Évolution des mammifères

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Biarmosuchia

The Biarmosuchia were the most primitive and pelycosaur-like of the therapsids.

Dinocephalians

Dinocephalians ("terrible heads") included both carnivores and herbivores. They were large; Anteosaurus was up to 6 m (20 ft) long. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs. In many ways they were very primitive therapsids; for example, they had no secondary palate and their jaws were rather "reptilian".

Anomodonts

The anomodonts ("anomalous teeth") were among the most successful of the herbivorous therapsids — one sub-group, the dicynodonts, survived almost to the end of the Triassic. But anomodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw and it is generally agreed that they had beaks like those of birds or ceratopsians. 

Theriodonts

The theriodonts ("beast teeth") and their descendants had jaw joints in which the lower jaw's articular bone tightly gripped the skull's very small quadrate bone. This allowed a much wider gape, and one group, the carnivorous gorgonopsians ("gorgon faces"), took advantage of this to develop "sabre teeth". But the theriodont's jaw hinge had a longer term significance — the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear.

The gorgonopsians still had some primitive features: no bony secondary palate (but other bones in the right places to perform the same functions); sprawling forelimbs; hindlimbs that could operate in both sprawling and erect postures. But the therocephalians ("beast heads"), which appear to have arisen at about the same time as the gorgonopsians, had additional mammal-like features, e.g. their finger and toe bones had the same number of phalanges (segments) as in early mammals (and the same number that primates have, including humans).

Cynodonts

The cynodonts, a theriodont group that also arose in the late Permian, include the ancestors of all mammals. Cynodonts' mammal-like features include further reduction in the number of bones in the lower jaw, a secondary bony palate, cheek teeth with a complex pattern in the crowns, and a brain which filled the endocranial cavity.

Multi-chambered burrows have been found, containing as many as 20 skeletons of the Early Triassic cynodont Trirachodon; the animals are thought to have been drowned by a flash flood. The extensive shared burrows indicate that these animals were capable of complex social behaviors.

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The origin and early radiation of the therapsid mammal‐like reptiles: a palaeobiological hypothesis

The replacement of the basal synapsid pelycosaurs by the more ‘mammal‐like’ therapsids in the Permian was an important event in the history of tetrapods because it initiated the eventual transition to the mammals. It is also an example of taxon replacement in the fossil record that is unusually amenable to explanation, based on a combination of analysis of the biological significance of the inferred character changes, with the stratigraphic, palaeogeographic and palaeoecological circumstances of the time. An hypothesis is presented in which the origin of the therapsids resulted from a correlated progression of character evolution leading to higher levels of metabolic activity and homeostatic regulation of the body. It was a response to the availability of a seasonally arid, savanna‐like biome. The subsequent explosive radiation of therapsids was associated with habitat expansion made possible by the Mid‐Permian development of geographical continuity between that biome and the temperate biomes. The final extinction of the pelycosaurs was a case of incumbent replacement by the new therapsid lineages.

The tempo of the origin and early diversification of the therapsids

The hypothetical ancestral therapsid has been reconstructed as an actively hunting predator, of medium body size, which also proved to have had the potential to give rise to a range of different kinds of carnivores and herbivores. The earliest appearance of possible therapsids in the fossil record is the Russian early Kazanian (Fig. 3), dated approximately 267 Ma (Lucas, 2004). Unfortunately they are very poorly preserved and fragmentary (Efremov, 1954; Chudinov, 1983), leaving their true identification in doubt. However, by the later Kazanian‐early Tatarian of Russia, about 265 Ma, at least seven therapsid lineages are known to have existed (Ivakhnenko, 2003a). Carnivores are represented by biarmosuchians, basal gorgonopsians, and anteosaurid (brithopodid) dinocephalians. There were also the large, herbivorous estemmenosuchids, and three kinds of small herbivores, basal anomodontians and the as yet little known niaftasuchids and nikkasaurids. In the contemporary South African record there are also the more progressive, larger carnivorous therocephalians and herbivorous dicynodont anomodontians (Rubidge, 1995).