What is Morganucodon?

Morganucodon is an early mammaliaform genus that lived during the late Triassic period. It first appeared about 205 million years ago. Unlike many other early mammals, Morganucodon is well represented by abundant and well preserved, though in the vast majority of cases disarticulated, material. Most of this comes from Glamorgan in Wales (Morganucodon watsoni), but fossils have also been found in Yunnan Province in China (Morganucodon oehleri) and various parts of Europe and North America. Some at least closely related animals (Megazostrodon) are known from exquisite fossils from South Africa.

Morganucodon is the type genus for the order Morganucodonta, a group of generally similar mammals or pre-mammals from Late Triassic to Middle Jurassic time of the close relatives. All were small and mainly insectivores. Of the small bodied relatives, Morganucodon is the best preserved and best understood find.

There is controversy about classifying Morganucodon as a mammal or as a non-mammalian mammaliaform. Some researchers limit the term "mammal" to the crown group mammals, which would not include Morganucodon and its relatives. Others, however, define "mammals", by the possession of a special, secondarily evolved jaw joint between the dentary and the squamosal bones, which has replaced the primitive one between the articular and quadrate bones in all modern mammalian groups. Under this definition, Morganucodon would be a mammal.