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Re: a lot of fog (halkieriids)



I know this case quite well. Simon Conway Morris is IMO one of the most brilliant of the world's invertebrate paleontologists. However, in this particular case, I think he has his character polarities backwards.
Halkieriids are sluglike animals with two separate dorsal shells (one anterior and one posterior). He argues (and illustrates in fig. 86) that halkieriids evolved into brachiopods by decreasing the relative size of the body and folding it up between the shells.
As I have explained to him and others, this seems highly unlikely, and it makes far more sense that the converse is true. Namely that a lineage of brachiopods "unfolded" and the relative size of the body increased. The shells of halkieriids persisted because they probably protected vulnerable areas of the body.
I think Simon is skating on thin ice in this case, and I have told him so. The Crucible of Creation is a wonderful "must-have" book in my opinion. However, I would take Simon's halkieriid-to-brachiopod hypothesis with a huge grain of salt. It makes little sense to me and runs counter to a lot of other data. But that is beyond the purview of this list, so I'll leave at that.
----- Cheers, Ken
******************************************
David wrote:
BTW, do you know the strange case of Simon Conway Morris: Crucible of Creation? The author finds, by means of various "halkieriids" that I drop here, the following

--+--Mollusca
   `--+--Annelida
       `--Brachiopoda



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