echidna, lay eggs. Unfortunately, we have no record of mammal eggs, and the fossil record is utterly opaque in telling when the first placental mammals occurred. But there is indirect evidence that is telling. DNA work (based on the molecular clock) indicates that the divergence of major groups of placental mammals—such as divergence into insectivores, carnivores, and artiodactyls, among many other major groups, happened between 100 million and 60 million years ago. Thus, some of the major divisions of mammals predated the Cretaceous extinction. But prior to that extinction all of these forms were small, and not just in the late Cretaceous. All Mesozoic mammals were small, from the first in the Triassic until those of the latest Cretaceous.

One possibility for this small size was so as not to compete with the dinosaurs. No dinosaurs occupied the rodent niche in the Mesozoic, and, as we know all too well today, there is a good living to be had if one is rat-sized or smaller. Under this scenario, mammals did not compete with dinosaurs for ecological reasons. But it may be that other reasons were involved. Perhaps in the hot, low-oxygen world (at least of the late Triassic until the end of the middle Jurassic), a larger, warm-blooded, highly active mammal—an animal that needed to eat much more than a cold-blooded form—was just too energetically expensive to exist. Here, as in Chapter 9, is the idea that dinosaurs were a really different kind of beast than anything we know today and, at least until the Cretaceous, were the only kind of animal that worked really well in the peculiar early and mid-Mesozoic conditions on Earth.

OXYGEN AND THE SIZES OF MAMMALS

We know that the disappearance of the dinosaurs unleashed a torrent of evolution, producing, in short order, many mammalian taxa. And for the first time, true mammals of larger size evolved. Thus, after the Cretaceous extinction, after the smoke had cleared and the dinosaurs were no longer around, large mammals did begin to appear. But it took a while. Santa Barbara paleontologist John Alroy has meticulously studied the sizes of mammals through time. Through dint of hard work in the library and among numerous dusty museum drawers, he tabulated average size for over 2,000 kinds of mammals from the late Meso-



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