There has been a long controversy about whether or not to classify it as a mammal.
Some prefer to include it in a broader clade called Mammaliaformes.

Morganucodon is regarded as very basal (= primitive).
Its lower jaw has some of the bones found in its synapsid ancestors in a very reduced form.
Mammals have a jawbone composed solely of the dentary.

The primitive jaw joint between the articular and quadrate bones is still  found in Morganucodon.p107/112 In modern mammals these bones moved into the middle ear and become part of the ear ossicles as malleus and incus, Morganucodon is unusual because, apart from still having the primitive hinge, it has also evolved the derived mammalian one and so has a double jaw joint.

Its tooth replacement and determinate growth, are mammalian features.p324/346 Earlier synapsids replace their teeth throughout life, as sauropsids also do.
This is known as "polyphyodonty", while mammals grow only two sets of teeth, in "diphyodonty".
There has been a long controversy about whether or not to classify it as a mammal. Some prefer to include it in a broader clade called Mammaliaformes.
Morganucodon is regarded as very basal (= primitive). Its lower jaw has some of the bones found in its synapsid ancestors in a very reduced form. Mammals have a jawbone composed solely of the dentary.
The primitive jaw joint between the articular and quadrate bones is still  found in Morganucodon.p107/112 In modern mammals these bones moved into the middle ear and become part of the ear ossicles as malleus and incus, Morganucodon is unusual because, apart from still having the primitive hinge, it has also evolved the derived mammalian one and so has a double jaw joint.
Its tooth replacement and determinate growth, are mammalian features.p324/346 Earlier synapsids replace their teeth throughout life, as sauropsids also do. This is known as "polyphyodonty", while mammals grow only two sets of teeth, in "diphyodonty".
Morganucodon