Gorilla horses with claws - Chalicotheres

A selection of the extinct perissodactyls known as chalicotheres, to scale.

Some prehistoric species have modern analogues with a host of common features, even if they are not remotely related. Others present a problem of being related to living animals but not actually resembling any. Chalicotheres are the classic conundrum, having various anatomical similarities in common with horses, giraffes, bears, anteaters and primates. They are actually true ungulates, despite possessing clawed digits instead of hooves, and are a member of the odd-toed perissodactyls. In fact early chalicotheres in Eocene Asia were difficult to tell apart from the forest dwelling ancestral horses and brontotheres, their closest relatives. Some fossils have moved from one group to another due to incorrect placement.

All of the family were browsers, never developing high-crowned teeth for abrasive grasses. As a result they are restricted to woodland and forest habitats, and were never very diverse in species. Two major branches divide the family, based around different locomotion. Schizotherines had the more conventional bodyshape which walked on the soles of foot. The claws on the forelimbs were retractable to keep the tips from wearing, a feature convergent with felids. Strong hindlimbs and an elongated pelvis allowed schizotherines to rear upright for added height. A lack of upper incisors and canines, convergent with bovids, and other adaptations to the skull, is highly suggestive of a protrusible tongue. More bizarre are the chalicotherines. Very long forelimbs contrasting with short hindlimbs gives chalicotherines a distinctive sloped back and upright walk. Like anteaters and ground sloths, they were able to protect long curved claws by walking on the thickened knuckles.

In the past it was thought that the long claws were used to dig roots and tubers from the ground, however the lack of wear disproved this idea. Now it is generally agreed that the claws were similar to those of the extinct ground sloths and used to drag foliage to their mouths. Not all chalicotheres ate the same foods, though. Schizotherines such as the North American Tylocephalonyx and Moropus tended to have low numbers of scratches on teeth (indicating softer leaves) but higher gouging (caused by twigs) with alot of grit thrown in. More fruit was eaten by the European Schizotherium while the chalicotherines Chalicotherium and Anisodon ground their teeth on hard nuts and seeds. Chalicotherines were more forelimb-dominated, pulling branches close towards their mouths. Extra weight-bearing pads on their hips and hindlimbs suggesting that they also spent considerable time sitting vertically while feeding. Those claws inevitably doubled as formidable defensive weapons.

The best known species from good fossil material are the Miocene Moropus and Anisodon. Moropus was one of the largest species reaching up to 5 metres into the canopy when rearing up on to it's hindlimbs. Nestoritherum was one of only two species which survived into the Pleistocene.

[Note: Have had to make a major change to the image. I double-checked references to find that Nestoritherium was actually a knuckle-walking chalicotherine with a short skull, despite the fact that it is regularly quoted as being a schizotherine.]