Merycoidodontoidea

Merycoidodontoidea, sometimes called "oreodonts" or "ruminating hogs", is an extinct superfamily of prehistoric cud-chewing artiodactyls with short faces and fang-like canine teeth. As their name implies, some of the better known forms were generally hog-like, and the group has traditionally been placed within the Suina (pigs, peccaries and their ancestors), though some recent work suggests they may have been more closely related to camels.Spaulding, M., O'Leary, M.A. & Gatesy, J. (2009): Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution. '' PLoS ONE'' no 4(9): e7062.
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/ref> "Oreodont" means "mountain teeth", referring to the appearance of the molars. Most oreodonts were sheep-sized, though some genera grew to the size of cattle. They were heavy-bodied, with short four-toed hooves and comparatively long tails.

The animals would have looked rather pig- or sheep-like, but features of their teeth indicate they were more closely related to camelids. They were most likely woodland and grassland browsers, and were widespread in North America during the Oligocene and Miocene. Later forms diversified to suit a range of different habitats. For example, ''Promerycochoerus'' had adaptations suggesting a semiamphibious lifestyle, similar to that of modern hippos.

Taxonomy

The two families of oreodonts are the Merycoidodontidae (originally known as Oreodontidae) which contains all of the advanced species, and the Agriochoeridae, smaller, primitive oreodonts. Together they form the now-extinct suborder Oreodonta. Oreodonts may have been distantly related to pigs, hippopotamuses, and the pig-like peccaries. Indeed, some scholars place Merycoidodontidae within the pig-related suborder Suina (Suiformes). Other scholars place oreodonts closer to camels in the suborder Tylopoda. Still, other experts put the oreodonts together with the short-lived cainotheres in the taxonomic suborder Ancodonta comprising these two groups of extinct ancodonts. All scholars agree, however, that the oreodont was an early form of even-toed ungulate, belonging to the order Artiodactyla. Today, most evidence points towards the oreodonts being tylopods, along with camels, xiphodonts, and protoceratids.

Over 50 genera of Oreodonta have been described in the paleozoological literature. However, oreodonts are widely considered to be taxonomically oversplit, and many of these genera may prove to be synonymous. The last researchers to fully review oreodont taxonomy, C. Bertrand Schultz and Charles H. Falkenbach, have been criticized for erecting excessive numbers of genera, based in part on apparent anatomical differences between different specimens that were actually taphonomic deformations due to postburial forces. Undeformed skulls would be placed in one genus, while skulls crushed from side to side would be placed in a second genus and skulls crushed from front to back would be placed in a third genus. Researchers are beginning to restudy oreodonts and synonymize many genera, but only a few groups have been reviewed.

Natural history

This diverse group of stocky prehistoric mammals grazed amid the grasslands, prairies, or savannas of North and Central America throughout much of the Cenozoic era. First appearing 48 million years ago (Mya) during the warm Eocene epoch of the Paleogene period, the oreodonts dominated the American landscape 34 to 23 Mya during the dry Oligocene epoch, but they mysteriously disappeared 4 Mya during the colder Pliocene