And it’s not what was expected:
In 1999, researchers uncovered a 66-million-year-old fossil of a bizarre mammal in Madagascar. The creature was about the size of an opossum and may have looked more like a badger or a beaver—but it’s the ancestor of none of them. The animal’s anatomy was so confounding, researchers named it Adalatherium hui, using a Malagasy word for “crazy” and the Greek word for “beast.” …
Adalatherium’s teeth are the strangest part of the fossil. Its front teeth are long and curved like a rodent’s, but otherwise its teeth are unlike any modern animal’s, paleontologist Guillermo Rougier says in a statement. Rougier specializes in using teeth to classify mammals, so the strange fossil presented a challenge.Theresa Machemer , “66-Million-Year-Old ‘Crazy Beast’ Finds a Taxonomical Home” at Smithsonian Magazine
It’s now considered a member of an extinct group, Gondwanatherians. Paper. (open access)
It lived at the time that the continent Gondwana was breaking up:
Its discovery challenges previous assumptions that mammals were generally very small – the size of mice – at this point in their evolutionary history.
Researchers say this individual animal weighed 3kg (6.6lbs) and had not reached its full adult size.Reuters, “‘Crazy beast’ lived among last of dinosaurs” at BBC