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-
zoic and Cenozoic of North America. His work showed an increase in body size through the Cenozoic. But details of the overall size increase indicate that it happened at two different times and rates, rather than as a smooth and continuous increase. The first size increase occurred in the first few million years after the Cretaceous extinction and seems to have been a response by mammals to filling in now-emptied ecological niches. When the herbivorous dinosaurs died out, there was nothing around to browse higher bushes and trees and it became advantageous to grow larger. Large size also lends protection against predation and with the loss of the large, medium, and small dinosaur carnivores, a host of mammal groups began to enlarge. But a much larger increase in size happened much later, in an interval of 50 million to 40 million years ago in the Eocene Epoch. What caused this kind of size increase? One possibility is that it was at least enabled by a rise in oxygen.
The Berner oxygen curve suggests there was a rapid increase in atmospheric oxygen soon after the Cretaceous mass extinction of 65 million years ago. At the same time, mammal size increased. Coincidence? Probably not. As for dinosaurs, it may have been that rising oxygen had something to do with mammalian reproduction. A 2005 study by geochemist Paul Falkowsky suggested that the first appearance of "placental mammals" (the vast majority of mammals today, all of which have live births and nurse their young) could not take place until a critical level of rising oxygen was reached. Their argument was that prior to that time, there was insufficient oxygen within the placenta of pregnant female mammals to nurture developing embryos. This happened in the late Cretaceous.
A key aspect to understanding the evolution of mammals is discovering when the placental form of reproduction first occurred, for there may be a crucial and potentially limiting aspect to placental reproduction related to oxygen. The mother's arterial blood (which is oxygenated) mixes with venous blood (which is enriched in carbon dioxide) in the placenta. Fetal blood picks up its oxygen load from this admixture, and thus the fetus is exposed to oxygen levels that are lower than that of the mother's arterial blood. If the oxygen level of the mother's blood is already depressed because of living at higher altitude or, in the Mesozoic, living at lower atmospheric oxygen levels, the life of the fetus would be endangered.
Today our best model for understanding the life of animals in the lower-oxygen past comes from the study of animals at high altitude. Mammals can readily survive up to 28,000 feet; humans can certainly live, if only for a short time, atop Mt. Everest. But no mammal reproduces above 14,000 feet, which corresponds to the oxygen levels of the early Jurassic. And these are animals that have had 65 million to 100 million years to refine the placental system. The first evolved placentas would surely have been less efficient in delivering oxygen. This would seemingly suggest that the placental system of reproduction was not possible until oxygen blood levels had risen to, perhaps, Cretaceous levels—above 15 percent and perhaps approaching 20 percent. It thus looks as if the oxygen-level increase of the late Cretaceous—an oxygenation event that at least in part helped spark the major diversity increase in dinosaurs—also allowed the first evolution and successful implementation of a new kind of reproductive pattern, placental development (itself but one kind of live birth).
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