This 20 August 2020 video says about itself:
To figure out how Thrinaxodon and Broomistega became entombed together, scientists looked at the burrow itself, along with their fossilized bones. And it looks like their luck ran out, when a behavior that usually would’ve helped them survive just didn’t work.
Thrinaxodon was a mammal-like reptile. Broomistega was an amphibian. They lived during the Triassic in South Africa.