Lambdotherium

Lambdotherium was small compared to later perissodactyls, and was lightly built and likely adapted for fast movement.^[3]

The skull of Lambdotherium is dolichocephalic (elongated).^[3] The skull bears an enlarged sagittal crest, which meets with a sizable lambdoidal crest towards the back of the skull. The occiput in itself is tall and narrow. The shape of the rostrum is tubular, which widens greatly past the second premolar.^[4] The orbit rests above a gap in the first and second molars, and there is a small infraorbital foramen (hole below the eye socket) in the maxilla above the third premolar.^[4] The maxillary bone prevents the premaxillary bone from meeting with the nasal bone.^[4] The jaws are generally slender and elongated.^[8] In terms of proportions, the skull has been compared to Eohippus.^[3]

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.^[8][6] The molars of Lambdotherium are all similar in shape and undifferentiated, with all of them having a V-shaped crest on the tooth.^[3] The crowns on the molars are low.^[6] The first lower premolar, unlike the upper premolar, is not present. The lower fourth premolar is shaped similarly to the molars, which are lophiodont.^[8] The enamel of the last molar is generally smooth and there is no cingulum.^[3]

Lambdotherium is lightly built, and was likely cursorial in life, as the feet and limbs are adapted for running.^[3] The radius is 90% the size of the humerus, with the radius measuring about 103 mm (4.1 in). 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.^[3]

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.^[3][9]

Lambdotherium lived alongside a multitude of other animals, including the primitive horse Eohippus, the oxyaenid Oxyaena, the phenacodont Phenacodus and the pantodont Coryphodon.^[3] Lambotherium also coexisted with omomyid primates. The environments Lambdotherium lived in were likely open habitats, interspersed with forests along water bodies.^[9] The flora of the region is speculated to have been similar to the flora of the Woolwich Formation in England, which preserves figs (Ficus), the locust tree (Robinia), the tulip tree (Liriodendron), and Grevillea.^[3] Direct fossils of plants from the Wasatch formation confirm the presence of Metasequoia and Averrhoites affinis. The environments Lambdotherium lived in were mostly tropical and subtropical in climate, which, in the case of the Wasatch formation, allowed Paleocene plant communities to persist into the Eocene.^[10]