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this post was submitted on 16 Nov 2024
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Ah! So the same as the rest of their hard parts, I suppose. I suspected as much, but couldn't seem to find any paper that explicitly stated this.
Thank you!
In regards to use of calcite vs. chitin: doing a quick search: https://www.ualberta.ca/en/earth-sciences/facilities/collections-and-museums/treasury-of-trilobites/index.html
Were trilobites also unique in using calcite in addition to chitin in their exoskeleton? Do any extant arthropods use calcite in any significant way?
From my searching, it would seem that trilobites used calcite to reinforce their hard parts as well. Crustaceans have amorphous calcium carbonate reinforced exoskeletons through mineralization, but no calcite land arthropods do not seem to have this mineralization tho.
I'm no expert. Just like using Google Scholar to answer questions I find interesting.
What I find interesting, then, is what advantage the trilobites may have gained by using a basic mineral for the lenses vs. organic chitin. Chitin must have a transparent form in order to function for the eyes in modern creatures? Hmm.
I read in one paper that trilobites may have actually formed some kind of dual-layer in their lenses to compensate for the double-refraction property of calcite.
Seems to be easier focusing with the calcite as well as durability in the scratchy sand environment.
Additionally, more derived/later species had aspherical lenses to compensate for the spherical aberration and dual layer organic→calcite lenses to allow for more of a refractive gradient to counteract the double imaging of calcite alone.
Modern arthropods probably done use it on land because of an inability to accumulate it in an efficient manner. Water arthropods like crustaceans probably done because it's metabolically expensive to do these things when chitin alone can make for useful eyes.
Evolution is more of a "good enough" kinda situation.