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Re: a lot of fog (halkieriids)
> Halkieriids are sluglike animals with two separate dorsal shells (one
> anterior and one posterior).
*Halkieria* has that. AFAIK others don't... may depend on whether *Wiwaxia*
is included. All have sclerites = sort of scales made up of little tubes
that, according to Morris (Conway-Morris? I forgot :-] ), look a lot like
annelid bristles (chaetae/setae).
> He argues (and illustrates in fig. 86) that
> halkieriids evolved into brachiopods by decreasing the relative size of
> body and folding it up between the shells.
Brachiopods and phoronids fold up their bodies during ontogeny to arrive at
a U-shaped gut which is much more useful for their sessile filter-feeding
lifestyle. Would explain why (a) molecular phylogenies, when resolved, like
such a grouping, and (b) why on the planet brachiopods have annelid
bristles, among other things.
> As I have explained to him and others, this seems highly unlikely,
> it makes far more sense that the converse is true. Namely that a lineage
> brachiopods "unfolded" and the relative size of the body increased.
And then started to creep around again? Are you serious? IMHO that's a
gigantic reversal without an apparent reason and therefore against all
> shells of halkieriids persisted because they probably protected vulnerable
> areas of the body.
Exactly those where there were no sclerites :-)
> I think Simon is skating on thin ice in this case,
Hm. Homologizing the long "spiny" dorsal sclerites of *Wiwaxia* with annelid
notochaetae looks more like a speculation than a hypothesis at the moment,
though I can't think of evidence against. But arguing that the ventralmost
row of sclerites were used for creeping, much like the ventral scales of
snakes, and homologizing them with annelid neurochaetae sounds very good,
> It makes little sense to me and runs counter to a lot
> of other data.
What data? (offlist, please)