Origin and evolution of Mammals

Early mammals evolved from reptiles some 200 million years ago. The history of mammalian origins is a complex story that is only slowly unfolding. The Permo-Triassic mammal-like reptile faunas of southern Africa, and the Jurassic mammalian faunas of Europe and North America, both discovered in the last century, have been central to the development of theories on the reptile-mammal transition.

There are enough evidences from extinct reptiles and mammals for this universally accepted view that mammalians had a reptilian ancestry. Feathers and hair evolved from reptilian scales and all three contain keratin.

For one thing, reptiles are cold blooded, which means that their body temperatures changes with the temperature around them.

In many cases this means that during the winter reptiles remain dormant because their body temperature is reduced to a point that precludes much activity. But even in warmer climates most reptiles must feed during the daylight hours, when it is warmer, and are less active during the night, when the temperature drops.

In contrast to reptiles, early mammals had the advantage of being warm-blooded. Because they have a constant body temperature regardless of their environment, mammals can compete with reptiles by remaining active throughout the night and throughout the year.

There are several special features that are linked to warm-bloodedness. One is the bones inside the nose and snout, called the turbinates. These bones increase the distance that air travels into the body, allowing it to warm up on the way in. There is also the bony palate which separates the mouth from the nose and allows for continuous breathing, even while eating.
Origin and evolution of Mammals

The first mammals

The Permo-Triasic mammal-like reptile faunas of southern Africa, and the Jurassic mammalian faunas of Europe and North America, both discovered in the last century, have been central to the development of theories on the reptile-mammal transition.

In the more than 500 million year long history of vertebrates, mammals do not appear until quite late, some 225 million years ago.

The first mammals had come into a world dominated by reptiles.  For one thing, reptiles are cold blooded, which means that their body temperatures changes with the temperature around them.

In contrast to reptiles, early mammals had the advantage of being warm-blooded. Because they have a constant body temperature regardless of their environment, mammals can compete with reptiles by remaining active throughout the night and throughout the year.

In early Triassic times, when Thrinaxodon was evolving in the direction of mammals, a new group of reptiles appeared on the scene; it was the dinosaurs or ‘ruling reptiles’ that came to dominate the animal world.

By late Triassic times, when the first mammals like Megazostrodon were scuttling around the undergrowth, large dinosaurs, such as Massospondylus were common. For the next 70 million years dinosaurs of every shape and size, from creatures no bigger than a domestic cat to the largest land animals the world has ever seen, took over the Earth.

Although the first Mezonoic mammal was discovered as early as 1764 in England, its significant was not understood until more than 100 years later, in 1871, when Sir Richard Owen (1804-1892) published his great opus, the ‘Monograph of the Fossil Mammalia in the Mezonoic  Formations.’
The first mammals