Mammals Evolved

The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the pennsylvanian sub period of the late carboniferous period. In fact the first mammals evolved from a population of vertebrates called therapsids mammal like reptiles at the end of the triassic period and coexisted with dinosaurs throughout the mesozoic era.

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About 320 million years ago give or take a few million years the first true reptiles evolved from amphibians.

Mammals evolved. But part of this folktale has a grain of truth. In fact they nearly made it into the cretaceous and definitely coexisted with many of the major dinosaurs. The first mammaliaforms evolved from the cynodonts during the early norian age of the late triassic about 225 mya.

Cetaceans are fully aquatic marine mammals belonging to the order artiodactyla and branched off from other artiodactyls around 50 mya million years ago. The evolution of cetaceans is thought to have begun in the indian subcontinent from even toed ungulates 50 million years ago over a period of at least 15 million years. Interestingly the malleus and incus of modern mammals evolved from the lower jaw bone of the immediate predecessors of mammals the mammal like reptiles of the paleozoic era known as therapsids.

During the evolutionary succession from early therapsid to cynodont to eucynodont to mammal the main lower jaw bone the dentary replaced the adjacent bones. They do not breathe underwater as fish do so their respiratory systems had to protect the body from the surrounding water. Early mammals were small about the size of a small rodent.

Mammals evolved on land so all aquatic and semiaquatic mammals have brought many terrestrial adaptations into the waters. The cynodonts were the only mammal like reptiles to survive to the jurassic. Cetaceans are thought to have evolved during the eocene or earlier.

Warm bloodedness is believed to have first evolved among the cynodonts a late but successful group of mammal like reptiles from which the mammals evolved. During the jurassic the mammals remained small and mainly nocturnal living beneath the metaphorical feet of the great dinosaurs. The therapsids members of the subclass synapsida sometimes called the mammal like reptiles generally were unimpressive in relation to other reptiles of their time.

Valvular nostrils and an intranarial larynx exclude water while breathing and swallowing. The evolution of the mammalian condition mammals were derived in the triassic period about 252 million to 201 million years ago from members of the reptilian order therapsida. Mammals evolved from therapsids in the late triassic period as the earliest known mammal fossils are from the early jurassic period some 205 million years ago.

By the mid triassic there were many synapsid species that looked like mammals. With their scaly skin and semi permeable eggs these ancestral reptiles were free to leave rivers lakes and oceans behind and venture deep into dry land.

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