Morganucodon

Morganucodon is an early mammaliaform genus.The name comes from a Latinization of Morganuc, "South Glamorgan in Domesday Book", the county where it was discovered by Walter Georg Kühne.Walter G. Kühne 1949–1950. "On a Triconodont tooth of a new pattern from a Fissure-filling in South Glamorgan", Proceedings of the Zoological Society of London, 119, pages 345-350.Jenkins F.A. Jr. and A.W. Crompton 1979. "Triconodonta". Chapter 4 pages 74-90 of Mesozoic Mammals: the first two-thirds of mammalian history, edited by Jason A. Lillegraven, Zofia Kielan-Jaworowska and William A. Clemens. University of California Press: Berkeley, Los Angeles & London. p79 It lived during the latest Upper Triassic or earliest Jurassic.p299

Unlike many other early mammals, Morganucodon is known from abundant and well preserved material. Most of this comes from Glamorgan in Wales (Morganucodon watsoni).

According to Kemp, it is impossible to say whether the rock fissures (cracks), where the remains were found, are latest Triassic or earliest Jurassic.Kemp T.S. 2005. The origin and evolution of mammals. Oxford University Press. p142

Morganucodon first appeared about 205 million years ago.  "It was a small animal with a skull 2-3 cm in length and a body length of about 10 cm [4 inches]. In general appearance it would have looked like a shrew or mouse".p143

 Distribution 
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.Kielan-Jaworowska, Zofia; Richard L. Cifelli and Zhe-Xi Luo 2004. Mammals from the Age of Dinosaurs: origins, evolution, and structure, Columbia University Press, New York. p21/33, 174

 Physiology 
From their size and teeth, they may have been insectivores.Benton, Michael J. 2005. Vertebrate paleontology. 3rd ed, Oxford: Blackwell. p300
The small size suggests some kind of temperature regulation, which in turn suggests insulation in the form of fur. Nocturnal activity in an animal of this size virtually requires an increased internal production of heat.p124

Brain size is another factor. It is certain that early mammals did have enlarged brains relative to their size. Reconstructions of the brain size has shown this.Kielan-Jaworowska Z. Brain evolution in Mesozoic mammals. Contrib. Univ. Wyom. Spec. Pap. 3 21-34. Apparently, brain size in Mesozoic mammals lay within the lower range of brain size for living mammals. This is an overall increase of four or more times the volume of basal amniote brains. Very likely this involved the development of the neocortex, "one of the most striking of all mammalian characteristics".p120

 Is it a mammal? 
 thumb | right | Morganucodontidae and other transitional forms had both types of jaw joint: dentary-squamosal (front) and articular-quadrate (rear).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.Kermack K.A; Frances Mussett and H.W. Rigney 1981. The skull of Morganucodon. Zoological Journal of the Linnean Society, 71,  1-158.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,means juvenile growth has a definite stopping-point are mammalian features.van Nievelt, Alexander F.H.  and Kathleen K. Smith 2005. To replace or not to replace: the significance of reduced functional tooth replacement in marsupial and placental mammals. Paleobiology, 31.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".

 References 

Category:Theriodonts