February 10, 2014
I’m still reading this book, The Eye: A Natural History, by Simon Ings. I keep getting sidetracked by all the interesting things I find in it and so my amazon.com wish list is getting longer with books I want to read. So, the trilobite, what an ancient critter. There were huge numbers of them, a prolific species living in the oceans between 250 and 500 million years or so ago…but now extinct so there are no living ones today. Apparently the Permian–Triassic extinction event killed practically everything, I mean upwards of 90% of every living animal it’s estimated, around 250 million years ago, eventually died. The little trilobites weren’t fit for whatever nasty events happened that long ago (look up Siberian Traps). But we have their fossils and in particular there’s one genus of trilobite called Phacops that was studied by Dr. Kenneth Towe, who is a paleobiologist at the Smithsonian Institution, which had something about it’s eyes that interested me.
These Phacops trilobites and other trilobites had eyes unlike most animals except our present day brittle star (which is similar to a star fish). It had something like compound eyes found in present day arthropods like insects. How vision worked for something that lived hundreds of million years ago is pretty hard to know but eyes are basically light detectors attached to a nervous system. Eyes were and are soft tissue in animals past and present so they are not the kind of stuff that usually fossilizes. But the eyes of a trilobite had a mineral crystal for it’s focusing lens and it’s no longer thought that the crystal developed because the trilobite fossilized. It’s clear that that was what the trilobite had in it’s eyes when it was alive. Guess what you can do with it now? You can look through it! You can look (with a microscope or other setup) through an animal’s eye that lived about 400 million years ago. Crazy!
The crystal lens in the Phacops eye is made of calcite, a calcium carbonate mineral that is optically clear, like glass. We don’t know why or how the genetics worked in this organism to produce this. But what I also find interesting is what this mineral, calcite, can also do optically. Have you ever heard of Iceland Spar or heard of the optics word, birefringence? I’d never heard of Iceland Spar before (I want some) but I do know about birefringence. Birefringence is an effect that occurs in a transparent material where it causes an image to double. I think the pictures below will explain what birefringence does. Also look at the photographs Dr. Towe took looking through a Phacops crystal lens. Back in 1973 he took pictures of smiley faces that he drew and a picture through a Phacops lens of a building across the street from his office in Washington, DC. It’s also interesting to note that in our own eyes we have a crystalline lens (we call it that….unless you’ve had cataract surgery) but it’s a cellular structure, not a calcite crystal rock. It’s not at all related to a rock, but we still call it a crystalline lens. There are a lot of different types of eyes in our world that suit a particular animal and the trilobites had a very unique type of optic material.
What an amazing feat though. Finding a fossil, finding the tiniest little particle, a crystal, and then being able to look through it. It’s just incredible what we can find if we spend some time to look at the details or even the larger perspective how things existed on this planet through the study of geology and fossils and my favorite, astronomy (of course, the other sciences too). There will be a lot of astrobiologists and other scientists looking at what might exist on other planets and how those organisms, if we find any life, live in their particular environment. The trilobite could be somewhere living on another planet. Who knows?
Thank you to Dr. Kenneth Towe for corresponding with me and providing some of his research regarding the Phacops trilobite.