing is trying to determine how much of that diversity may be artificial - how many species are diagnosed by differences that we should consider differences between individuals, rather than between actual biological species?

At the American Museum of Natural History in front of a few nice oreodont skeletons.

The Evolution of Trunks in Oreodonts

Trunks (or, a vestibular proboscis) are a convergently evolved morphology that have shown up across Mammalia. While traditionally people think of trunks as being for food manipulation like in the elephant, many modern mammals have trunks that are not flexible enough to move food into their mouths and these trunks are used for other purposes. For example, a dik dik antelope uses its trunk for counter current heat exchange, and an elephant seal uses its trunk to increase its volume and its sex appeal. I am particularly interested in understanding how, why, and when organisms evolve their trunks. However, modern organisms with trunks like tapirs, elephants, saiga, and dik-diks, either do not have deep enough fossil records or do not have ancestors without trunks in the fossil record, making it difficult to study the evolution of trunks in these groups.

Fortunately, Oreodonts have evolved trunks at least three times in three different groups, and have many examples of taxa without trunks and with semi-trunked morphologies. They provide an excellent canvas for testing hypotheses about the evolution of trunks, and so myself and my collaborators are currently creating a phylogenetic tree of the superfamily to better understand overall trunk evolution.

Schultz and Falkenbach diagrams of, from top, Merycochoerus, Brachycrus, Ustatochoerus, and Merychyus. Our research indicates that the first three individually evolved trunks but for different adaptive purposes.

Sexual Dimorphism and Bite Marks in Promerycochoerus

Promerycochoerus is a large pig-sized genus of oreodont with unusually wide and dramatic zygomatic arches. I and my colleagues have discovered that these zygomatic arches show considerable signs of sexual dimorphism and are covered in signs of bite mark, periosteal reaction, and even broken and re-healing bones. Myself and my colleagues are currently evaluating whether this group of oreodonts, which is commonly found preserved in groups, may indicate a change from solitary to herd behavior in oreodonts as a group. This project has been temporarily put on hold while I revise the genera involved, and will pick up post systematics work.

To the right is a graphic showing the location of bite marks, bone brusing, bone infection, and other injuries in over 50 different specimens. Background images are from Schultz and Falkenbach.

Systematics of Oreodonts

Oreodonts are abundant and diverse, and have often been described without a great deal of attention to modern variation and its implications for how to diagnose a species in the fossil record. I am currently working on genus-scale systematic revisions of different groups, in particular Promerycochoerus, Eporeodon, and Agriochoerus.

To the right are the casts of 10 different skulls, which have previously been described as 8 different species, all from the same location and time period. I think they all are individual variants of one species.

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