Mimoperadectes houdei, a new species of peradectid marsupial formally described by the authors of the new PLoS ONE paper, Cranial Anatomy of the Earliest Marsupials and the Origin of Opossums, is too small to count as megafauna but it certainly is very charismatic. This week’s PLoS ONE featured image is taken from Figure 6 of this published paper and shows a reconstruction of Mimoperadectes houdei by Jorge González (La Plata, Argentina).
Most species of North American mammals known only from their teeth, but in the new article, Inés Horovitz of the University of California, Los Angeles, and colleagues describe a 55 million-year-old skull from the new species, discovered in the Bighorn Basin in Wyoming. High-resolution, CT imaging allowed them to provide some detailed descriptions of the animal’s internal anatomy, particularly its ear, which can provide clues about the animal’s locomotion. The discovery of the new species allowed the researchers to show that peradectids, a family of marsupials known from fossils found mainly in North America and Eurasia, were close relatives of living opossums, which originated and live primarily in South America. The researchers also analyse the exceptionally well preserved skeletons of Herpetotherium, a herpetotheriid marsupial from North America, also reconstructed in Figure 6.
The findings lead the authors to conclude that the evolutionary split between the ancestor of opossums and the ancestor of all other living marsupials occurred at least 65 million years ago—this is the approximate age of the oldest known peradectid and represents an older calibration point for the evolution of marsupials than researchers could use confidently before. The authors also suggest that North America played an important role in early marsupial evolutionary history and it may have even been the centre of origin of living marsupials and opossums.
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