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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
the oxygen high of the Carboniferous-early Permian. Just such a finding has recently been made. In 2005, paleontologist Matthew Powell of Johns Hopkins University compiled voluminous data on the fates of various marine invertebrates during this oxygen high. He discovered very low rates of both origination and extinction. In other words, few new taxa appeared, and those that were already present rarely went extinct. The marine world was composed of an assemblage of virtual living fossils, which are characterized by long ranges (they existed for long periods of time) and produced very few new species.
Why did this happen? Powell invoked the presence of the late Paleozoic glaciation as the cause:
The Paleozoic history of marine life was interrupted in late Paleozoic time by a conspicuous interval of sluggish diversification and low taxonomic turnover. This unique interval coincided precisely with the late Paleozoic Ice Age.
Powell went on to suggest that the cool climate was the cause of this slow interval of evolution. Yet other times of glaciation seem to contradict this. One of the greatest extinctions of all time, that of the Ordovician (the mass extinction discussed in Chapter 4), has been blamed on the glaciation by most experts, and noted paleontologist Steve Stanley has suggested that cooling is a killer and cause of mass extinction. In our different view the sluggish evolution seen during the late Paleozoic is related to the high level of oxygen.
So how did all of this relate to the group we belong to and the group most people are interested in—the vertebrates?
OXYGEN AND LAND ANIMALS—REPTILES AND THEIR EGGS
Conquest of the land by chordates, our lineage, required many major adaptations. The most pressing was a way to reproduce that allowed development of the embryo in an egg out of water. The amphibians of the Pennsylvanian and Permian presumably still laid eggs in water, and thus they could not exploit the resources of land regions that were without lakes or rivers. The evolution of what is termed the amniotic egg solved this, and it was this egg that ensured the existence of a stock of vertebrates now known as reptiles. The evolution of the amniotic