An old reconstruction of Paraceratherium, based on partial skeletons and modern rhinos.
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Art from Granger and Gregory, 1935.
An old reconstruction of Paraceratherium, based on partial skeletons and modern rhinos.

The Last of the Rhinoceros Titans

In paleontology, size matters. The lifestyles of the large and charismatic often gain far more attention those of smaller, equally-strange creatures that thrived alongside the leviathans during prehistory. The most massive dinosaurs, of course, are the recipients of such scale-dependent adoration, and the same is true of the great extinct rhinoceros Paraceratherium (or “Indricotherium“, or “Baluchitherium“, or “Dzungariotherium“, but I’ll get to that in a moment.)

Stretching over 26 feet long, and often said to weigh as much as five elephants, Paraceratherium has traditionally been heralded as the largest mammal ever to tromp over the Earth. The enormous rhino is practically required to make appearances in books, documentaries, and museum displays about fossil mammals. Yet, as paleontologist Donald Prothero demonstrates in his new book Rhinoceros Giants, old misconceptions about Paraceratherium cling to our imagination even as paleontologists are slowly piecing together a more complete picture of the superlative mammal.

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For paleontology aficionados, the majesty of Paraceratherium is self-evident. The rhino was far larger than any alive today, and was an iconic member of mammal faunas that roamed Eurasia between about 35 and 20 million years ago. But Prothero spends no time trying to draw in those who are not already enamored with the titan. The first chapter of the relatively slim book – “Quicksand!” – jumps right into the romantic tales of fossil rhino discoveries in Mongolia during the American Museum of Natural History’s celebrated 1920s expeditions to the region. There is no introduction to what the giant rhinos were, or even why we should care at all that they existed.

For more than the first half of the book, in fact, Paraceratherium only appears as scattered fragments that puzzled and inspired successive generations of paleontologists. Prothero recounts the lives of fossil mammal researchers such as Walter Granger, Henry Guy Ellcock Pilgrim, Clive Forster Cooper, and Zhou Ming-Zhen, among others, in detail before diving into the geological particulars of where Paraceratherium bones are found and where the giant fit in the wider rhino family tree. While a giant rhino without a horn might look odd compared to living species, Prothero points out that Paraceratherium belonged to a major and totally-extinct group of rhinos, and that most fossil rhinos don’t show any evidence of horns at all. Modern rhinos might look prehistoric, but they’re actually quite different from their varied predecessors.

Through copious background details, Prothero celebrates great and lesser-known names in the history of paleontology, as well as geological and taxonomic nitty gritty. The problem is that the audience for all these particulars isn’t clear. In some sections, such as a lengthy review of taxonomic rules, Prothero lays out the basics of how species are named in textbook fashion for those unacquainted with such arcana. Yet an earlier chapter on the geology of Eurasia contains passages that only a trained researcher or especially avid avocational paleontologist would understand (“…the Hsanda Gol Formation consists of about 80 m of redbeds, floodplain mustones, and gray fluvial sands (Figs. 3.7, 3.8) that intertongue with the underlying Ergilin-Dzo where the Houldjin gravels are not present.”)

Frustratingly, Rhinoceros Giants often misses that sweet spot of readily-accessible science prose that Prothero has struck in other books such as Evolution: What the Fossils Say and Why it Matters. This problem is not unique to Rhinoceros Giants, but is rife within academic press books that are meant for popular audiences but are not composed or edited to meet the needs of such an audience. Rhinoceros Giants will primarily appeal to professional paleontologists and avocational fossil fans already familiar with many of the terms and concepts found within.

Yet, for fossil fans who are able to navigate the book, Rhinoceros Giants offers a valuable summary of what researchers presently understand about Paraceratherium. After laying out the historical and conceptual foundations of the book in the first half, Prothero begins to approach Paraceratherium proper. The complicated story of the mammal’s name is first on the agenda.

Paleontologists are generally agreed that Paraceratherium is the single valid name for all the huge fossil rhinos that roamed from eastern Europe through Mongolia during the Oligocene epoch. Yet, as a kid, I remember books and documentaries using names such as “Indricotherium” and “Baluchitherium” for the same animal. Prothero untangles the mess of names, created by incomplete knowledge of early finds and the idiosyncratic process of applying names to extinct animals. Paraceratherium has priority of place as a name, Prothero argues, and all the skeletons of the biggest rhinos so far known fall within the range of variation for this single genus.

With the beast’s rightful name established, Prothero proceeds to reconfigure the giant. Although, compared to other large mammals, Paraceratherium no longer seems quite so titanic. The rhino was tall and had a long, deep neck, but revised weight estimates have moved Paraceratherium away from oft-cited estimates of 30 tons towards a more modest 15-20 ton range. That’s still quite impressive, but comparable to estimates for large mammoths and other big fossil elephants. Paraceratherium just had a bit more vertical reach thanks to a long neck and, Prothero suggests, a prehensile lip. That’s a way of reaching high for food different from the elephantine method of extending a flexible trunk into the canopy.

Those two modes of browsing for food – having a long neck versus an extendable trunk on a head set lower to the ground – might have had something to do with why there are still elephants, but no enormous rhinos. Aside from reviewing the relatively small literature on the natural history of Paraceratherium, currently thought to have been a high browser that chewed soft tree leaves, Prothero also considers the various hypotheses for why the rhino’s lineage fizzled out. Most of these proposals – such as the wrongheaded notion that the rhinos suffered an “inadaptive” decline that drove them to extinction through a kind of evolutionary self-destruct – have themselves gone extinct. Instead, Prothero prefers the proposal of P.V. Putshkov and others that archaic elephants moved into Paraceratherium habitat and literally changed the landscape by stripping and pushing over the trees that the huge rhinos relied on. The arrival of a different kind of herbivore might have triggered ecological changes that drove the rhinos to extinction.

Paleontologists are as yet working with an outline of the evolution and extinction of Paraceratherium. Even the basic construction of the mammal’s skeleton is only partly known – reconstructions and restorations are based on parts from animals of different sizes and ages. Researchers are still striving to complete the skeletal structure of Paraceratherium. And despite being the most mega of the Cenozoic’s megafauna, we know considerably less about the natural history of Paraceratherium than many of the much older dinosaurs. Fossil mammals just don’t get the same research attention that Mesozoic celebrities do. Rhinoceros Giants is a handy synthesis of what paleontologists now know about the giant rhinos, but there are more mysteries about Paraceratherium than answers. Perhaps the book will inspire further work into Paraceratherium and other hefty mammals that are often obscured in the shadow of dinosaurs.