Evolutionary history of the siréniens

The Siréniens are marine mammals which, like the Cétacé S, do not need to regain the dry land at one unspecified period of their existence. However, the origin of these Mammifère S is necessarily terrestrial. As several characteristics of their anatomy point out those of the elephants, the scientists think that the herbivorous ancestor of the siréniens who became watery is different from that of the Cetacea which was in addition carnivorous.

The origin

The separation of the group of the siréniens with the others Tethytheria is older than that of the Cetacea and the Artiodactyla (10 My) or of that of the marine Caniformia (2-4 My).

Prorastomus , oldest of the found fossils of siréniens is a semi-watery animal which could still evolve/move on the dry land and was gone back old to 50 My. During the Eocene the number of fossils is diversified enough. These animals lived close to the coasts, along the rivers and the estuaries. The Prorastomidae and Protosirenidae were size of a pig with legs adapted to the amphibious medium. Towards the end of the Eocene, others siréniens having more the pace of Dugongidae appeared, their legs before having become fins and the tail transformed into horizontal caudal fin. Displacements were done by from top to bottom undulation like the Cetacea. The Trichechidae seem most modern of the siréniens and resulting from the Dugongidae . One has many fossils which correspond at several stages of the back reduction of the members to the disappearance of the Pelvis.

Analyzes fossils

Although one counts the teeth to identify many species of mammals, their number does not vary significantly among the siréniens. The parts most easily identifiable of the skeletons are the jaw lower and the cranium, especially the frontal cranial bones. Except for a pair of upper incisors similar to defenses, the front teeth (incisors and canines) miss, except at the first siréniens.

Genesis of this theory

Prorastomus was discovered in Jamaica.

The genetic analyzes of the Années 1990 show that Siréniens are Afrotheria and thus very far away from the Perissodactyla and Artiodactyla although they were arranged with the Proboscidea in the Ongulé S. the siréniens thus did not evolve/move starting from mammal with primitive shoes like Condylarths .

Cladogramme simplified: ─o Afrotheria └─o Paenungulata ├─o Hyracoidea └─o Tethytheria ├─o Proboscidea └─o Sirenia

See too

History of the Cetacea

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