Among the vertebrates, a picture of gradual change is seen for mammals, with drastic reductions occurring only in the marsupials. The boundary also does not seem to have been a barrier for turtles, crocodiles, lizards, and snakes, all of which came through virtually unscathed. The dinosaurs did become extinct, and much argument has centered on whether this was abrupt or occurred after a slow decline. In this context, it must be noted that there is only one area where a dinosaur-bearing sedimentary transition across the boundary can be seen, and that is in Alberta, Canada, and the northwestern United States. Records of dinosaurs in this area during the upper part of the Cretaceous period show a gradual decline in diversity, with a drop from thirty to seven genera over the last eight million years. Although explanations of the extinction of dinosaurs have ranged from mammals eating their eggs to terminal allergies caused by the rise of flowering plants to the current ideas about bolide impacts, the answer may be climate related. A major regression of the oceans occurred at this point, resulting in a drop in mean annual temperatures and an increase in seasonality. The bolide impact may have served as the death blow to taxa (animals in classification groups) that were already declining.
The main period of evolutionary expansion in the Phanerozoic era is at the base of the Cambrian period, 544 million years ago. Termed the "Cambrian explosion," it marks the development of all the modern phyla of organisms, and as many as one hundred phyla may have existed during the Cambrian period. This period seems to have lasted only about 5 million years, and the subsequent history of animal life consists mainly of variations on the anatomical themes developed during this short period of intense creativity. This period is represented in the fossil record by the remarkably well-preserved Burgess Shale fauna of British Columbia, which has been extensively described, and faunas of similar age from China and Greenland. Why the Cambrian explosion could establish all major anatomical designs so quickly is not clear. Some scientists believe that the lack of complex organisms before the explosion had left large areas of ecological space open, and when experimentation took place, particularly with the advent of hard skeletons, any novelty could find a niche. Also, the earliest multicellular organisms may have maintained a genetic flexibility that became greatly reduced as organisms became locked into stable and successful designs. Why some of the innovations were successful in the long term and others were not is unknown, as no recognized traits unite the successful taxa. It has even been suggested that success may be due to no more than the luck of the draw.
In contrast, the recoveries after the major extinctions at the end of the Permian and Cretaceous periods did not result in the development of new phyla. The earliest Triassic ecosystems were more vacant than at any time since the Cambrian period, yet no new phyla or classes appear in the Tri-assic period. This suggests that despite the overwhelming nature of the extinctions, the pattern was insufficient to permit major morphological innovations, in part probably because no adaptive zone was entirely vacant. Hence, despite the fact that the mass extinction at the end of the Permian period triggered an explosion in marine diversity described as the Mesozoic marine revolution, persisting species may have limited the success of broad evolutionary jumps.
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