Revisiting a Fossil Friend

Some fossils feel like old friends. As happy as I am to encounter petrified skeletons I’ve never seen before in museum halls, there’s something comforting about rounding a corner to find a cast of AMNH 5027 – the first Tyrannosaurus I ever met – or see Stenomylus, a little North American camel that I’ve encountered in exhibits from New Haven to Los Angeles.

The ancient rhino Teleoceras recently joined the club, too. Paleontologists have exhumed so many of these squat, tubby rhinos that almost every major museum in America seems to have one. And, because of its stubby-legged, barrel-shaped body, the 17-4.5 million year old herbivore has often been cast as “the rhino that lived like hippo”. A life ambling between lake and shore seemed to be the only sort that made sense for such an oddly-proportioned mammal, and I repeated the traditional story when I tweeted a photo of the beast during a visit to Harvard’s Museum of Natural History.

But looks can be deceiving. Paleontologist Bobby Boessenecker – expert on aquatic and semiaquatic fossil mammals – jumped in on Facebook to point out a 1998 study that cast Teleoceras in a different way. (This is the nice thing about being friends with paleontologists – they’ll catch your errors and will immediately proffer citations to get you back on track.) According to the Paleobiology paper by Bruce MacFadden, at least some of the rotund rhinos stayed high and dry.

Different species of Teleoceras used to range across the span of North America, from Oregon to Florida. In some places – like the 10 million year old Ashfall Beds of Nebraska – they’ve been found by the dozens. MacFadden chose to focus on specimens found in the 9.5 to 4.5 million year old rock of the sunshine state, as well as those of its neighbor Aphelops – a rhino with longer legs that has often been reconstructed as a relatively swift species that browsed for soft plants in open habitats. And to investigate this “tale of two rhinos”, MacFadden turned to geochemical traces locked inside the rhinos’ teeth.

You are what you eat. That’s the basis for a branch of fossil research that focuses on geochemical isotopes. In short, different pathways in plant photosynthesis leave distinct signatures of the isotope 13C. That trace ends up becoming incorporated in teeth as they grow, and, by comparing these signatures in fossil teeth to those of living animals with known diets, paleontologists can narrow down what prehistoric mammals were munching on. Similar logic applies to other geochemical indicators. The water prehistoric animals imbibed, for example, left geochemical signposts in the form of 18O, and this isotope can be used to determine whether an animal in question had a more or less aquatic lifestyle.

By analyzing these carbon and oxygen isotopes in the fossil rhinos, MacFadden was able to check what had often been assumed on the basis of skeletal shapes. What he found didn’t fit the traditional Teleoceras tale.

In the oldest part of the sample, in teeth 9.5 and 7 million years old, MacFadden didn’t find much difference in diet between Teleoceras and the leggier Aphelops. They had similar carbon isotope signatures, indicating a diet of what are called C3 plants that included trees, forbs, shrubs, and some grasses.

By 4.5 million years ago, though, the rhinos were definitely picking off different parts of the menu. Aphelops appears to have been a browser, nibbling on soft plants, while Teleoceras was a “mixed feeder” with a large proportion of C4 grasses – which had spread into the region by that time – in its diet. This seemed to be in accord with evidence from elsewhere. At Nebraska’s Ashfall site, for example, plant material found around the throats and stomachs of Teleoceras hinted that the rhino snarfed down bushels of grass.

From diet, Teleoceras seemed more at home out in the grasslands than in the water – an idea mooted decades before by paleontologist William Diller Matthew. And the oxygen isotope signatures seemed to cinch it. Teleoceras didn’t have an oxygen isotope profile like that of a hippo, but instead seemed to match mammals that spend most of their time in the terrestrial realm. “While the δ18O results cannot rule out the possibility that Florida Teleoceras spent a small percentage of its daily life cycle in water,” MacFadden wrote, “the data do not support the previous hypothesis that this species was principally aquatic.”

In place of the old model, MacFadden proposed that Florida’s ancient rhinos avoided competition for the salad bar by eating different plants – Aphelops being more of a browser, like today’s black rhino, and Teleoceras pioneering the grazing lifestyle embodied by the modern white rhino. The low-slung appearance of Teleoceras wasn’t because it lived like a hippo. Stumpy legs were an old, common trait amongst the rhinoceros lineage it belonged to, and it didn’t have to be particularly tall if – by MacFadden’s estimation – 60% of the herbivore’s diet was grass. Teleoceras was not so much a hippoceros as a living lawnmower.

Reference:

MacFadden, B. 1998. Tale of two rhinos: isotopic ecology, paleodiet, and niche differentiation of Aphelops and Teleoceras from the Florida NeogeneTale of two rhinos: isotopic ecology, paleodiet, and niche differentiation of Aphelops and Teleoceras from the Florida NeogeneTale of two rhinos: isotopic ecology, paleodiet, and niche differentiation of Aphelops and Teleoceras from the Florida NeogeneTale of two rhinos: isotopic ecology, paleodiet, and niche differentiation of Aphelops and Teleoceras from the Florida NeogeneTale of two rhinos: isotopic ecology, paleodiet, and niche differentiation of Aphelops and Teleoceras from the Florida Neogene. Paleobiology. 24 (2): 274-286.

Prothero, D. 2005. The Evolution of North American Rhinoceroses. Cambridge: Cambridge University Press. pp. 94-124