The Evolution of Mammalian Sight and Hearing

Abstract

The Middle Jurassic gives glimpses of the evolving mammals. From recent discoveries (see Chapter 7 for details) there is evidence of a much wider range of mammalian life than was seen in the Upper Triassic and Lower Jurassic, but unfortunately the new material is almost entirely teeth with but a few jaw fragments. It is impossible to reconstruct the skull of a Middle Jurassic mammal in the same way as the Lower Jurassic Morganucodon. Nevertheless these later mammals show one quite fundamental difference from the earlier forms: the majority of them have lost the reptilian quadrate jaw articulation. Clearly this fundamental step in the evolution of mammals happened between the Middle and Lower Jurassic, and we must consider why this change came about.