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Morganucodon comes from the Latin for "Glamorgan tooth". This early mammaliaform lived in the late Triassic. It appeared about 205 million years ago. It is represented by abundant and well preserved material, but most of this is disarticulated. It was discovered in South Glamorgan by Walter Georg Kühne [2] and has acquired the nickname Morgie in the Smithsonian National Museum of Natural History.[3]

Morganucodon was a small, plantigrade animal with a moderately long tail. Kemp (4) said it looked like a shrew or mouse with a skull was 2-3 cm long and a presacral body length of about 10 cm. It may have had glands used for grooming, so probably had fur like more advanced Mammaliaformes [5]. It probably ate insects and other small animals and grew fairly quickly to adult size. Unlike its therapsid ancestors, it probably lived a rather short life, like most small mammals today [6]. Its eggs were probably small and leathery, as in monotremes [7]. The teeth grew in mammalian fashion, with deciduous teeth replaced by permanent teeth, retained throughout the rest of the animal's life [8]. Unlike most later mammals, the upper and lower molars did not occlude properly when they first met; as they wore against each other; their shapes were modified by wear to produce a precise fit [9].

Morganucodon is the type genus for the order Morganucodonta, of generally similar mammals or pre-mammals from the Late Triassic-Middle Jurassic [10]. All were small and mainly insectivorous. There is controversy about whether Morganucodon was a mammal or a non-mammalian mammaliaform. Some researchers limit the term "mammal" to the crown group mammals, which would not include Morganucodon and its relatives. Others define "mammals" as possessing a special, secondarily evolved jaw joint between the dentary and the squamosal bones, replacing the primitive one between the articular and quadrate bones in all modern mammalian groups. Under this definition, Morganucodon would be a mammal, but its lower jaw retains some bones from its non-mammalian ancestors in a very reduced form rather than being composed solely of the dentary. 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 found in Morganucodon [11].

Most Morganucodon material comes from Glamorgan, Wales (M. watsoni), but fossils have been found in the Yunnan Province in China (M. oehleri), in various parts of Europe and North America and some at least closely related animals (Megazostrodon) are known from South Africa [1].


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