Macrauchenia

Description

Macrauchenia had a somewhat camel-like body, with sturdy legs, a long neck and a relatively small head. Its feet, however, more closely resembled those of a modern rhinoceros, and had three hoofs each. It was a relatively large animal, with a body length of around 3 metres (9.8 ft) and a weight up to 1042.8 kg.

One striking characteristic of Macrauchenia is that, unlike most other mammals, the openings for nostrils on its skull were atop the head, leading some early scientists to believe that, much like a whale, it used these nostrils as a form of snorkel. Soon after some more recent findings, this theory was rejected. An alternative theory is that the animal possessed a trunk, perhaps to keep dust out of the nostrils. Macrauchenia's trunk may be comparable to that of the modern Saiga antelope.

One insight into Macrauchenia's habits is that its ankle joints and shin bones may indicate that it was adapted to have unusually good mobility, being able to rapidly change direction when it ran at high speed.

Macrauchenia is known, like its relative Theosodon, to have had a full set of 44 teeth.

Paleobiology

Macrauchenia was a herbivore, likely living on leaves from trees or grasses. Carbon isotope analysis of M. patachonica's tooth enamel, as well as analysis of its hypsodonty index (low in this case; i.e., it was brachydont), body size and relative muzzle width suggests that it was a mixed feeder, combining browsing on C3 foliage with grazing on C4 grasses. Scientists believe that, because of the forms of its teeth, Macrauchenia ate using its trunk to grasp leaves and other food. It is also believed that it lived in herds like modern-day wildebeest or antelope, the better to escape predators.

When Macrauchenia first arose, it would have been preyed upon by the largest of native South American predators, terror birds such as Andalgalornis, and carnivorous sparassodontids such as Thylacosmilus. During the late Pliocene/Early Pleistocene, the Panama Isthmus formed, allowing predators of North American origin, such as the cougar, the jaguar, the South American giant short-faced bear, and the saber-toothed cat, Smilodon, to emigrate into South America and replace the native forms.

It is presumed that Macrauchenia dealt with its predators primarily by outrunning them, or, failing that, kicking them with its long, powerful legs, much like modern-day vicuña or camels. Its potential ability to twist and turn at high speed could have enabled it to evade pursuers.

History of discovery

Macrauchenia was first discovered on 9 February 1834 at Port St Julian in Patagonia (Argentina) by Charles Darwin, when HMS Beagle was surveying the port during the voyage of the Beagle. As a non-expert he tentatively identified the leg bones and fragments of spine he found as "some large animal, I fancy a Mastodon". In 1837, soon after the Beagle's return, the anatomist Richard Owen revealed that the bones including vertebrae from the back and neck were actually from a gigantic creature resembling a llama or camel, which Owen named Macrauchenia patachonica. In naming it, Owen noted the original Greek terms Μακρος (large or long), and αυχην (neck) as used by Illiger as the basis of Auchenia as a generic name for the llama, Vicugna and so on. The find was one of the discoveries leading to the inception of Darwin's theory. Since then, more Macrauchenia fossils have been found, mainly in Patagonia, but also in Bolivia, Chile and Venezuela.

The related genus Cramauchenia was named by Florentino Ameghino as a deliberate anagram of Macrauchenia.

Evolution

Macrauchenia appeared in the fossil record some 7 million years ago in South America (in the Miocene epoch). It is likely that Macrauchenia arose from either Theosodon or Promacrauchenia. Notoungulata and Litopterna were two ancient orders of ungulates which occurred only in South America. Most of these species became extinct through competition with invading North American ungulates during the Great American Interchange, after the establishment of the Central American land bridge. The litopterns Macrauchenia and Xenorhinotherium and the large notungulates Toxodon and Mixotoxodon were among the only South American ungulate genera to survive the Interchange. These last endemic South American hoofed animals died out at the end of the Lujanian (10,000-20,000 years ago) at roughly the time of arrival of humans at the end of the Pleistocene, along with numerous other large animals on the American continent (such as American proboscideans, horses, camelids, bears, and cats). As Macrauchenia was the last of the litopterns, the genus' extinction marks the close of the South American ungulate dynasty.

An analysis based on collagen sequences obtained from Macrauchenia and Toxodon found that litopterns along with notoungulates form a sister group to perissodactyls, making them true ungulates.