Big possum that lived 60 million years ago unearthed in Texas - Biology - Paleontology & Fossils - phys.org

Reconstruction of Swaindelphys. Credit: Kristen Tietjen

They say everything's bigger in Texas. And that appears to be true, at least in the case of a group of ancient near-marsupials scientists call Swaindelphys.

Paleontologists from the University of Kansas have described for the first time a species of Swaindelphys discovered in Texas' Big Bend National Park, though the ecosystem was drastically different in the Paleocene, when it thrived, than today.

Dubbed Swaindelphys solastella, the new species is much larger than similar species of Swaindelphys known from that period.

Their report detailing the ancient species, which was gigantic by the standards of Swaindelphys but still about the size of a modern hedgehog, appears in the Journal of Vertebrate Paleontology.

Lead author Kristen Miller, doctoral student at KU's Biodiversity Institute and Natural History Museum, spent a year examining specimens collected decades ago in West Texas by the late Judith Schiebout, a paleontologist whose career was spent at Louisiana State University.

Some of the fossils collected at Big Bend by teams led by Schiebout had never been thoroughly studied, including molars that piqued Miller's interest. She wanted to find out what kind of metatherians—the group that includes living marsupials and their extinct relatives—the Texas fossils represented.

"I compared them to a lot of other marsupials from around the same time period to see what they're most closely related to," Miller said. "It was a lot of morphological comparisons."

The researchers initially thought the fossils were either survivors of a group of large Cretaceous metatherians that somehow made it through the Cretaceous-Paleogene extinction event or that they were the oldest member of a group of Eocene metatherians that showed up a few million years later.

Miller's analysis eventually showed that both ideas were wrong. The Texas specimens belong to a surprisingly large species of Swaindelphys.

"Not only are they the largest metatherians from this time period, but they're also the youngest and located at the most southern latitude," Miller said.

Miller's doctoral advisor and co-author, Chris Beard, senior curator with KU's Biodiversity Institute and Foundation Distinguished Professor, said the first fossil mammals of the Paleocene age in Big Bend were first described decades ago.

"But our work is aimed at uncovering some of the smaller and harder-to-find fossil mammals that lived in Big Bend at that time," Beard said. "The new fossil we're describing is notable because it's the largest marsupial—in terms of body size—found so far in the North American Paleocene.

"Since everything is bigger in Texas, this is perhaps not surprising."

The KU researchers said their study of Swaindelphys potentially informs scientific study of early primates that inhabited the same ecosystems in Texas. Indeed, because Swaindelphys was in so many ways like early primates, their behavior and ancient distribution is seen by paleontologists as a collateral way to understand primate history.

For this reason, the research into Swaindelphys solastella—including analysis of specimens from the LSU and University of Texas at Austin collections—and new fieldwork in Big Bend National Park was supported by The Leakey Foundation, a donor-supported nonprofit organization with a mission "to uncover the story of human evolution and share this knowledge with the world."

Along these lines, the KU researchers said the distribution patterns of Swaindelphys could indicate what kinds of natural features and barriers constrained the geographical spread of species in this time period, including early primates.

"It's during the Paleocene, so it would have been warmer than it is now—probably more on the tropical side," Miller said. "In place of desert terrain seen today, there was a lot more vegetation and probably lots of rivers and streams. We find these fossils in what we call fluvial deposits— so, deposits from ancient river systems."

The investigators are interested in the differences in the kinds of fossil species found in more northern zones—like Wyoming and Alberta, Canada—compared to southern areas like the U.S.-Mexico border in the vicinity of Big Bend National Park.

"What's interesting about the localities in Texas is we have some taxa we'd call 'anachronistic'—things we don't expect to see in Texas during the time these fossils were deposited," Miller said. "The fossil record in places like the Bighorn Basin in Wyoming is very complete. It's a really nice stratigraphic sequence spanning millions of years, so it's easy in the Bighorn Basin to compare fossils from different localities.

"We call it biostratigraphy—you basically use the fossils to understand what time period you're in. If you have certain taxa, you know it has to be from a specific time period."

But outside of the Bighorn Basin, the picture gets murkier, according to the KU paleontologists. They said it's harder to pinpoint the time periods associated with fossils. Miller and Beard wondered if some kind of geographic barrier was behind the difference.

Working with colleagues from KU's Department of Geology, they've identified "an ancient high point or divide in the landscape, in southern Wyoming, that seems to correspond with the shift we see," Miller said.

"North of that ancient divide, we see the classic Bighorn Basin taxa in their expected time periods," she said. "But south of that, in river drainages that originate in the central Rockies and areas farther to the south, things start to go a little wacky. What we're proposing is that this shift in river drainages marked the boundaries where ancient species of marsupials and primates lived."

Miller and Beard think ancient landscapes posed obstacles to species distribution during the Paleocene—some taxa couldn't cross rivers and high points, while others could. Miller plans to investigate the question with follow-up research.

"That's our working hypothesis, and it's something I'll be looking into later in my dissertation," she said. "We want to see if we can nail down, quantitatively, whether there's a significant difference on either side of that potential barrier." 

by Brendan M. Lynch, University of Kansas

edited by Gaby Clark, reviewed by Robert Egan

Source: Big possum that lived 60 million years ago unearthed in Texas