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 currently controversy about whether or not to classify 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", as a group, 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.
Nevertheless, its lower jaw retains some of the bones found in its non-mammalian ancestors in a very reduced form rather than being composed solely of the dentary.
Furthermore, the primitive reptile-like jaw joint between the articular and quadrate bones, which in modern mammals has moved into the middle ear and become part of the ear ossicles as malleus and incus, is still to be found in Morganucodon.
Morganucodon also suckled (it may have been the earliest animal to do so), had only two sets of teeth and grew rapidly to adult size and stopped growing thereafter, all typical mammalian traits.

Phylogeny
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 currently controversy about whether or not to classify 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", as a group, 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. Nevertheless, its lower jaw retains some of the bones found in its non-mammalian ancestors in a very reduced form rather than being composed solely of the dentary. Furthermore, the primitive reptile-like jaw joint between the articular and quadrate bones, which in modern mammals has moved into the middle ear and become part of the ear ossicles as malleus and incus, is still to be found in Morganucodon. Morganucodon also suckled (it may have been the earliest animal to do so), had only two sets of teeth and grew rapidly to adult size and stopped growing thereafter, all typical mammalian traits.
Phylogeny
Morganucodon