The oreodonts: the tylopods’ successful venture

Although their family is properly known by the horrid mouthful Merycoidodontidae, the term oreodont can still be used to refer to them. Contrary to popular belief, oreodonts were not fond of Oreos; their name means “mountain tooth”, in reference to the location where they were found*. They were also among the most successful of North American Tertiary ungulates, and in some areas their bones are so common that they are used as index fossils.

*Which, of course, leaves open the question: why were the cookies called Oreos?

While oreodonts are seen as rather pig-like (hence the predominance of hyus and choerus suffixes to their names), they are generally classified as tylopods, related to the camelids and the protoceratids. The oreodonts proper belong to the Merycoidodontidae, and are united with the more basal Agriochoeridae in the Oreodontoidea, and oreodontoids as a whole are also referred to as oreodonts (Lander, 1998). The skull of oreodonts is quite derived as compared to their pig-like postcranial skeleton. Their diagnostic characters include a complex-crested P1-3, selenodont molars, a lacrimal fossa, a postorbital bar, and an absent pollex (Lander, 1998). Other odd features include clawed, four-toed feet and sharp canines, possibly for intraspecific display, combat, and defense.

Picture of Merycoidodon by Heinrich Harder, from earlyimage.

As with many speciose groups, the phylogeny of oreodonts is a tangled mess, with many species that may be obliterated by synonymy. The affinities of several genera remain unknown. Schultz and Falkenbach (1968) included some 50 genera, but more recent revisions (e.g. Lander, 1998) greatly reduced this number to around 20 genera, in a varying number of subfamilies.

Regardless of their classification, oreodonts were certainly extremely successful animals. As mentioned before, the sheer quantity of fossils discovered suggests the presence of vast herds roaming the landscape. Their reign spanned most of the Tertiary, starting with the late Eocene all the way to the start of the Pliocene. Oreodonts were endemic to North America, evolving and going extinct there. The Oreodon Beds of the White River Badlands are named for the abundance of oreodont fossils.

Picture shows the skull of the oreodont Merychyus, from Stevens (1970).

Judging by the large herds known to have existed, oreodonts are also believed to have been social animals. Their large canines and wide gape are both similar to the arrangements in hyraxes, hippos, and pigs, and are indicatory of animals that gape for display (Greaves, 1972; Janis, 1983). In addition, some structures, such as skull tubercles and the disputed* nasal bosses of Leptauchenia, are possibly sexually dimorphic, and may have been used for display. Stevens (1970), for instance, compares them to the warts on the extant warthog, but does not elaborate further on their function.

*They may have been caused by crushing of the skull during fossilization, as with the supposed nose horn of Ornitholestes.

The reconstruction depicts Merycoidodon culbertsoni, the best-known and one of the most common oreodonts. Pic by myself.

Oreodonts successfully occupied a wide range of niches as well. Merycoidodon and Eporeodon were classic grazers. The barrel-bodied, short-legged Promerycochoerus and Merycochoerus were amphibious. Leptauchenia, long assumed to be amphibious due to its high eyes and nostrils, was recently placed in deserts (Prothero, 2005; Mihlbachler and Solounias, 2006); Sespia also probably roamed arid lands. Brachycrus, with its tapir-like snout, likely led a similar lifestyle in forested areas. The impressive diversification of the oreodonts was part of a great radiation of Miocene ungulates, which suggests that the ecosystems at the time were far more productive than those today (Janis et al., 2000). Most species were medium to small in size (indeed, many cases of dwarf oreodonts are known). The largest species was the cow-sized Eporeodon.

Image shows the browser Merychyus (top), the amphibious Promerycochoerus (bottom left), and the tapir-analogue Brachycrus (bottom right), from Wikipedia.

Most oreodonts had relatively short legs compared to their body. Given that they also had claws, it has been suggested that many species were burrowers. At least some of the smaller species have since then been confirmed as burrowers, with the discovery of a Miniochoerus burrow with the oreodonts inside it.

For all their success, however, oreodonts were among those ungulates that did not make it through the Pliocene. With their passing, they left us neither descendants nor proper analogues for their behavior.


References

Greaves, W. S. (1972) Evolution of the Merycoidodont Masticatory Apparatus (Mammalia, Artiodactyla). Evolution, Vol. 26, No. 4,pp. 659-667

Janis, C. M. (1983) Muscles of the masticatory apparatus in two genera of hyraces (Procavia and Heterohyrax). Journal of Morphology, 176, 61-87

Janis, C. M.; Damuth, J.; and Theodor, J. M. (2000) Miocene ungulates and terrestrial primary productivity: Where have all the browsers gone? PNAS, Vol. 97, no. 14

Lander, B. (1998) "Oreodontoidea", in Janis, C.M.; Scott, K.M.; and Jacobs, L.L. Evolution of Tertiary mammals of North America. Cambridge University Press, 402–425

Mihlbachler, M. C. and Solounias, N. (2006) Coevolution of Tooth Crown Height and Diet in Oreodonts (Merycoidodontidae, Artiodactyla) Examined with Phylogenetically Independent Contrasts. Journal of Mammalian Evolution, Vol. 13, No. 1

Prothero, D. R. and Sanchez, F. (2005) Review of the leptauchenine oreodonts (Mammalia: Artiodactyla). New Mexico Museum of Natural History and Science Bulletin.

Schultz, C. B. and Falkenbach, C. H. (1968) The phylogeny of the oreodonts, parts 1 and 2. Bulletin of the American Museum of Natural History, 109, 373-482

Stevens, M. S. (1970) Merychyus verrucomalus, a New Species of Oreodont (Mammalia, Artiodactyla) from the Middle Miocene Runningwater Formation. American Museum Novitates, No. 2425