The announcement of a “Saber-Toothed Squirrel” fossil has been making the rounds on a number of high-profile news sites this week – primarily due to its resemblance to the Scrat from Ice Age. This discovery was published in the Nov. 3rd edition of Nature under the far less exciting title: “Highly specialized mammalian skulls from the Late Cretaceous of South America”. As this new species is not actually a squirrel and not really saber-toothed either, here is the what, when, where and why:
What: Scientists from the University of Louisville uncovered two partial skulls from an extinct mammalian group, the Dryolestoids. Rather than being squirrels, Dryolestoids belonged to a lineage that also gave rise to modern-day marsupial and placental mammals. More specifically, Dryolestoidea is a sublegion within the legion of Cladotheria. The other sublegion of Cladotheria is Zatheria, which gave rise to the supercohort Theria. Theria includes modern-day marsupial and placental mammals. The bottom lines is that these critters were running around way before the first squirrel came into existence. Additionally, these guys are an extinct offshoot of the lineage that lead to marsupials and placental mammals, most likely not a direct ancestor.
When: The fossils are about 100 million years old from the Late Cretaceous period. The Cretaceous period spans about 145 to 65 million years ago. It was preceded by the Jurassic period and ended with the K-T extinction, which is when non-avian dinosaurs met their end. The Cretaceous period was followed by Cenozoic Era, which is also known as the Age of the Mammals. This name refers to the massive diversification of mammalian species during this time period. So this creature would have lived alongside dinosaurs when mammals were much rarer than they are today.
Where: The fossils were uncovered in what is now the Rio Negro providence of Argentina, which is on the northern edge of Patagonia. Patagonia is known for being rich in fossils of small vertebrates and dinosaurs from the late Cretaceous and Cenozoic Era. During the late Cretaceous, this area was part of the Gondwana landmass, which included present day South America, Africa, India, Madagascar and Australia.
Why: The discovery of this fossil is big deal because it so rare to find mammalian fossils, especially in such good condition, from this era and also, this location. Most pre-Cenozoic mammalian fossils consist of a few teeth or jaw fragments and are found in present day North America, which was a part of Laurasia. Not only is this fossil is a complete skull, but is was from Gondwana. In fact, this is only the second semi-complete mammalian skull from the Mesozoic Era that has been found in present day South America.
Rougier, Guillermo W., Sebastián Apesteguía & Leandro C. Gaetano (2011) Highly specialized mammalian skull from the Late Cretaceous of South America. Nature. 479: 98-102. [link to abstract]
de Muizon, Christian (2011) Palaeontology: Fresh light on southern early mammals. Nature. 479: 51-52. [link to abstract]