Moropus (meaning "slow foot") is an extinct genus of large perissodactyl mammal in the chalicothere family. They were endemic to North America during the Miocene from ~20.4–13.6 Mya, existing for approximately . Moropus belonged to the schizotheriine subfamily of chalicotheres, and has the best fossil record of any member of this group; numbers of individuals, including complete skeletons, have been found. The type species of Moropus, M. distans, was named by Othniel Charles Marsh in 1877, alongside two other species, M. elatus and M. senex. Three more species have been named since. Others have been named, but have either been invalidated for one reason or another, or reclassified to other genera.
Among the largest chalicotheres, some specimens of Moropus stood at the shoulder. One specimen had an estimated body mass of . Its dental anatomy was similar to ruminants, suggesting a similar method of cropping vegetation. Retracted nasal bones suggest a long upper lip, and a wide gap between the lower incisors and cheek teeth, called a diastema, would provide room for a long tongue to extend from the mouth at an angle. Together, the upper lip and tongue may have been used to pull down vegetation. Though not as adept at bipedalism as the related Chalicotherium, it may nonetheless have reared up on two legs to browse on vegetation, using its claws to hook into the bark of a tree or using them to pull down leaves that would otherwise have been unreachable. Moropus may have been sexually dimorphic, with the males being larger than the females.
Taxonomy
Early history
The first of the remains currently assigned to Moropus was a partial right maxilla (YPM 10030), uncovered at some point prior to 1873. In that year, the specimen was described YPM 10030, initially mistakenly attributed to Lophiodon. After its discovery, multiple more complete specimens were discovered in the Miocene strata of the John Day Fossil Beds of Oregon. In 1877, Othniel Charles Marsh formally described the specimens, assigning to them the genus name Moropus. The type species of Moropus, M. distans, was based only on fragments of the hind foot. Two other species, M. elatus and M. senex, were also described. At first, Marsh believed that Moropus belonged to the order Edentata, which historically included any mammal that lacked incisor teeth. Though he noted affinities with the African Ancylotherium, he opted to erect a new family, Moropodidae, to exclusively include Moropus.
In 1913, Olof August Peterson named a new species of Moropus, M. hollandi, from limb elements recovered in 1901, at first mistakenly assigned to M. elatus. In a 1913 monograph on chalicothere taxonomy, Moropus in particular, Peterson and William Jacob Holland recognised two additional species, M. matthewi and M. merriami, and reassigned Moropus to Chalicotheriidae. He assigned the specimen to Moropus, and named a new species, M. cooki (after Harold Cook, who discovered it) based on it. However, Peterson and Holland considered M. cooki a junior synonym of M. elatus. Later, in 1975, Margery Chalifoux Coombs suggested that M. petersoni was instead the same taxon as M. elatus, and that its differences could be explained through sexual dimorphism . In 1935, Soviet palaeontologist K.K. Flerov named an Asian species of Moropus, "M." betpakdalensis from Kazakhstan. This taxon has since been reassigned to a genus of its own, Borissiakia. Another purported Asian Moropus, "M." huangheensis, has also been reassigned to Borissiakia.
Taxonomy
Chalicotheres are part of the order Perissodactyla, which includes modern equines, rhinoceroses, and tapirs, as well as extinct groups like brontotheres. As the early evolution of perissodactyls is still unresolved, their closest relatives among other perissodactyl groups is obscure. A 2004 cladistic study alternatively recovered Ancylopoda as sister to all modern perissodactyls (which includes Equoidea and Ceratomorpha), with the brontotheres basal to both.
In their 1914 monograph on chalicotheres, Holland and Peterson listed three subfamilies: Moropodinae (Ancylotherium, Moropus, and Nestoritherium), Macrotheriinae (including Chalicotherium, Circotherium, and Macrotherium) and Schizotheriinae (Pernatherium and Schizotherium). William Diller Matthew instead split chalicotheres into just two subfamilies, Chalicotheriinae and Eomoropinae. The former was divided into two clades based on whether their teeth were brachydont (short-crowned) or hypsodont (high-crowned): Moropus fell into the latter category. In 1935, Edwin H. Colbert retained this system, though divided Chalicotheriinae into the tribes Chalicotheriini and Schizotheriini. Moropus is currently classified under Schizotheriinae.
Description
left|thumb|Size comparison between M.elatus and a human
Some species of Moropus, such as M. elatus, were among the largest chalicotheres, Smaller specimens have been described as being about the size of a tapir.
Skull
Moropus<nowiki/>' skull was fairly small compared to its body. It was narrow, and bore high nasal bones. The snout had a spoon-shaped tip, a characteristic common to selective browsers. It suggests the presence of mobile lips and possibly a long tongue. The lower incisors protruded forwards, and the premaxilla is toothless, similar to in modern ruminants. This would have formed a cropping mechanism for processing vegetation. There was a diastema (gap) separating the incisors from the cheek teeth, which would have allowed the tongue to extrude from the mouth. The cheek teeth (the premolars and molars) were robust, covered in thick enamel, and strongly rooted. The first upper premolar is absent, like other chalicotheres. The second upper premolar was triangular, with the protocone and tritocone (cusps) having fused into a single structure, mostly comprising the former. The third upper premolar is more quadrate in shape, and has one tubercle rather than two. The fourth upper premolar is slightly larger but otherwise very similar. In 1943, Swiss palaeontologist Samuel Schaub suggested that the related Ancylotherium used its forelimbs to pull down vegetation, much as in chalicotheriines.