The end of the Ordovician and the colonization of the land

Abstract

The much improved fossil record of the Phanerozoic allows for a more detailed analysis of changes in animal and plant assemblages. Disappearance of some animals and plants, and the proliferation of new types can be at least partly quantified. Sudden extinction and rapid development of new groups of animals provide a picture of evolutionary history which suggests that there were rather spectacular extinction events as well as both sudden and gradual emergence of new forms. The extinction events are typically the most striking, with significant percentages of extant species wiped out in a very short time, even almost instantly. Extinction events were usually followed by rapid development of new types to fill the vacated ecological space, and this has happened several times over the last 500 million years. One of the most interesting developments in the early Phanerozoic was the proliferation of animals during the Ordovician, followed by a significant extinction event at or near its close. This was soon followed by the colonization of dry land previously inaccessible to life adapted to aquatic living. This innovation opened up an enormous area of ecological space and was a key step toward the Earth as we know it. An important part of geologic study is to understand what processes may be responsible for both extinction events and the way in which long-term geological changes may result in biological “opportunities”.