Morganucodonta

Model of Megazostrodon at the Natural History Museum in London

• Eurasia, North America, South Africa

The Morganucodonta are a group of extinct mammal ancestors ( Mammaliaformes ) that lived in the Upper Triassic and Lower Jurassic.

Description

Morganucodonta were small ( approximately 10 to 12 inches long ), probably externally shrew-like animals. In the construction of their TMJ they show transitional features between the synapsids ancestors of mammals and the actual mammals, as is still the primary TMJ articular between Os and Os quadratum recognizable. But as today's mammals they had four different types of teeth: incisors, canines, premolars and molars. The most three molars each jaw half each had three sharp cusps, the occlusion ( the contact of the teeth of the upper and lower jaw ) was well developed. The construction of the teeth suggests insects or other small animals as food.

As with today's mammals and unlike older forms such as Sinoconodon there was a one-time change of teeth, the molars only appear when the permanent dentition. Since this change of teeth is brought evolutionarily with suckling in conjunction, it is conceivable that these animals suckled their young. There is, however, assumed a gradual evolution of the teats and the concomitant suckling behavior of pups over a state as we find him at the monotremes ( Monotremata ), in which first only glandular zones are formed, the licking of the hatched from the egg pups be, in the case of the described dentition of Morganucodonta also into account the possibility that the unique dentition could be related to the molars in the permanent dentition evolutionary history in a completely different context.

Also the rest of the body is consistent with that of mammals are largely the same, although they still have in the construction of the cervical vertebrae, the shoulder girdle and pelvis are some transitional features. It is likely to have traded with them for nimble, ground-dwelling animals.

History of development and external system

Finds the Morganucodonta are known from the Upper Triassic to the lower Jurassic ( about 210-175 million years). Fossils have been found in Europe, Asia, Southern Africa and North America, suggesting a nearly worldwide distribution of this group of animals.

They are counted in a number of animals showing advanced mammal -like features, but still differ in details from today's mammals and therefore as Mammaliaformes ( mammal -like ) or as Mammalia sensu lato are summarized ( in the broad sense ). Whether they are already referred to as mammal or even as a mammal ancestors, is largely matter of definition. Discarded, however, is one belonging to the Triconodonta, a group of mammals that was characterized by dreihöckrige molars, but which has turned out to be not a natural group.

Inside systematics

The Morganucodonta be divided into two families, the Morganucodontidae and Megazostrodontidae. However, some researchers consider the Megazostrodontidae for more closely related to the Docodonta than with the Morganucodontidae.

The best-known genus of Morganucodontidae is Morganucodon (synonym Eozostrodon ), other genera are Erythrotherium from South Africa, Hallautherium (named after the site Hallau ) and Helvetiodon from Europe ( Switzerland ), Indotherium from India and Brachyzostrodon and Wareolestes from Western Europe. ( The latter two genera are sometimes also attributed to the Megazostrodontidae. )

The Megazostrodontidae include, in addition to the eponymous Megazostrodon still Dinnetherium from Arizona (USA) and Indozostrodon from India.