Re: Origin of body plans (phyla)

Arthur V. Chadwick (chadwicka@swau.edu)
Mon, 19 Oct 1998 16:28:55 -0700

At 05:07 PM 10/19/98 -0400, David wrote:
>>David, you are well aware that the trilobites are the first found metazoans
>>in most places in the world.
>
>Not anymore. There is a usually pre-trilobite basal Cambrian fauna of
>"small shelly fossils" and a late Precambrian fauna including some "small
>shelly fossils", Ediacaran-type organisms, and the like. I do not know
>what exact age the rumored Precambrian trilobites were supposed to be. If
>they do prove to be really from the Precambrian, it would push the origin
>of trilobites back again, but right now there are a lot of earlier
>metazoans than trilobites.

Trilobites are the first found metazoan forms in most places around the
world. The exceptions are exceptions and are clearly not lobopods.

>> Thus you are assuming "descendants" and "transitional", and then
>>using this assumption to justify your assumption that the evolutionary
>>history of the trilobites (or "arthropods") involves the lobopods, which
>>appear after the first arthropods. Does this strike you as a bit
>>tautological?
>
>A bit, but not entirely. Intermediate forms clearly do exist.
>Intermediate forms may reflect transition, convergence, or creation of
>intermediate forms. Many intermediate forms fit exactly how they would be
>expected to if they are transitional forms. Thus, it is not unreasonable
>to suppose that intermediate forms for which additional data are lacking
>may also be transitional.

Not if they first appear as fossils after their supposed progeny. I would
look for another theory. But then that's me.
Art
http://biology.swau.edu