Morganucodonta
| Morganucodonta | ||||||||||||
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Model of the Megazostrodon in the Natural History Museum in London |
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| Temporal occurrence | ||||||||||||
| Upper Triassic to Lower Jurassic | ||||||||||||
| 210 to 175 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Morganucodonta | ||||||||||||
| Kermack , Musset & Rigney , 1973 | ||||||||||||
The Morganucodonta are a group of extinct mammalian ancestors ( Mammaliaformes ) that lived in the Upper Triassic and Lower Jurassic .
description
Morganucodonta were small (around 10 to 12 centimeters long), outwardly probably shrew-like animals. In the structure of their temporomandibular joint they show transition features between the synapsid ancestors of the mammals and the actual mammals, so the primary temporomandibular joint between the articular and quadratic bones can still be recognized. Like modern mammals, they already had four different types of teeth: incisors , canines , premolars and molars . The mostly three molars of each half of the jaw each had three sharp cusps, the occlusion (the contact between the teeth of the upper and lower jaw) was well developed. The structure of the teeth suggests insects or other small animals as food.
As in today's mammals and in contrast to older forms such as Sinoconodon , there was a one-time change of teeth, with the molars only appearing when the teeth are permanent. Since this change of teeth is evolutionarily associated with suckling, it is conceivable that these animals suckled their young. However, since a step-by-step evolution of the teats and the associated suckling behavior of the young animals is assumed via a condition as we find it in the monotremes, in which initially only glandular fields are formed that are licked by the young animals hatched from the egg In the case of the described tooth change of the Morganucodonta, the possibility must also be taken into account that the one-time tooth change with molars in the permanent set of teeth could also have a completely different evolutionary history.
The rest of the physique also largely corresponds to that of mammals, although they still show some transitional features in the structure of the cervical vertebrae , the shoulder girdle and the pelvis . They were probably nimble, ground-dwelling animals.
Development history and external systematics
Morganucodonta finds are known from the Upper Triassic to the Lower Jurassic (around 210 to 175 million years ago). Fossils have been found in Europe, Asia, southern Africa and North America, which suggests that this group of animals is almost worldwide.
They are counted among a number of animals that have advanced mammal-like characteristics, but still differ in details from today's mammals and are therefore summarized as Mammaliaformes (mammalian-like) or as Mammalia sensu lato (in a broader sense). Whether they are already referred to as mammals or still as mammalian ancestors is largely a question of definition. Belonging to the Triconodonta , a group of mammals that was characterized by three-humped molars , but which has turned out to be no natural group, is rejected.
Internal system
The Morganucodonta are divided into two families, the Morganucodontidae and the Megazostrodontidae. However, some researchers consider the Megazostrodontidae more closely related to the Docodonta than to the Morganucodontidae.
The best-known genus of Morganucodontidae is Morganucodon (synonym Eozostrodon ), other genera are Erythrotherium from South Africa, Hallautherium (named after the place where it was found Hallau ) and Helvetiodon from Europe (Switzerland), Indotherium from India and Brachyzostrodon and Wareolestes from Western Europe. (The latter two genera are sometimes also assigned to the Megazostrodontidae.)
In addition to the eponymous Megazostrodon, the Megazostrodontidae also include Dinnetherium from Arizona (USA) and Indozostrodon from India.
literature
- Thomas S. Kemp: The Origin & Evolution of Mammals. Oxford University Press, Oxford 2005, ISBN 0-19-850761-5 .
Web links
- Morganucodontidae and Megazostrodontidae on Mesozoic Mammals ( Memento from September 10, 2012 in the Internet Archive )