It is the earliest known example of several features possessed only by mammals , including the middle - ear structure characteristic of modern mammals and a relatively large brain cavity .
Hadrocodium might have been the first animal to have a nearly fully mammalian middle ear .
Hadrocodium is the earliest known animal of several features possessed only by mammals , including the middle - ear structure characteristic of modern mammals and a relatively large brain cavity .
Hadrocodium might have been the first animal to have a nearly fully mammalian middle ear .