Pass the Twigs, Please – Toothwear Indicates the Variety of Chalicothere Diets

Chalicotheres were just plain weird. With horse-like heads, but bodies which appeared to be equal parts gorilla and giant ground sloth, these herbivorous mammals have stymied paleontologists for well over a century. Did they dig for roots and tubers with their massive claws, or did they use their long arms to bring leafy tree branches into reach? Perhaps they used their formidable forearms to excavate their own waterholes in times of drought, or maybe they literally shredded small trees into easily-digestible pulp. No one has been able to figure out the paleobiology of these animals with any certainty, leaving scientists to frustratedly ask “Fucking chalicotheres; how do they work?

Part of the reason why chalicotheres have been so difficult to figure out is a lack of modern analogs. These animals were perissodactyls – a once more-diverse group of hoofed mammals with an odd number of toes represented by only a handful of horse, tapir, and rhino species today – and there is no living animal which shares the peculiar mish-mash of anatomical features seen in chalicotheres. Nevertheless, the teeth of these beasts may provide some clues as to their habits. Some well-studied chalicotheres – such as Moropus from North America – lacked upper incisors and canines, and at a superficial level their dental arrangement corresponded to that of artiodactyls (even-toed ungulates) which are known to feed on leaves and other soft vegetation. Gross anatomy alone can’t solve this puzzle, however, and so scientists Gina Semprebon, Paul Sise, and Margery Coombs took a closer look at the teeth of chalicotheres in search of clues about their diet.

The basis for the new Journal of Mammalian Evolution study by Semprebon and colleagues is dental microwear – the pattern of pits and scratches which were made on an animal’s teeth shortly before they died. Different tooth wear patterns have been used to investigate the diets of fossil mammals at varying levels of resolution, and microwear, especially, has been used in comparison to data from living mammals to figure out whether prehistoric species were grazers, browsers, or mixed feeders.

In order to piece together what was on the chalicothere menu, the paleontologists closely studied casts made from the teeth of a wide swath of these mammals under stereomicroscopes. The point of this was to detect and record the presence of pits, gouges, and scratches which could be compared to data sets from other animals. The species selected by the scientists ranged from early forms like the ~47 million year old Eomoropus of Asia to the ~9 million year old Ancyclotherium from Europe (although the last of the chalicotheres disappeared only about 800,000 years ago, and it should be noted that species from Africa were not included in this study).

At a broad level, the results of the examinations confirmed what had been expected. Chalicotheres showed no sign of being grazers and their wear patterns most closely corresponded to those of browsers. Still, not all chalicotheres ate the same things. North American forms – such as Moropus and Tylocephalonyx – had a low number of scratches on their teeth but a high number of gouges, indicating a diet of soft leaves mixed with twigs and bark. Tooth wear patterns on these chalicotheres also indicated the presence of significant levels of grit in their food, and, combined with details of the environments in which their bones are found, Moropus and Tylocephalonyx may have been “dirty browsers” that supplemented their soft-food diet with twigs, bark, and dirt-covered plants when the dry season put pressure on them.

Chalicotheres elsewhere relied on different food sources. Some species from prehistoric Europe – such as Schizotherium – ate fruit more frequently than the North American types, while Chalicotherium and Anisodon ate so many seeds, nuts, and hard fruits that they ground their teeth down just like a grazer would (the difference only being apparent by looking at the microscopic wear patterns). All the chalicotheres included in the study turned out to be browsers, but they were browsers of different sorts.

Since Semprebon and her co-authors sampled a range of taxa from Europe, Asia, and North America over a span of about 40 million years, their study also provides an outline of the way chalicothere diet has changed. The group’s evolution began in Asia with forms such as Litolophus which lacked claws and fed on soft fruits. It was only after the groups spread to other continents that groups in different continents began to specialize in different types of browse, meaning that regional differences in food sources were probably very important to chalicothere ecology and evolution.

But what about all those hypotheses concerning the natural history of chalicotheres? Many of those hypotheses were speculative and focused on what the animals could have been doing with their claws, and the truth is that we still don’t entirely know. Some hypotheses – such as the suggestion that chalicotheres primarily dug up roots and tubers – can be set aside since the tooth wear data clearly identifies them as browsers, but figuring what is plausible and what actually occurred is extremely difficult.

The authors of the new paper point out that their findings are consistent with the idea that chalicotheres used their claws to pull down branches and reach succulent browse hanging from tree limbs. I see no reason to doubt that hypothesis. Chalicotheres were clearly browsers and they were certainly capable of reaching high to bring resources down to mouth-level. Nevertheless, what chalicotheres used their claws for and why those claws evolved in the first place are two separate issues. Parsing the origins and early evolution of chalicotheres will rely on the discovery of further fossil evidence, but I am glad that scientists are finding new ways to probe the lifestyles of these strange, extinct mammals.

Top image: The North American chalicothere Moropus, from the American Museum Journal.

References:

Semprebon, G., Sise, P., & Coombs, M. (2010). Potential Bark and Fruit Browsing as Revealed by Stereomicrowear Analysis of the Peculiar Clawed Herbivores Known as Chalicotheres (Perissodactyla, Chalicotherioidea) Journal of Mammalian Evolution DOI: 10.1007/s10914-010-9149-3

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