Sternaspid "shells"

Ken Kinman kinman at HOTMAIL.COM
Sat Aug 17 03:44:58 CDT 2002


     I guess I just have a much broader view of what can be called a shell,
and that we shouldn't restrict it to those which have just calcium
carbonate, since the shells of some brachiopods (lingulids and relatives)
are calcium *phosphate* and chitin.  Just because calcium carbonate is a
widely used as a mineralizer of shells that does mean all animals have to do
so.
     More importantly, Bartolomaeus says that the anterior third of
Sternaspid's "shield-like plates" are much less densely packed with the iron
particles.  There are clearly other materials in these plates, and I
wouldn't be surprised if they included chitin and/or calcium compounds.
     Sipunculids, which are probably even more primitive than sternaspids,
have a shield at the posterior end which is partially calcified.  Basically,
a sipunculid is probably a more primitive mollusc-annelid intermediate, but
in which the shell (or shells) have been lost.  Sternaspids are probably
more advanced in having segmentation, but still retain the shells.  That
they incorporate iron into their shells does not disqualify them as being
shells.
     Call them plates or valves (if these more neutral terms will get me off
the hook), but I still think they are homologous to the shells of bivalve
molluscs.  The anterior end of the bivalve's body has greatly expanded (and
I would say the same thing happened to sipunculids), but remember that their
ancestors may be very primitive forms of bivalve molluscs that are extinct
and more primitive than even living protobranch bivalves.
     And remember that virtually all bivalves of the PreCambrian probably
had unmineralized shells (perhaps similar to the softer ones found on some
sternaspids).  That some forms like sternaspids would mineralize with iron
is not particularly surprising.  A similar kind of iron particles are found
in the radular teeth of some chitons.  And remember that this iron material
is also found in the chaetae of Sternaspis, and that doesn't disqualify
these structures from being real chaetae.
      Therefore, I will continue to refer to Sternaspid paired shields as
"shells", and also assume that they are homologous to bivalve shells,
whether they are directly descended from bivalve molluscs or not (but my
intuition is that they are bivalve descendants).  I may be rather
imaginative, but it is based in a very wide reading of the literature that I
have access to.
           ------ Cheers,
                       Ken
*****************************************
>From: Kirk Fitzhugh <kfitzhug at NHM.ORG>
>Reply-To: Kirk Fitzhugh <kfitzhug at NHM.ORG>
>To: TAXACOM at USOBI.ORG
>Subject: Re: Bivalves; and Feduccia newsrelease
>Date: Fri, 16 Aug 2002 13:10:27 -0700
>
>At 07:52 PM 8/16/02 +0000, Ken Kinman:
>>I am particularly interested in the shells of sternaspid polychaetes
>>(perhaps remnants that offer a clue that polychaetes might have evolved
>>from bivalves??).
>
>
>Sternaspid polychaetes do not have shells. There are some contiguous
>posterior segments with sclerotinized tissue, forming what is called a
>"ventral shield."
>
>KF
>
>------------------------------------------
>
>FELIX QUI POTUIT RERUM COGNOSCERE CAUSAS.
>
>------------------------------------------
>Kirk Fitzhugh, Ph.D.
>Associate Curator of Polychaetes
>Research & Collections Branch
>Los Angeles County Museum of Natural History
>900 Exposition Blvd
>Los Angeles CA 90007
>Phone:   213-763-3233
>FAX:     213-746-2999
>e-mail:  kfitzhug at nhm.org
>------------------------------------------



_________________________________________________________________
Chat with friends online, try MSN Messenger: http://messenger.msn.com




More information about the Taxacom mailing list