Poebrotherium wilsoni 1848
In 1848, Joseph Leidy received a package from Samuel D. Culbertson containing fossils collected by a relative (Alexander Culbertson) in the White River Badlands of South Dakota. The fossils contained the skull pictured below, as well as fragments of the right front leg adjacent to the elbow. These fossils were the first mammalian fossils sent to the Leidy from any of the great fossil beds of the American West.
Side view of the skull of Poebrotherium wilsoni. This image is a detail from a lithograph published in Ancient Fauna of Nebraska (1853). Click here or on the image to view the full lithograph.
At firstPoebrotherium presented a puzzle for Leidy. He knew it was an artiodactyl (2), but he was unable to determine its family by the time he published his Ancient Fauna of Nebraska in 1853. The skull had a mix of features. Most of these features indicated that it was a primative deer, but some were more similar to those of camels or even peccaries.
Despite the fact that Poebrotherium was the first fossil received from the White River Badlands, only a few teeth and jaw fragments became available during the next two decades. Nonetheless, Leidy had determined that it was indeed an early camel by the time he published Extinct Mammaliam Fauna of Dakota and Nebraska in 1869 (3).
Comparision of the skulls of Poebrotherium (left) and the modern llama, Lama glama, (right).The shaded area of the Poebrotherium skull represents the part of the skull present in the fossil examined by Leidy and shown earlier in this page.
Part of Leidy's difficulty in identifying Poebrotherium as a camel resulted from the limited fossil material at his disposal. (It would be several decades before scientists could study more complete specimens.) But part of the difficulty was because Poebrotherium is one of the oldest and most primitive camels known. Modern camels have evolved a number of dental adaptations for plant feeding. One adaptation is the specializations of the front teeth for plucking plants and of the cheek teeth for grinding them. Another is the space separating these two types of teeth. The comparision of the skulls of Poebrotherium and the modern llama shown in the picture above demonstrates the extent of this specialization in the modern species.
Another important aspect of camel evolution is specialization of their limbs. Like many modern ungulates, camels have legs specialized for efficient locomotion in open country. These specializations include the enlogation of the lower legs and feet as well as the reduction in the number of toes. However, camels started evolving these specializations millions of years earlier than did other ungulates. Camels have a unique type of foot, with pair of splayed toes and a pad at the back to supports the camel's weight. Poebrotherium had the splayed pair of toes, but hadn't yet evolved the pads.
Modern camels are represented by six species (including 3 totally domesticated species), and are found in Asia, North Africa and the Andes of South America. But most of the camel's evolutionary history was restricted to North America. It wasn't until the Pliocene that camels expanded into Eurasia and gave rise to the Bactrian (2-humped camel) and the Dromedary (1-humped camel). Llama-like camels, which had originated in North America, entered South America during the Pliocene. Camels survived in North America until the end of the Pleistocene (Ice Age).
Notes:
- Etymology:
Poebrotherium (POE-broe-THEER-ee-um) = grass-eating beast
wilsoni (WILL-son-eye) = named in honor of Dr. Thomas R. Wilson
[go back] - Artiodactyls,
or even-toed ungulates, are the most diverse and abundant large mammals living
today. There are more than 180 living species and include pigs, peccaries, hippopotamuses,
camels, giraffes, deer, antelope, goats, sheep and cows. The name (even-toed ungulates)
refers to the even number (2 or 4) of weight-bearing toes; the weight-bearing
axis of the limb passes between the third and fourth toes.
[go back] - Leidy
had named other extinct American camels by the time he wrote his 1869 monograph.
These include Camelops kansanus in 1855 and Procamelus gracilis in 1858.
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