As the spread of Mesozoic flora proceeded, so did development of animal life. Largely aquatic during the Paleozoic, Mesozoic fauna developed any number of land-based forms of a reptilian nature. The best known among them are known today as Dinosaurs.
Based on The Record of the Rocks there were several classes of reptiles as follows:
And then there were the birds, with the earliest known variety being the Archaeopteryx.
Little is known as to why most of the reptiles disappeared. Any number of theories ranging from Meteors to Ice Ages have been advanced. All that is really known is that their disappearance is rather sudden. Another thing that has been advanced consists of competition with mammals, except that there is precious little evidence of mammals having existed during that period. Indeed, it isn't until the Cretacious Period that The Age of Mammals dawns upon the Earth.
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