North America was center of origin for all living marsupials

In a new research, a scientist from the University of Florida has traced the evolution of the modern opossum back to the extinction of the dinosaurs, and finding in the process evidence to support North America as the center of origin for all living marsupials.

The study shows that peradectids, a family of marsupials known from fossils mostly found in North America and Eurasia, are a sister group of all living opossums.

The findings are based in part on high-resolution CT scans of a 55 million-year-old skull found in freshwater limestone from the Bighorn Basin of Wyoming.

"The extinction of the dinosaurs was a pivotal moment in the evolution of mammals," said Jonathan Bloch, study co-author and associate curator of vertebrate paleontology at UF's Florida Museum of Natural History.

"We're tracing the beginnings of a major group of mammals that began in North America," he added.

Opossum-like peradectids first appeared on the continent about 65 million years ago, at the time of the Cretaceous-Paleogene extinction event, which killed the dinosaurs.

"North America is a critical area for understanding marsupial and opossum origins because of its extensive and varied fossil record," said lead author Ines Horovitz, an assistant adjunct professor at the University of California, Los Angeles.

"Unfortunately, most of its species are known only from teeth," he added.

The study also analyzes two 30-million-year-old skeletons of Herpetotheriidae, the sister group of all living marsupials.

Based on fossil evidence from the skull and two skeletons, the study's authors concluded that the evolutionary split between the ancestor of opossums and the ancestor of all other living marsupials occurred at least 65 million years ago.

The study describes a new peradectid species, Mimoperadectes houdei, based on a relatively complete fossil skull.

The high-resolution CT scan of the skull gave researchers a large amount of information about the animal's internal anatomy.

"The ear, in particular, provides researchers with information on skull anatomy and clues about the animal's locomotion," Bloch said.

The scan showed that the new species shared enough common traits with living opossums to indicate an evolutionary relationship.

According to Bloch, some predictions about that relationship could have been made from fossil teeth, "but this provides a much stronger foundation for that conclusion."

"In parts of North America today, opossums are one of the most commonly observed mammals around," he said.

"This fossil skull shows its roots going back to the extinction of the dinosaurs. This is literally the fossil that shows us the ancestry of that animal," he added.