Re: Skeleton of trilobites
David Campbell (bivalve@mailserv0.isis.unc.edu)
Fri, 30 Oct 1998 15:21:34 -0400
>Jonathan Clarke wrote:
>>
>> Dear Arthur & All
>>
>> The key phrase I used was "mineralised skeletons". Many crustaceans lack
>> mineralised skeletons, but get by very well with chitinous ones. My
>> understanding of the trilobite skeleton was that it consisted of > chitin
>>mineralised by calcite.
>
>I am not a trilobite expert, but I do teach undergrad paleontology on
>occasion and what Jonathon said about trilobite cuticles did not ring
>right so I looked it up in Clarkson (the text I use). He says that
>unlike most arthropod skeletons that are chitin and sometimes
>mineralized the cuticle is "largely of low magnesium calcite ..
>arranged in microcrystalline needles. .. they are, "set in an
>organic base whose nature has yet to be determined" (p.346) but he
>indicates that chitin has NOT been detected.
>
>If I am not mistaken the nature of their cuticle may be why they have
>unusual calcite lenses with interesting corrections for the nature of
>calcite. As far as I can recall no other invertebrates use calcite
>lenses. I don't know how that helps the argument but it means that
>going from a soft to hard exoskeleton could not be simple mineralization
>of a chitin's skeleton.
Some other arthropods do have calcite in their skeletons (most ostracods,
many decapods, etc.) At least ostracods have calcite over the eye. The
unusual feature of trilobites is that they have good eyes with calcite over
them. If you have encountered Iceland spar, you know that calcite crystals
have odd optical effects in two planes. Trilobites have the calcite over
the eye aligned so that the c axis (if I remember correctly) is the one
they look along and could see well (in the species that had eyes).
Ostracods, on the other hand, are probably badly astigmatic.