Members of the metatherian genus Swaindelphys were previously known from Swain Quarry in south-central Wyoming and the Nacimiento Formation in the San Juan Basin, New Mexico, and now, from the Black Peaks Formation of west Texas.
Life reconstruction of Swaindelphys. Image credit: Kristen Tietjen.
The newly-identified species lived in what is now Texas during the Paleocene epoch, some 60 million years ago.
Named Swaindelphys solastella, the animal was much larger than similar species of Swaindelphys, but still about the size of a modern hedgehog.
It was a type of metatherian, a group that includes living marsupials and their extinct relatives.
“Not only are they the largest metatherians from this time period, but they’re also the youngest and located at the most southern latitude,” said Kristen Miller, a doctoral student at the University of Kansas.
The fossilized remains of Swaindelphys solastella were found in the Black Peaks Formation in Big Bend National Park, west Texas.
“The first fossil mammals of the Paleocene age in Big Bend were first described decades ago,” said University of Kansas Professor Chris Beard.
“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.”
“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.”
According to the authors, 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 discovery of Swaindelphys solastella is described in a paper published this month in the Journal of Vertebrate Paleontology.
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Kristen Miller et al. Biogeographic and biostratigraphic implications of a new species of Swaindelphys (Mammalia, Metatheria) from the Paleocene (Tiffanian) Black Peaks Formation, Big Bend National Park, Texas. Journal of Vertebrate Paleontology, published online June 17, 2025; doi: 10.1080/02724634.2025.2500501
