Surprising marsupial relative lived in ancient Texas

The Paleocene period, which followed the extinction of the dinosaurs, left behind many fossil traces across North America. Among these remnants, a remarkable discovery has emerged from what is now Big Bend National Park in Texas.

Paleontologists from the University of Kansas (KU) have identified and named a previously unknown species of ancient near-marsupial, calling it Swaindelphys solastella.

Marsupial relative in Texas

This small but significant creature lived millions of years ago during a time when the region was far different from today – lush, tropical, and covered with flowing rivers.

Despite the richness of the fossil record from that era, Swaindelphys solastella stood out due to its unusual size and unique features.

While most Swaindelphys were small, this one was the size of a modern hedgehog – giant for its kind. The research team, led by doctoral student Kristen Miller, analyzed old fossil collections to reach this conclusion.

The fossils were gathered decades ago by Judith Schiebout from Louisiana State University (LSU) but had never been fully examined.

Solving a marsupial mystery

Miller focused on molars that caught her attention, comparing them to those of many metatherian species – extinct relatives of marsupials.

The researchers initially thought the fossils were either leftovers from Cretaceous-era survivors or early members of a later Eocene group. Miller’s deep analysis proved both theories wrong.

“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.

Her adviser Chris Beard, a senior curator at KU, highlighted the importance of this find. He said that while Big Bend’s Paleocene mammals have been known for decades, this discovery sheds light on the larger ones often overlooked.

Insights into primate evolution

Beard also connected this marsupial’s story to early primates. The Leakey Foundation, which supported the research, saw the relevance.

“These marsupials are probably ecological analogues of early primates,” Beard said. That connection might help scientists better understand how early primates behaved and spread.

The Leakey Foundation usually funds research in Africa but found this project compelling.

“When the Leakey Foundation found out we wanted to do this kind of research, they said, ‘This sounds interesting,’” Beard explained. That led to a deeper look into fossils held by the University of Texas at Austin and LSU.

Fossils from a warmer Texas

Miller noted the climate differences. “It would have been warmer than it is now – probably more on the tropical side,” she said.

Unlike today’s desert terrain, Paleocene Texas had vegetation, streams, and rivers. The fossils were found in fluvial deposits, left behind by ancient river systems.

The team compared fossils from other places like Wyoming and Alberta, Canada. Some species found in Texas didn’t quite fit the timeline. Miller called them “anachronistic.”

That made them question the accuracy of fossil dating in southern regions, compared to places like the Bighorn Basin in Wyoming, where the fossil record is much clearer.

Where animals could live

To explain this, the KU researchers studied the region’s ancient topography. With help from the Department of Geology, they proposed that a high point in southern Wyoming might have divided marsupial species.

“North of that ancient divide, we see the classic Bighorn Basin taxa in their expected time periods,” Miller 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.”

This divide might explain why some species stayed north while others lived only in the south. Physical features like rivers or mountains may have acted as barriers to migration.

Texas marsupial and mammal evolution

Miller plans to continue testing this idea. She’s looking to confirm whether species differences on either side of the divide are statistically significant.

“That’s our working hypothesis, and it’s something I’ll be looking into later in my dissertation,” she said.

Through careful fossil analysis, unexpected connections, and geological insights, the story of Swaindelphys solastella offers a fresh glimpse into mammal evolution.

And true to Texas style, this ancient creature proved that even marsupials could go big.

The study is published in the Journal of Vertebrate Paleontology.

Image Credit: Kristen Tietjen

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