Steropodon was one of the earliest mammals. It possessed mammal things like hair and a neocortex (a part of the mammal brain that deals with sensory perception, spatial reasoning, conscious thought, and language). It will not let you forget that it discovered those things before they went mainstream.
Steropodon is a goofy-looking platypus ancestor. It belongs to a group of creatures called monotremes, which are are mammals that lay eggs. The only ones that are still around today are platypuses and anteaters. This mammal is estimated to have been about the size of a cat. That may sound small, but it actually makes it one of the largest of its time period. Most Cretaceous mammals were tiny and nocturnal.