giant sloth. Teeth found in the same de- 

 posits were assigned to the Perissodactyla, 

 the order that includes horses, tapirs, and 

 rhinoceroses. Not until 1890 did the 

 French paleontologist Henri Filhol realize 

 that the horselike animals never seemed to 

 have feet nor the slothlike animals a head. 

 He concluded that the claws and teeth be- 

 longed to a single beast. Instead of having 

 toes like their horse and rhino relatives, 

 chalicotheres had hooklike claws. 



I first became interested in chali- 

 cotheres as a graduate student some 

 twenty years ago, and my work involved 

 not only their morphology but also their 

 habits and natural history. Most early 

 speculations about how chalicotheres used 



their claws came down on the side of dig- 

 ging; chalicotheres were envisioned claw- 

 ing through earth in search of water or ed- 

 ible roots. As more fossils came to light, 

 thought shifted, and chalicotheres were 

 seen as browsers of leaves. I spent a lot of 

 time comparing chalicothere skeletons 

 with those of possibly analogous diggers 

 and browsers, both living and fossil. Dig- 

 gers generally have strong forelimb mus- 

 cles and short, forceful forearms and 

 hands, enabling the animal to move earth 

 easily. Chalicotheres have long forelimbs 

 that are not particularly muscular. They 

 also lack vertebral, pelvic, and hind limb 

 modifications usually found in habitual 

 diggers, and their teeth are relatively low 

 crowned, with no signs of the heavy wear 

 they would sustain if chalicotheres had 

 chewed on a diet of coarse, gritty roots. In- 

 stead, the teeth are like those of animals 

 that feed on leaves and twigs. I concluded 

 that chalicotheres were not diggers, or at 

 least that digging was not the major func- 

 tion of their claws, and that they browsed 

 rather than grazed or grubbed for a hving. 

 Two basic designs of chalicotheres ex- 

 isted. One, exemplified by the Old World 

 genus Chalicotherium, had gorillalike 

 proportions and may have engaged in 

 something like knuckle walking. The 

 other, exemplified by Moropus, which in- 

 habited North America in the Miocene, 

 some twenty-four to eight million years 

 ago, had a longish neck and was shaped 

 rather like an okapi (a giraffe relative that 

 today lives in African rain forests). Moro- 

 pus could extend and lift its claws clear of 

 the ground to keep from blunting them 

 when it walked. The hind limbs of both 

 creatures were shorter than the forelimbs 

 and had weight-bearing adaptations not 

 found in the forelimbs. Both groups of 

 chaUcotheres could probably stand up on 

 their hind limbs as they browsed. The 

 clawed digits on the hands may have func- 

 tioned like hooks, helping the animal to 

 support itself against tree trunks or to pull 

 branches down to mouth level. Possibly 

 the claws served as occasional weapons 

 for defense or, in the case of breeding 

 males, for intimidation of competitors. 



Chalicothere fossils are relatively rare, 

 and the animals were probably never par- 

 ticularly numerous. The evidence suggests 

 that Chalicotherium may have lived in 

 forests, while Moropus inhabited more 

 open environments, perhaps tree-lined 

 areas around streams or water holes. Large 

 concentrations of chalicothere fossils are 

 found in only three places worldwide — in 

 the Czech Republic, in Kazakhstan, and in 

 northwest Nebraska. In the 1920s, the 

 American Museum of Natural History 

 collected eighteen skeletons of Moropus 

 from what is now Agate National Monu- 

 ment in Nebraska. In the 1970s, I exca- 

 vated fossils from a nearby quarry in 

 which more than 50 percent of the total 

 specimens belonged to Moropus. This 

 creature was the largest animal living in 

 the early Miocene assemblage in the 

 Agate area. It shared its habitat with 

 medium-sized and small rhinos, three- 

 toed horses, and large piglike beasts 

 known as entelodonts. Preying on all of 

 these were "bear-dogs" and early canids. 

 Small camels and a sheeplike group called 

 oreodonts roamed nearby. 



The Agate fossils shed some new light 

 on chalicothere life style. When I exam- 

 ined the specimens of Moropus at the 

 American Museum, I found two size 

 groups: the larger ones probably repre- 

 senting males; the smaller, females. Such 

 sexual dimorphism, which is also found in 

 chalicotheres from Eurasia and Africa, is 

 common when animals breed in a group. 

 The bony dome on the skull of another 

 North American chalicothere, known as 

 Tylocephalonyx, may have been used in 

 low impact butting, a behavior common 

 today in male giraffes and many other 

 hoofed animals when they compete for fe- 

 males. Thus, chalicotheres may have gath- 

 ered in at least seasonal groupings. 



Reconstructing the lives of chali- 

 cotheres expands our knowledge of mam- 

 mal evolution and of the variety of ecosys- 

 tems during the Age of Mammals. The 

 existence of a clawed, herbivorous chali- 

 cothere, for which there are now no exact 

 biological equivalents, opens a window on 

 a world that is not quite like our own. 



The chalicothere Moropus easily fends off two snarling 



Dapheonodon bear-dogs by simply raising its long front limb 



equipped with six-inch claws. Its mate grooms their offspring 



nearby. Other fauna of this North American Miocene 



environment are camels, three-toed horses, sheeplike 



oreodonts (far left), and piglike entelodonts (upper right). 



Detail of painting by Jay lulatternes; courtesy of the Smithsonian Institution 



71 



