Ancient American Horses

oreodon

In 1848, Joseph Leidy reported on a pair of jaw fragments collected by Alexander Culbertson (2) from the White River Badlands of South Dakota. Leidy noted that the fossil's molars resembled those of ruminants (3) in that they had pairs of crescent-shaped lobes. He named it Merycoidodon culbertsonii.

oreodont teethTop and side views of a lower jaw fragment of Merycoidodon culbertsonii are shown to the right. Top and side views of a single molar tooth are shown to the left. Note the two pairs of crescent-shaped lobes (yellow) in the top view of the single molar. The jaw fragment is a detail from a 1853 lithograph from "Ancient Fauna of the Nebraska Territory". Click here or on the fossil to view the full lithograph.

Soon Leidy received many additional specimens from a number of collectors. The quantity was so great that by the time he published On the Extinct Mammalian Fauna of Dakota and Nebraska in 1869 he guessed that he had seen the fossils of about 500 individuals. In that publication he wrote:

"From the comparative abundance of its remains, we may suppose it to have existed in great numbers, and to have lived together in large herds, which once roamed over the extensive prairies and through the dense forests of ancient Nebraska."

Although he originally named this animal Merycoidodon in reference to its ruminant-like teeth, Leidy preferred the name Oreodon, which he coined in 1853. By that time he seen enough specimens to conclude that it was a true ruminant rather than a ruminant-like mammal. The name was changed back to Merycoidodon in 1902 (4). It and its relatives are still commonly called oreodonts.

oreodon skulls 1853 Plate V 1853 Plate III 1853 Plate II 1869 Plate VI
Three skull fragments (top row and lower left) and a reconstruction (lower right) of Merycoidodon culbertsonii. These images came from four different lithographs published in the "Ancient Fauna of Nebraska" (1853) and On the Extinct Mammalian Fauna of Nebraska and Dakota (1869). Click on each skull to view the corresponding lithographic plate.

Leidy recognized three species of Merycoidodon from the White River Badlands. Merycoidodon culbertsonii accounted for nearly 90% of the specimens he examined. A smaller species, Merycoidodon gracilis, accounted for most of the rest, while a larger species, Merycoidodon major, accounted for only 1%. Leidy also described other oreodonts from the White River Badlands and from elsewhere in the American West.

Merycoidodon culbertsonii was about 4 feet, six inches (1.4 m) long. It had a stout body with relatively short legs, with five toes on the front leg and four on the back. It was a primitive ruminant, but it shared many features with the pig. Leidy referred to Merycoidodon and other oreodonts as "ruminating hogs."

Oreodonts were the most successful ungulates (hoofed mammals) of the North American Oligocene. Thousands of their fossils have been recovered from the White River Badlands and hundreds more have been collected from other western localities. Despite this success, oreodonts were apparently confined to North America.

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Notes:

  1. Etymology:
    Merycoidodon (MARE-ee-COY-doe-don) = ruminant-like tooth
    culbertsonii (CULL-bert-SO-nee-eye) = named in honor of the Culbertson family
    Oreodon (OH-ree-oh-don) = mountain tooth
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  2. Alexander Culbertson worked for the American Fur Company. He had originally sent the Merycoidodon (and Poebrotherium) fossils to his family in Pennsylvania. They were later forwarded to Leidy in Philadelphia.
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  3. Ruminants are a group of even-toed ungulates (Artiodactyls) that include camels, giraffes, deer, antelopes, cows, sheep and goats. (Ungulates are hoofed mammals.) The name ruminant refers to the first chamber (rumen) of their multi-chambered stomach. The rumen contains microorganisms that ferment the plant matter. This in turn increases the amount of nutrients that the ruminant can extract from its food. All ruminants also have the characteristic pairs of crescent-shaped lobes on their molar teeth.
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  4. Biologists have adopted a set of rules for naming organisms. One of them is the law of priority. In other words, the first name used in describing a new species is the official name. Even though Leidy preferred the name Oreodon, Merycoidodon came first.
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