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Re: a lot of fog (halkieriids)



On Thu, 14 Feb 2002 18:48:20  
Tim Williams wrote:

>"The Crucible of Creation" is a marvellous book - I'd highly recommend
>it.  

Certainly.  It's a great little volume (I can almost fit it in my pocket), and 
I list it among my favorite paleo books.  IMHO, it's the best book on the 
Burgess Shale and animal evolution out there.  


 David Marjanovic wrote:
>>      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).

Conway-Morris says that the two shells were either absent or greatly reduced in 
_Wiwaxia_.  He then goes on to say that a specimen of _Wiwaxia_ shows a 
possible vestigial shell.  But, this specimen was disarticulated.  The best 
photos of articulated _Wiwaxia_ specimens that I have seen look like bundles of 
sclerites, with no obvious shells.  But, of course, better specimens are needed.

>> 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.
>
>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.

Yep.  One of Conway-Morris' major arguments is that the primitive extant 
brachiopod _Neocrania_ is active as a juvenile, possessing normal numbers of 
setae but without a shell.  However, the areas that will later house the shell 
are visible in the juvenile.  During ontogeny, as David mentioned, _Neocrania_ 
begins to secrete a shell, and actually folds about its middle so that the area 
that will eventually start to secrete the brachial valve directly overlies the 
position of the soon-to-be-secreted pedicle valve.    So, basically, a juvenile 
_Neocrania_, sans shell, looks like a halkieriid (a long, annelid-like animal 
with bristles and possessing areas of future shell growth that DO NOT overlie 
each other).

>>      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.
>
>And then started to creep around again? Are you serious? IMHO that's a
>gigantic reversal without an apparent reason and therefore against all
>parsimony.

I suppose what HP Kinman mentions is definitely possible.  But, the 
observations in extant _Neocrania_ support Conway-Morris' view.  The halkieriid 
folding is more parsimonious than brachiopod unfolding.  But, some additional 
observations of other extant brachiopods would be nice (both articulates and 
inarticulates).  I wonder if a study is underway...  It would be interesting to 
see if the ontogenic pattern seen in _Neocrania_ is the norm among living 
brachiopods.

>> The
>> shells of halkieriids persisted because they probably protected vulnerable
>> areas of the body.
>
>Exactly those where there were no sclerites :-)

Yup.  I wonder why the sclerites weren't "strong" enough to protect those 
areas, though.  Perhaps the "gut" region was just too vital and weak to be 
protected by mere mortal sclerites :-)

>>      I think Simon is skating on thin ice in this case, (Ken)
>
>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,
>IMHO. (David)

I don't know if it's speculation.  The observable similarities between the two, 
and the still rudimentary cladograms, make it seem more like hypothesis to me.  
It certainly seems plausible.  Only additional fossil evidence can falsify it, 
though...or perhaps some molecular studies using extant brachiopods and 
annelids.

>> It makes little sense to me and runs counter to a lot
>> of other data.
>
>What data? (offlist, please)

I'd also be interested in hearing what you have to say offlist, Ken!

Regards,
Steve

****
Steve Brusatte
Dino Land Paleontology
http://www.geocities.com/stegob
****


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