(endothermy) allows animals using it to stay at a constant, warm temperature no matter how cold it gets. However, this can work against animals in very hot climates, as it is more difficult to cool off than warm up. Cold-blooded animals match the temperature around them. They are sluggish in the cold. There is a third kind of metabolism, found in animals so big that they are largely unaffected by daily swings in temperature, such as daytime and nighttime. The very large dinosaurs presumably used this system.

Other important aspects that might be related to the environmental conditions in which the various clades evolved and then lived include the presence or absence of scales, hair, and feathers. With thermoregulation pathways, the question of whether or not dinosaurs were warm-blooded has been the most discussed and most controversial of all. The fact that each of these characteristics, thermoregulatory systems and body covering, is primarily either physiological or involves body parts that only rarely leave any fossil record (such as fur) is in large part responsible for the controversies. We know that all living mammals and birds are warm-blooded, with the former having hair and the latter feathers, just as we know that all living reptiles are cold-blooded, with no hair or feathers. The status of extinct forms remains controversial. Of interest here is how oxygen concentration primarily and characteristic global temperatures secondarily affected thermoregulation or characteristic body coverings in animal stocks of the past. Let’s look at each of the three stocks in more detail with this overarching question in mind.

Diapsids

Openings are used to lighten the reptile skull, and their number (or lack of) is a convenient way of differentiating the three major stocks of “reptiles.” Anapsids (ancestors of the turtles) had no major openings in their skulls; synapsids (ancestors of the mammals) had one; and diapsids (dinosaurs, crocodiles, lizards, and snakes) had two. The earliest member of the diapsids is known from the latest Pennsylvanian rocks around 305 million to 300 million years ago, and it was small in size, about 8 inches in total length. From the time of their origins until



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