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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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Gomphotherium

Mounted specimen on display at the America Museum of Natural History, NYC

Reconstruction by Charles Knight. 

When: Miocene to Pliocene (~12 - 3.5 million years ago)

Where: North America, Europe, Asia, and Africa 

What: Gomphotherium is a four tusked extinct proboscidean. Unlike modern elephants which only have enlarged upper incisors as their tusks, Gomphotherium and its kin had enlarged upper and lower incisors. Neither set of tusks grew as large as living elephants, but the lower jaw was heavily modified and elongated to support the lower tusks. If you look at the photograph of the mounted specimen above, you can see that the actual bone of the mandible extends to almost the tip of the upper tusk. Based on the structure of the skull of Gomphotheriumit is thought the animal had a trunk, though again not one as log as the living species of elephants. Gomphotheriumis on the small side compared to the mammoth and mastodon in the photo with it, and also is a bit smaller than the living african elephant, but about the same size as the asian elephant - standing about 10 ft (3.2 meters) tall at the shoulder. These fourtuskers were proportioned very differently from the asian elephant, however. Their legs were much shorter in proportion to their body. The genus Gomphotheriumoriginated in North America, but spread throughout most of the world before going extinct in the Pliocene.  

Gomphotheriumin the group Gomphotheriidae (shocking I know). Gomphotheres ranged almost world-wide for over ten million years, and it is possible the last one died less than 10,000 years ago. I say only possible as relationships of gomphotheres, and really proboscideans as a whole, are really not well understood. Gomphotheriidae may be a paraphyletic series of taxa (not a ‘real’ group), with some taxa more closely related to the living species than others. Basically if you are interested in paleontology the study of proboscideans is an area that desperately needs more people in it. You also get to look at other cool extinct forms like Deinotherium


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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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