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Macrauchenia

Skeleton on display at the American Musuem of Natural History, NYC. 

Reconstruction is part of the traveling exhibit Extreme Mammals which started at the American Museum of Natural History. 

When: Late Miocene to Late Pleistocene (7 million to 20,000 years ago)

Where: South America 

What: Macrauchenia is a hoofed mammal from South America. This animal has been known to science for a very long time. The first fossils were found by none other than Charles Darwin when he was traveling on the Beagle. He gathered up the fossilized  vertebra and limb bones and brought them back to England, where they were studied by Richard Owen, who coined the name Macrauchenia (meaning ‘long neck’), and supposed the whole animal would have resembled a llama. Later fossil finds, including several almost complete specimens, confirmed that Macrauchenia  did somewhat resemble a llama, with its slender legs and long neck. However, it was very diffent in some critcal areas, such as having 3 hoofed toes per foot and a mobile trunk. How do we know this animal had a trunk from just the bones? In living mammals with long trunks (such as the elephant and the tapir) the skull is transformed for the musculature that allows such a structure to move, and the skull of Macrauchenia has many features which closely match that of these modern trunked species. 

So with this long llama-like neck and the tapir-like trunk, how does Macrauchenia fit into the mammal family tree? That is a subject of much debate, but it is certain that it is not especially closely related to either artiodactyls (llamas) or  perissodactyls (tapirs).  Macrauchenia is in the order Litopterna, a group of mammals which is only found in South America. Litopterna is assuredly an order of placental mammals, but its exact placement relative to the other major clades is uncertain at this time. It has been suggested they, and other South American ungulate groups, may fall somewhere close to Afrotheria

South America  was isolated from all others for millions of years, in 'splendid isolation’, during which time the mammals upon it radiated to fill all available niches, and this resulted in dozens of cases of convergent or parallel evolution. There are ungulate fossil froms known from South America that closely resemble not only llamas but also horses, rabbits, and even elephants! Carnivorous forms got in on the act too, such as Thylacosmilus, which looked very simular to the Saber-toothed 'tigers’ of North America. Many South American natives went extinct during the great faunal interchange, but Macrauchenia survived until the end of the last glacial period. There is hope that one day we might recover some ancient DNA of this animal, which would be very helpful in determining where it falls in the great family tree of placental mammals. 

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Thylacosmilus - marsupial sabertooth

Skull on display at the American Museum of Natural History.

Reconstruction by Mauricio Antón. 

When: Late Miocene to Late Pliocene (~10 million to 3 million years ago)

Where: South America, most fossils are from Argentina, but it was fairly widespread.  

What: Thylacosmilus at first glance looks much like the famous Smilodon (sabertooth tiger). But a closer  look starts to reveal many significant differences, such as the extremely large flanges on its lower jaw, which protect the large canines, and the lack of any teeth anterior to these massive teeth. This animal is very far removed from Smilodon, it is not even a placental mammal. It is a metatherian (marsupials are the living metatherians) more closely related to kangaroos, koalas, wombats, etc than to the saber-toothed tiger.  The similarity between Thylacosmilus and Smilodon is an excellent example of convergent evolution - two distantly relating forms converging upon a simular morphology and life habitat.  There are an three other examples of mammals that have developed saber-teeth- in fact most of the last 65 million years had some large cat-like saber-toothed mammal present, the modern biota is the outlier. 

Thylacosmilus went extinct roughly 3 million years ago, closely coinciding with the formation of the land bridge linking the Americas. Animals from North America emigrated south and those from South America journeyed north; this fauna exchange is referred to as the Great American Interchange. It is at this time we start to find Smilodon fossils in South America. It is thought that the arrival of this relatively larger predator (the largest Smilodon was twice the size of the largest Thylacosmilus) may have been what drove the only known marsupial saber-toothed form to extinction. 

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