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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Mysteries of the diceratheriine rhinos

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This amazing fossil represents the diceratheriine rhino Subhyracodon occidentalis from the Late Eocene and Early and Middle Oligocene of the USA. Subhyracodon seems to have been ancestral to the better known Diceratherium*, a very long-lived diceratheriine that appeared in the Early Oligocene and persisted into the Middle Miocene. Diceratherium is well known for being one of the first rhinos to exhibit sexual dimorphism in horn and tooth form, so it may have had a complex social life (or, a more complex social life than earlier kinds of rhino, anyway). These were the first big-bodied North American mammals to be ecologically important and abundant following the extinction of the brontotheres. They were also the first rhinos to have horns, though their horns are low, paired bony knobs only present in males, not tall keratinous spikes present in members of both sexes.

* Not to be confused with Diaceratherium, an Oligocene teleoceratine rhino from Europe.

Skeleton of Subhyracodon occidentalis. Image in public domain.

Like so many fossil rhinos, Subhyracodon has a confusing nomenclatural history and often went by the name Caenopus in the older rhino literature. Dental variation previously led to an over-splitting of Subhyracodon; Prothero (1998) argued that these could mostly be regarded as populational variants.

The fossil specimen shown at the very top was discovered in the Brule Formation in Wyoming (this is a cast, photographed at the Oxford University Museum of Natural History, UK). It’s a baby, reportedly not recovered from the body of a mother, but found on its own. Its total length is 76 cm. This might represent a bit of a mystery: how did a baby in a foetal position get outside its mother’s body? Was it dragged out of a corpse by a predator? Or is it not an embryo at all, but a newborn that curled up into a foetal position to sleep? I don’t know and wonder if anybody does. As an adult, Subhyracodon was a big, reaching 4 m in total length.

Rhino phylogeny is vast and sprawling: there are competing phylogenetic hypotheses, of course, but diceratheriines appear to be the sister-group to the major clade that includes aceratheriines, teleoceratines and rhinocerotines (Cerdeño 1995, Prothero 1998). It is an aim of mine that I will one day get to comprehensively discuss rhino diversity. Until then, here’s a highly simplified cladogram to whet your appetite.

Artwork by Ely Kish (Paraceratherium, representing Indricotheriidae) and Zdenek Burian (Hyracodontidae and Amynodontidae).

And, finally...

This graphic shows the number of rhinos killed in South Africa >alone< during 2012.

If you're interested in, or like, rhinos, you're surely aware that 2012 was a horrific year as goes rhino conservation and welfare. An unbelievable 455 African rhinos had been killed by October 2012, surpassing the total for 2011 (448) and 2010 (333). Annual kills were previously on the order of 12-14 animals, so you can appreciate how things have spiraled into total crisis. The demand for horn is apparently being driven by wealthy socialites in Vietnam who believe that spicing drinks with rhino horn powder will increase their sexual performance.

We need to act hard and decisively to stamp this out. I wish there was a way of poisoning exported rhino horn, but that would involve interacting with the black market. What about flooding the market with a fake alternative? What about educating the morons who fuel the trade? Things are so bad that I also wonder whether the best thing might be to capture all surviving wild rhinos and relocate them on another continent (North America or Australia). This would require a huge amount of funding - 'all' you need to do is divert some of the more than 1000 BILLION DOLLARS the world spends annually on military funding (seriously: human insecurity has made the world a screwed-up place). Anyway, things aren't all bad as goes rhino conservation - there have been successes in India and elsewhere. Check out, or consider supporting, bodies like the International Rhino Foundation. Remember also to check out and use the excellent Rhino Resource Center.

For previous Tet Zoo articles on rhinos, see...

And seeing as brontotheres were mentioned here as well, check out...

Refs - -

Cerdeño, E. 1995. Cladistic analysis of the family Rhinocerotidae (Perissodactyla). American Museum Novitates 3143, 1-25.

Prothero, D. 1998. Rhinocerotidae. In Janis, C. M., Scott, K. M. & Jacobs, L. L. (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, pp. 595-605.

The views expressed are those of the author and are not necessarily those of Scientific American.

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