Wednesday, September 19, 2012

Prehistoric Rhinos - An Underrated Evolutionary Saga: World Rhino Day Part 2

Paraceratherium, a giant browsing rhinoceros from the Oligocene;
(By Kevin Yan)

Previously, I explained my great dislike for statements claiming that the 5 extant species of rhinoceros are prehistoric survivors. The rhinos we see today represent a single surviving lineage, a mere twig in the rhino family tree. This post will look at some of these actual prehistoric rhinos, and we'll see that over the last 50 million years, rhinos took on many different shapes and sizes.

Our story begins sometime during the Early Eocene, some 55 million years ago. At this point in Earth's history, temperatures were much higher, and much of the continents, including the polar regions, were covered in dense forests. Eurasia and North America were connected via land bridges and island chains across the Atlantic and Pacific Oceans. It was in such an environment that the first perissodactyls evolved. They flourished as small herbivores, feeding on leaves and fruit in these vast jungles, and rapidly diversified into many different forms.

Hyrachyus sp.
Hyrachyus skull;
(Photo by California Academy of Sciences Geology)

One of these was Hyrachyus, a small herbivore found in Eurasia and North America. Some palaeontologists think that this creature possibly represents the ancestral form that would eventually give rise to rhinos, although there are others who think that it's more closely related to tapirs. The ancient ancestors of rhinos, tapirs, and horses all look so similar at this stage, that we need more fossils to get a clearer picture of early perissodactyl evolution in the Eocene.

Ancient tapiroids
The larger skeleton is that of a Hyrachyus, while the smaller skeleton in the foreground is that of Orohippus, an early horse, Smithsonian Natural History Museum;
(Photo by Tapir Girl)

Reconstruction of Hyrachyus;
(Photo by Carl Wozniak)

Regardless of whether or not Hyrachyus belongs to the rhino family tree, what we do know is that by the Middle Eocene, rhinos had evolved. But these didn't look like our modern-day rhinos at all. In fact, they were so diverse that they are currently classified into 3 distinct families.


The Amynodontidae were a group of large-bodied rhinos that lived in Asia and North America, and are believed to have been browsers adapted to forests and swamps.

(By Hirokazu Tokugawa)

Many browsing herbivores have a prehensile upper lip, enabling them to grasp leaves and twigs, and studies of their skulls have shown that amynodonts possessed such an anatomical feature. Amynodon itself has been found in Middle Eocene deposits of Asia and North America, and appears to have been a relatively primitive-looking form.

Drawing of Amynodon skeleton;
(From American Museum Novitates)

(By Roman Yevseyev)

One particular group, represented by Cadurcodon from the Late Eocene of China and Mongolia, is believed to have further modified the upper lip into a proboscis, very much like that seen in tapirs.

Cadurcodon skull;
(Photo from Integrated Collection of Digital Educational Resources)

(From Natural History Museum Picture Library)

Another group of amynodonts, represented by Metamynodon from the Late Eocene and Early Oligocene of Asia and North America, appear to have become specialised for an amphibious lifestyle, very much like the unrelated hippos.

Metamynodon skeleton;
(From Wikipedia)

Most reconstructions of Metamynodon portray it as a hippo-like creature.

(By Charles R. Knight, 1896)

(By Heinrich Harder, c. 1920)

(By Zdeněk Burian, 1964)

At the end of the Middle Eocene, shifts in climate were causing great changes in the distribution and types of vegetation, and this continued into the Oligocene. In response to cooling and greater seasonal variation, the great forests of the Eocene were shrinking and thinning out, with increased aridity leading to the spread of open plains and savannas, covered in shrubs and bushes adapted to the drier conditions.

Presumably, the amynodonts were able to adapt at first, with some evolving to become grazers with teeth equipped to cope with tougher foliage, but they too eventually succumbed. All amynodonts in Asia and North America vanished after the Early Oligocene. A final amynodont, Cadurcotherium, lingered on in Europe during the Oligocene, and presumably managed to disperse back into Asia, where fossils have been found in end-Oligocene sediments in Pakistan. If the dates are accurate, this would mean that Cadurcotherium survived for around 10 million years after all its other relatives had become extinct. Still, it too eventually disappeared.


A second family of rhinos, the hyracodontids, adopted a very different lifestyle.

(By Charles R. Knight)

The early forms, like sheep-sized Hyracodon from the Middle Eocene to early Late Oligocene of North America, had long, slender legs with only 3 digits. This made it look very similar to the primitive horses that shared the open forests and woodlands.

(By Heinrich Harder)

(By Roman Yevseyev)

Drawing of Hyracodon skeleton;
(From Cambridge Natural History Mammalia)

Hyracodon skull
Hyracodon skull;
(Photo by ryanzz)

However, while the horses continued to adapt to life on the plains, becoming fleet-footed grazers, the hyracodonts, which remained browsers, took a rather different evolutionary path. Hyracodon persisted until the Late Oligocene, long after all the other small hyracodonts had gone extinct. But in the meantime, as the forests of central Asia vanished, with trees becoming more sparsely scattered or limited to well-watered areas, one group of hyracodonts had coped by growing larger. Presumably, this enabled them to travel greater distances between suitable patches of vegetation. And so, millions of years after the sauropod dinosaurs went extinct, a group of mammals finally evolved to rival their gargantuan size - these were the indricotheres.

(By Roman Yevseyev)

Indricotheres were hyracodont rhinos, classified under the subfamily Indricotheriinae, and their fossils are found in Central Asia. One of the earliest species, Forstercooperia, appeared during the Middle Eocene. The size of a cow, its fossils have been found in India, Pakistan, Kazakhstan, and China. At that time, this area would have been a lush forest, fringing the shores of the ancient Tethys Ocean. Later in the Middle Eocene, the larger Juxia appeared, followed by Urtinotherium in the Late Eocene.


Reconstruction of Juxia head;

(By Chen Yu)

It was in the Oligocene that the indricotheres achieved truly gigantic sizes, with Paraceratherium reaching heights of up to 4.8 metres at the shoulder, and weighing up to 16 tonnes.

Paraceratherium model, California Academy of Sciences;
(Photo by Rose Laurel)

Paraceratherium is known by 2 other names; some older sources will mention Baluchitherium or Indricotherium, and it's still unclear as to how many species of giant indricothere are known, and whether they are all similar enough to be considered the same genus. The name Paraceratherium has priority as it was named in 1911 from specimens found in Pakistan, with Baluchitherium being named in 1913 for fossils also from that country. Indricotherium was described in 1915, based on remains from the Caucasus Mountains in southern Russia. It's likely that Baluchitherium is synonymous with Paraceratherium, but whether the latter is the same creature as Indricotherium is still not confirmed.

In any case, fossils of this massive beast have mostly been found in Pakistan, China, Mongolia, Russia, and Kazakhstan, with possible remains from Turkey as well.

Indricotherium, an extinct herbivore mammal from the Oligocene
Moscow Paleontological Museum;
(Photo by cazfoto)

Paraceratherium dwarfs the largest living land mammal, the African bush elephant (Loxodonta africana);
(By SameerPrehistorica)

Paraceratherium is quite a famous prehistoric mammal, thanks to its epic size, and also due to it being the focus of an episode of the BBC's Walking With Beasts.

(By Chen Yu)

(By Paul Heaston)

Despite its hefty size, Paraceratherium retained the relatively gracile limbs of its smaller, more cursorial ancestors, which were adapted for running. Even so, it's highly unlikely that there were any other animals that would make an adult Paraceratherium run.

(By Mauricio Anton)

Its size and long neck mean that Paraceratherium is likely to have been a high-level browser, feeding at heights beyond the reach of other herbivores. In essence, this was a rhinoceros that fed like a giraffe, but grew larger than an elephant.

Зденек Буриан (35)
(By Zdeněk Burian)

(By Roman Yevseyev)

Most reconstructions depict Paraceratherium with some sort of prehensile upper lip, like that seen in many browsing herbivores. This would have helped in grasping twigs and leaves. So far, reconstructions depicting Paraceratherium with an elongated tapir-like proboscis are still speculative, although their presence cannot be completely discounted yet.

(From Wikipedia)

Indricotherium transouralicum
American Museum of Natural History;
(Photo by Ryan Somma)

(By Chen Yu)

Even though it was such an immense creature, Paraceratherium was initially known from scanty remains, bits and pieces that did not provide much information about its body proportions. In the 1920s, when a partially complete skull was found (and described as belonging to Baluchitherium), confirming that it was related to modern-day rhinos, it was restored as a giant, hornless rhino, with a shape roughly similar to its much smaller extant cousins.

(From July 1923 Issue of Popular Mechanics)

Indricotherium(largest land mammal ever) by Charles Knight at the Los Angeles Museum of Natural History 175A
(By Charles R. Knight)

Subsequently, as Brian Switek has written, it was realised that Paraceratherium was a much more gracile animal, resulting in an updated reconstruction that made it somewhat more slender.

1923 reconstruction of Paraceratherium;
(From American Museum Novitates)

1935 reconstruction of Paraceratherium;
(From American Museum Novitates)

Paraceratherium or Indricotherium
American Museum of Natural History;
(Photo by Ryan Somma)

Paraceratherium lived through the Oligocene, although some of the Late Oligocene forms have been identified by some paleontologists as belonging to a different genus altogether, known as Dzungariotherium. Others maintain that there aren't sufficient differences to warrant separation from Paraceratherium. Whatever the case, the largest of all the indricotheres lived during the Late Oligocene.

Unfortunately for the indricotheres, the dawn of the Miocene heralded their end. The cooling and drying of the Earth's climate did not abate, and the forests continued to shrink. The plains were being taken over by a new type of plant, one that had been present for some time, but had never dominated the landscapes until then: grass. The indricotheres, dependent on woodlands for their survival, vanished as these habitats gave way to open steppes in the early Miocene.

As mentioned earlier on, Carducotherium, the last of the amynodontid rhinos, also disappeared at this time. So the beginning of the Miocene saw the extinction of both the amynodont and hyracodont lineages. However, a final family of rhinos remained, and they continued to prosper and flourish - these were the rhinocerotids, or the 'true' rhinos, and they'll be the focus of the next post.