Members of the genus Metatherian Swaindelphys were previously identified in the Swain Quarry in southern Wyoming and the Nasimiento Formation in the San Juan Basin, New Mexico, and are now also found in the Black Peaks Formation in West Texas.

The newly identified species thrived in Texas during the Ale-Nocene period, around 60 million years ago.

It has been designated Swaindelphys Solastella; these animals were significantly larger than related species, yet still about the size of a modern hedgehog.

It belonged to Metatherian, a group encompassing living marsupials and their extinct relatives.

“We’ve dedicated a lot of effort to uncovering this world,” remarked Kristen Miller, a doctoral student at the University of Kansas.

Fossil remains of Swaindelphys Solastella were uncovered in the Black Peak Formation within Big Bend National Park, West Texas.

“The earliest fossil mammals from the Big Bend during the Ale Holocene epoch were initially documented decades ago,” stated Professor Chris Beard of the University of Kansas.

“However, our mission is to uncover some of the smaller, more elusive fossil mammals that inhabited Big Bend during that era.”

“The new fossil we are describing holds significance as it represents the largest marsupial ever identified from the Ale Holocene in North America, in terms of body size.”

“This isn’t surprising considering that everything tends to be larger in Texas.”

The distribution patterns of Swaindelphys may provide insights into the natural features and barriers that limited the geographical spread of species during this time, including early primates.

“During the Ale New Century, the climate would have been warmer, likely leaning towards tropical,” Miller explained.

“Instead of the desert landscape seen today, there would have been more vegetation, possibly alongside numerous rivers and streams.”

“These fossils are found in what are referred to as river sediments, stemming from ancient river systems.”

The discovery of Swaindelphys Solastella is detailed in a paper released this month in the Journal of Vertebrate Paleontology.

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Kristen Miller et al. Implications of biogeographical and biostratigraphic novel species. Swaindelphys (Mammals, Metatheria) from the Black Peaks Formation, Big Bend National Park, Texas. Journal of Vertebrate Paleontology, published online on June 17th, 2025. doi:10.1080/02724634.2025.2500501