A newly described species of ancient marsupial, Swaindelphys solastella, has been identified from fossils found in Texas’ Big Bend National Park.
The discovery reveals that this metatherian – a group of animals that includes modern marsupials and their extinct relatives – was the largest of its kind in North America during the Palaeocene, a period that followed the extinction of the dinosaurs around 66 million years ago.
The fossils were first collected decades ago by the late palaeontologist Judith Schiebout. They remained unexamined in collections until Kristen Miller, a doctoral student at the University of Kansas’ Biodiversity Institute and Natural History Museum, took a closer look at the remains, including some remarkably large molars. Her work has now led to the naming and formal description of the species in the Journal of Vertebrate Paleontology.
'Giant' marsupial
At about the size of a modern hedgehog, Swaindelphys solastella might not seem particularly large, but compared to its relatives it was a giant.
What makes this species especially remarkable is not just its size but also its location and age. The fossils – unearthed from outcrops of the Black Peaks Formation in Big Bend National Park – represent the most southerly and most recent example of the Swaindelphys genus, says Miller, pushing the boundaries of what scientists previously understood about metatherian distribution.
Chris Beard, Miller’s doctoral adviser and senior curator at the Biodiversity Institute, says this find helps fill in gaps about the smaller, lesser-known mammals of the Palaeocene. "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 Palaeocene. Since everything is bigger in Texas, this is perhaps not surprising."
The landscape in which Swaindelphys lived was very different than it is today. Instead of desert, Big Bend was once a lush, tropical environment filled with rivers and streams. These waterways, according to the team, played a key role in shaping where ancient animals could live.
"We find these fossils in what we call fluvial deposits – so, deposits from ancient river systems," says Miller.
Geographic barriers may also have shaped where different species thrived. With the help of geologists, the researchers identified what they believe to be an ancient high point in the landscape, possibly acting as a divide that limited species movement between northern and southern regions.
"North of that ancient divide, we see the classic Bighorn Basin taxa in their expected time periods," Miller says. "But south of that... things start to go a little wacky.”
Further research will investigate how landscape features such as rivers and ridges influenced species distribution and evolution during the Palaeocene. Miller plans to test whether this proposed geographic barrier truly created distinct faunal zones.
Top image: Big Bend National Park. Credit: Getty
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