Ausktribosphenidae

Ausktribosphenidae is a group name that has been given to some rather puzzling fossils which:^[1]

• appear to have tribosphenic molars, a type of tooth which is otherwise known only in Therians.
• come from mid Cretaceous deposits in Australia - but Australia was connected only to Antarctica, and placentals originated in the northern hemisphere and were confined to it until continental drift formed land connections from North America to South America, from Asia to Africa and from Asia to India (the late Cretaceous map at [1] shows how the southern continents are separated.
• are represented only by skull and jaw fragments, which are not very helpful.

Ausktribosphenidae and monotremes have been assigned to the larger group Australosphenida. But Australosphenida is a group which has been defined only in order to include the Ausktribosphenidae and monotremes - in other words, it may be just a fiction, unless someone finds a fossil which can sensibly be described as a "basal australosphenid" (member of the Australosphenida which belongs to neither Ausktribosphenidae nor monotremes).

References