Post by Joe Botting on Jun 29, 2005 5:24:23 GMT -5
Echinoderms are perhaps my favourite group of beasties, even including sponges. It's therefore a nice surprise to see a special volume of Geological Magazine devoted to the things, in honour of Chris Paul. It's got papers, among other things, on parablastoids, eocrinoids, and helicoplacoids, but the one I really wanted to draw attention to is by Andrew Smith, on 'carpoids.' the abstract is at:
www3.interscience.wiley.com/cgi-bin/abstract/110541515/ABSTRACT
For those unfamiliar with these fantastic beasts, they're probably the most controversial fossils on the planet. Depending on who you talk to, they're basal echinoderms, highly derived echinoderms, or somehere on the route to (or even within) the chordates (i.e. us).
Smith puts together a series of assumed characters that were present in the earliest echinoderms, based on comparisons with what molecular studies tell us are their closest relatives. He then looks for these characters in echinoderms, and finds them in the carpoids. Some of these intrepretations are similar to those of thingy Jefferies (the source of most of the controversy over the group!), but Smith has a different evolutionary arrangement. Rather than being the basal deuterostome stock that gave rise to all the existing phyla, Smith places them at the base of the echinoderms. The similarities with chordates are then shared through a series of largely unknown, soft-bodied ancestral deuterostomes with some resemblance to the hemichordates.
I'm not quite sure what this thread is for, except as a little celebration of how difficult the early 'echinoderms' are to deal with. It gets worse when you start looking at the numerous really obscure groups that turn up (often in only a handful of localities, if that) through the Lower Palaeozoic. They're incredibly diverse, and we still can't fit things together very well. For example, if Smith is right, we still don't know how to get from carpoids to edrioasteroids and/or helicoplacoids (the other earliest groups).
Oh, and if anyone has any ideas on what to do with 'problematicum A' in our faunal list, then all suggestions (humorous or otherwise) will be greatly received, because it's starting to give me a headache...
Joe
www3.interscience.wiley.com/cgi-bin/abstract/110541515/ABSTRACT
For those unfamiliar with these fantastic beasts, they're probably the most controversial fossils on the planet. Depending on who you talk to, they're basal echinoderms, highly derived echinoderms, or somehere on the route to (or even within) the chordates (i.e. us).
Smith puts together a series of assumed characters that were present in the earliest echinoderms, based on comparisons with what molecular studies tell us are their closest relatives. He then looks for these characters in echinoderms, and finds them in the carpoids. Some of these intrepretations are similar to those of thingy Jefferies (the source of most of the controversy over the group!), but Smith has a different evolutionary arrangement. Rather than being the basal deuterostome stock that gave rise to all the existing phyla, Smith places them at the base of the echinoderms. The similarities with chordates are then shared through a series of largely unknown, soft-bodied ancestral deuterostomes with some resemblance to the hemichordates.
I'm not quite sure what this thread is for, except as a little celebration of how difficult the early 'echinoderms' are to deal with. It gets worse when you start looking at the numerous really obscure groups that turn up (often in only a handful of localities, if that) through the Lower Palaeozoic. They're incredibly diverse, and we still can't fit things together very well. For example, if Smith is right, we still don't know how to get from carpoids to edrioasteroids and/or helicoplacoids (the other earliest groups).
Oh, and if anyone has any ideas on what to do with 'problematicum A' in our faunal list, then all suggestions (humorous or otherwise) will be greatly received, because it's starting to give me a headache...
Joe