Morganucodon

Morganucodon ("Glamorgan tooth") is an early mammalian genus which lived during the Upper Triassic. It first appeared about 205 million years ago. This has also been identified with Eozostrodon. "It was a small animal with a skull 2-3 cm in length and a presacral body length of about 10 cm [4 inches]. In general appearance it would have looked like a shrew or mouse."

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 the Yunnan Province in China (Morganucodon oehleri), in various parts of Europe and North America and some at least closely related animals (Megazostrodon) are known from exquisite fossils from South Africa.

There has been a long controversy about whether or not to classify it as a mammal at all and some authors prefer to include it in a broader clade called Mammaliaformes. Morganucodon is regarded as very primitive, for its lower jaw retains some of the bones found in its reptilian ancestors in a very reduced form, rather than being composed solely of the dentary. Furthermore, the primitive reptilian 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.

However, a consensus accepted by most scientists states that mammals as a group are defined by the possession of a special, secondarily evolved jaw joint between the dentary and the squamosal bones, which has replaced the primitive reptilian one between the articular and quadrate bones in all modern mammalian groups. Morganucodon is special in this respect, because apart from still having the primitive hinge, it has also evolved the derived mammalian one and thus features a double jaw joint. It is for this reason, and additional evidence pointing to diphyodonty and determinate growth, that Morganucodon is now usually considered a true mammal. (Reptiles repeatedly replace their teeth throughout life, known as "polyphyodonty", while mammals grow only two sets of teeth, in "diphyodonty".)

The name is often stated to mean "Morgan's tooth", but that would be Morganodon in Greek. Rather, it comes from a Latinization of Morganuc, "South Glamorgan in Domesday Book", the county where it was discovered by Walter Georg Kühne, so it means "Glamorgan tooth".

It has acquired the nickname Morgie in the Smithsonian National Museum of Natural History.

Classification
One classification of the close relatives (all extinct, variously from Late Triassic to Middle Jurassic) is:


 * Order Morganucodonta
 * Family Morganucodontidae
 * Genus ''Morganucodon
 * Genus Eozostrodon
 * Genus Brachyzostrodon
 * Genus Helvetiodon
 * Genus Erythrotherium
 * Genus Wareolestes