The curious chalicotheres

Suppose for a moment that you are walking across a dry, wind-swept landscape known to be rich in fossils. During your perambulations you notice a large fossilized claw sitting on the surface; what sort of animal could it be from? There is a lot you would have to know about the area, like how old the rock in that spot was, but it would seem reasonable that the claw belonged to a large predator.

Nature, of course, is not so straightforward. Panda bears have teeth and claws that reveal their carnivoran ancestry and yet they primarily eat plants. Giant sloths and the even more ancient therizinosaurs also had extraordinarily large claws on their hands but were herbivores. We focus on the claws of predators because we know the damage they can do, but a claw by itself tells us little about the animal it once belonged to.

There are few creatures that exemplify this anatomical lesson better than the extinct chalicotheres, relatives of horses and rhinoceros. Indeed, the chalicotheres were incongruous creatures that looked like a cross between a horse and a giant ground sloth, with long necks, a downward sloping back, and feet with tipped with claws instead of hooves. This set them apart from their other perissodactyl relatives, and their bizarre nature made it initially very difficult to determine just what sort of creatures they were.

The genus Chalicotherium was named in 1833, but it was not based on much. The 1851 edition of the Iconographic encyclopaedia of science, literature, and art, for instance, reveals that little more than the teeth of Chalicotherium had been found;

The genus Chalicotherium is known only by its dentition, which indicates an affinity with Anoplotherium. Two species have been found in the meiocene deposits of Germany, whose bulk, it is supposed, reached that of the rhinoceros.

Anoplotherium was an artiodactyl famously described by Cuvier from the gypsum quarries of Paris and it seemed that Chalicotherium was a similar, if larger, herbivore. Fossils of this genus remained frustratingly elusive, or so scholars thought. Elements of its limbs had already been found and attributed to a different animals called "Macrotherium" and "Ancylotherium" which paleontologists thought resembled giant anteaters or sloths. There was no reason to think a creature with teeth like an ungulate would have long arms like a giant sloth, but this is precisely what Charles Forsyth-Major proposed.

As a autobiography of Forsyth-Major's colleague W.B. Scott reveals other paleontologists were incredulous;

When ... Forsyth Major declared his belief that these two apparently unrelated animals [Chalicotherium and Ancylotherium] were one and the same, Weithofer and I scandalized and energetically dissented from such a heresy. However, we were anxious to learn Forsyth Major's reasons for his belief and asked him to explain. He replied substantially as follows: "No one had ever found the feet of Chalicotherium, or the skull, or teeth of Ancylotherium, yet they always occur together in the same beds; where you get one, you get the other." I admitted the force of this, but remained unconvinced.

Some weeks after this conversation, I was in Paris, where the eminent French palaeontologist, M. Filhol, invited me to come to the museum and see the new fossil mammals which he had been excavating at Sansan in the south of France. Among these was a complete skeleton, with the bones in their natural connections, which completely confirmed Forsyth Major's interpretation; the skull and teeth were those of Chalicotherium, while the feet were those called Ancylotherium! In a paper which I wrote shortly after my return home, I was happy to bear witness to Forsyth Major's independent discovery of this more interesting relation, thereby causing him to write me a very grateful letter.

Forsyth Major had been vindicated and Scott considered the story to be especially instructive of one of the hazards of paleontological work. During a lecture on paleontology at Wood's Hole in 1895 Scott said;

Extraordinary blunders have sometimes been committed in this work [of properly associating fragmentary fossil remains]. In the remarkable genus Chalicotherium the skull was at first referred to one mammalian order and the feet to another, and Forsyth-Major's suggestion that they all belonged together was received with incredulity.

Despite the discovery of a more complete skeleton, however, some naturalists were still unsure of just what sort of creature Chalicotherium was. It had some relationship to other hoofed mammals but how could an animal with claws be ranked among those with hooves? The 1904 Annual Report of the Geological Survey of Canada, for instance, had this to say about Chalioctherium;

Chalicotherium represents a distinct order and had a curious assemblage of characters amongst which were notably the possession of clawed feet, and teeth suggestive of the Perissodactyls.

Ernest Ingersoll, in the 1907 edition of the Life of Animals followed Henry Woodward's classification presented in a 1904 address to the Royal Microscopial Society and kept the chalicotheres within the group Ancylopoda. This was an anomalous group that Ingersoll placed in association with a few other groups that had defied stable classification. This largely had to do with the fact that they were unlike any living creatures and this apparently led Ingersoll to cast them as "losers";

None of the foregoing seem to have been successes, so to speak; that is, they were unable to change with the gradually altering conditions of climate and vegetation as time advanced, and were crowded out by the more adaptable progenitors of modern hoofed mammals.

The 1906 textbook Outlines of Zoology likewise remained vague on the systematic position of the group and simply stated that Chalicotherium and its allies were "remarkable type[s]" of perssodactyl. W.B. Scott's A History of Land Mammals in the Western Hemisphere published in 1913 would finally bring the chalicotheres closer to our present understanding. Despite the apparent existence of a complete skeleton in Paris since the late 19th century a large part of the systematic dilemma was that chalicothere remains were often fragmentary. America, in particular, contained little more than bits and pieces and it was difficult to tell whether the North American genera closely resembled those from Europe and Asia.

(I also have to wonder if, for whatever reason, the complete Chalioctherium from Paris was not as widely-reported or well-studied as it might have been. This would explain why so many naturalists seemed tentative or confused about what sort of creature it was.)

Strangely, however, it would be an American chaliocthere that would give naturalists a better look at this group. In 1877 O.C. Marsh described the chalicothere genus Moropus. It, too, was known from petrified bits and pieces, but by 1907 complete enough remains of it were found for O.A. Peterson to confirm that not only was Moropus a chalicothere, but that the chalicotheres represented a kind of "aberrant" type of perissodactyl.

Ten years later a complete Moropus skeleton was found and mounted in the American Museum of Natural History. It was featured in an article of Natural History by W.D. Matthew, and now that the general form of these creatures was known greater attention could be paid to the question of how they were using their claws. Matthew stated;

The only plausible suggestion that has been made is that the great claws were designed to aid the Moropus in scraping away sand in dry riverbeds or other suitable places to make a water hole where lie might drink. There is good reason to believe that the western country where lie lived was even then more or less arid, with a scanty water supply in the summer or seasons of drought. In Central Africa today the animals congregate in great numbers around the scattered water holes, and some of them may dig out the holes more or less with their paws. Our Moropus could do that sort of thing to great advantage, and the powerful claws often might enable him to dig down in a sandy riverbed to water that otherwise would be beyond his reach.

Unfortunately the paleobiology of Moropus and its relatives remains mysterious. They are certainly not the media darlings that dinosaurs are, and like many groups of fossil mammals they are probably only studied by a handful of paleontologists. There are still many unanswered questions about chalicotheres and how they lived, but this provides plenty of opportunity for any intrepid students of paleontology who are looking for a neglected group of organisms to study.