(January 23, 2014 at 3:29 pm)Pickup_shonuff Wrote: "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?
I think there are two question in your post:
Q1. What accounts for the sudden fossil appearence of metazoans.
Q2. What accounts for the fact that instead of start with a small number of simple body plans (phylums) and gradually diversify into larger numbers of body plans (phylums) over the entire fossil history of metazoans, metazoans instead appear to burst upon the scene with a very large number of body plans, but only a few survive a long time afterwards?
A1. This perception is increasingly attributed to fossilization bias (conditions prior to cambrian didn't favor fossilization, thus while fossils appears to abruptly increase at number of type at beginning of cambrian, animal life had really had a long, but poorly documented history on earth prior to cambrian). There are increasing amounts of fossil evidence since late 1980s to show complex animal life most probably existed not for millions, but hundreds of millions of years prior to beginning of cambrian. They left sparse fossil evidence, but they did leave some fossil evidence.
A2. The notion that there were many more basic body plans in Cambians fossils than in the modern world was popular in late 1980s and early 1990s, notably championed by Harvard Paleontologist Steven Gould.
But that position has been met with increasing skeptism since the 1980s. Many seemingly unusual fossils judged to lie outside of any currently existent body plans by during the 1980s by Gould are now recognized to either fit within existent phylums, or are very close relatives of currently existent phylums, and definition of existent phylum can be slightly and reasonably modified to acommodate them. All modern phylums still do appear to have already existed at the beginning of Cambrain, but this does away with the notion that many more phylums had existed in Cambrian than now. More recent analysis also seem to suggest diversity in fact remained low but gradually increasing through cambrian, and only achieve levels comparable to today after the end of cambrian. Also, genetic evidence suggests all the modern phylums didn't split off from each other more or less at the same around beginning of cambrian, but instead over a period of 300 million years or so before cambrian.
So to wrap up - Although the case isn't closed, it does seem there were:
1. No real explosion, only the appearence of an explosion.
2. A large number of phylums didn't all explode into existence at the same time and then get whittled down. Phylums arose slowly and it appears once a phylum is established, nothing yet happened on earth can cause a whole phylum to go extinct.
