The History Of Cenozoic Mammals

As we have seen, the ancestors of the mammals, the Paleozoic and early Mesozoic therapsids, were the dominant land animals until the Permian extinction, and even through the Triassic they maintained moderate diversity, if no longer being the most diverse and numerous of land animals as their Paleozoic forbearers had been. The first true mammals are found in rocks of late Triassic age—at about the time of the first dinosaurs, in fact. But while they may have both appeared at the same time, these two groups then went on to very different fates. The first dinosaurs were already relatively large animals for their time, about a meter long, but soon after they evolved into much larger sizes even before the end of the Triassic. But the first mammals may have been a tenth of the size or less of the first, meter-long dinosaurs, and they then stayed small—for a very long time. Why? This obvious question is all the more perplexing because some of the immediate ancestors of the first mammals, even forms that lived alongside them, such as the advanced cynodonts of the late Triassic, were the same size or larger than the early dinosaurs. The first mammals could have been larger, but they were not. Does this size limitation tell us something about their metabolism and constraints in the teeth of the low-oxygen high-heat interval?

Let's recount the ecological position of the first true mammals. It is the late Triassic and early Jurassic. The world is hot. And the world has oxygen levels as low as 10 percent and certainly less than 15 percent for tens of millions of years. Mammals then, as now, are presumed to be warm-blooded and furry. The biggest mystery is how they reproduced. Almost all mammals today are placental, where development takes place in the female to the point that the newborn is capable of existence outside the mother; a few are marsupial, where early birth prior to full development requires a period of time in a secondary pouch within the mother; and a very few, such as the platypus and 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 Meso-zoic, 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, warmblooded, 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.

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