"In the rise of the metazoans (animals), one would expect that soon after their appearance in the fossil record they would consist of a series of rather similar orders that would become increasingly more dissimilar to each other in the course of time. Yet the facts are astonishingly different from this assumption! When the metazoans appeared as fossils about 550 million years ago (admittedly they must have already existed for ca. 200 million years), they included four to seven bizarre body plans that soon became extinct. All the other Cambrian phyla survived, and what is quite unexpected, without a major revolution of the basic body plan. If we look at individual phyla, the same situation is encountered. The living classes of arthropods are already found in the Cambrian with the same body plans. But again there are a handful of strange types of arthropods in the Cambrian that do not exist today. I agree with those who conclude from this evidence that the variety of realized body plans was greater in the Cambrian than it is now. Furthermore, no fundamentally new body plan has originated in the 500 million years since the Cambrian." - Ernst Mayr, What Evolution Is, pg. 209
Thoughts? Ideas? Have any good theories been put forth to account for this?
Thoughts? Ideas? Have any good theories been put forth to account for this?
