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
