Around 60 million years ago, a large species of Swaindelphys—extinct marsupial-like animals—roamed through modern-day Texas’ Big Bend National Park. While they were only about the size of a hedgehog, the now-extinct species Swaindelphys solastella were “gigantic” in comparison to similar Swaindelphys from the same time period, according to researchers from the University of Kansas (KU). Swaindelphys solastella was the largest-known marsupials from the North American Paleocene. “North of that ancient divide, we see the classic Bighorn Basin taxa in their expected time periods,” she added. What we’re proposing is that this shift in river drainages marked the boundaries where ancient species of marsupials and primates lived.”In other words, the rivers and high points impacted the spread of certain species.