Lambdotherium

Lambdotherium is a perissodactyl mammal, grouped closest with the Palaeotheriidae. When originally discovered, it was assumed Lambdotherium was the first representative of the Brontotheriidae. This hypothesis was upheld by many researchers, but was disputed by Steven McKinney Wallace, who proposed Lambdotherium was not as close to the brontotheres as originally assumed, in 1980. This placement was supported by phylogenetic analysis done by Lucas et.al in 2004. He had also later classed Lambdotherium as a chalicothere. Henry Fairfield Osborn had attempted to move Lambdotherium out of the brontotheres and closer to Equidae in 1897, but this proposal was shut down by William K. Gregory. The name Lambdotherium is derived from the shape of the crests of the lower molars, while popoagicum is derived from the Popo Agie River, which is a tributary of the Wind River, close to where the fossils were found. == Description ==

Lambdotherium was small compared to later perissodactyls, and was lightly built and likely adapted for fast movement. In terms of proportions, the skull has been compared to Eohippus. AMNH 4863) Dentition Lambdotherium has chisel shaped incisors, which are shaped partly like a spatula and are separated from the canines by a diastema (gap).The incisors are arranged in a parabolic shape. Historically it had been assumed the first premolar wasn't present, though this idea has fallen out of favor with the discovery of the first premolar in some specimens, which is sharp, laterally compressed, and double rooted. The molars of Lambdotherium are all similar in shape and undifferentiated, with all of them having a V-shaped crest on the tooth. The crowns on the molars are low. The first lower premolar, unlike the upper premolar, is not present. The lower fourth premolar is shaped similarly to the molars, which are lophiodont. The enamel of the last molar is generally smooth and there is no cingulum. Appendicular skeleton Lambdotherium is lightly built, and was likely cursorial in life, as the feet and limbs are adapted for running. The radius is 90% the size of the humerus, with the radius measuring about . The scapula has a long neck, and the distal end of the humerus connects with the flattened articular surface of the ulna and radius.The olecranon process of the ulna is erect and shortened. There are 4 digits on the feet, with the median digit being the largest. The atlas vertebra assumes the typical perissodactyl form. == Paleoenvironment ==

, with the presence of Lambdotherium serving as an indicator of a biostratigraphic zone. Distribution Found from the lower Eocene, Lambdotherium is known from many western North American fossil-bearing formations. Typically fossils are found in formations that are part of the Lost Cabin locality. Fossils of Lambdotherium have been found in the Wind River Basin, the Bighorn Basin, the Beaver Divide and Huerfano Park, as well as the Wasatch Formation. Paleoecology Lambdotherium lived alongside a multitude of other animals, including the primitive horse Eohippus, the oxyaenid Oxyaena, the phenacodont Phenacodus and the pantodont Coryphodon. == References ==