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.
-- James Mahaffy (mahaffy@dordt.edu) Phone: 712 722-6279 Biology Department FAX : 712 722-1198 Dordt College, Sioux Center IA 51250