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Re: a lot of fog (halkieriids)



Sorry for not getting to this sooner. You can actually probably have peramorphic and paedomorphic changes happening simultaneously in an evolutionary lineage, depending on what part of the anatomy you look at.
One of the reasons I disagree with Simon Conway Morris on his polarity of "halkieriids--->brachiopods" is that he too easily dismisses lophophores and lophophore-like structures as being very homoplastic. And the molecular evidence is not nearly as definitive as it is being made out to be. And this "setae" thing is getting as complicated as protofeathers are. No simple answers.
Another very long-standing problem in zoology is the assumption that primitive bilaterians were some kind of motile worms (like flatworms). Many sessile filter-feeders may well be more primitive, and worms like flatworms may actually be relatively derived (and Vermes may be even more polyphyletic than we thought). I'll have to drag out my stuff on invertebrate evolution one of these days to refresh my memory (too busy now), but I particularly remember one article on bilaterian evolution by a Dr. Rieger in the 1991 book by Simonetta and Conway Morris (The early evolution of Metazoa). You might want to take a look at Rieger's ideas sometime.
------ Cheers, Ken
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Steve Brusatte wrote:
Okay, fine. I think both ideas are parsimonious. But, why exactly a paedomorphic change? Why not a peramorphic change like that seen in juvenile _Neocrania_? I'm curious to see what evidence you gathered from the literature (you can post it to the list or send it to me offlist when convenient...I know some of the best Olympic events are just about the begin! :-)




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