Good morning, John.  Trilobites are super weird and I want you to know more about them.  It's easy to forget that the moment we live in is just this one snapshot of this huge history of life on Earth, and yeah, we have to and should and do study now but also there's a lot to learn if we look at the past and it can just be fascinating as well, which is why we've teamed up with PBS Digital Studios to make a new show  called Eons, and it's about the entire history of life on Earth.  

So in most biology is what's happening right now.  Eons is everything else.  All of the life that has happened so far until now.  Turns out there's lots of good stuff to cover here.  Our first episode about trilobites, of course.  The emergence of trilobites happened around 500 million-ish years ago.  It was part of what's called the Cambrian explosion where life on Earth got way more complex.  They were the first true arthropods.  Arthropods now take up the 3/4 of the world's species, so basically, good on you, trilobites.  Arthropods basically are the segmented, exoskeleton covered things that you see all around you right now.  Spiders, butterflies, millipedes, crabs, like those things, too, and even though horseshoe crabs share some family resemblance with trilobites and some horseshoe crab species did co-exist with trilobites, they are old horseshoe crabs.  The last species of trilobites went extinct around 250 million years ago, but even though they did not last forever, they were extremely successful.  They existed for 270 years.  Primates, for comparison, have existed for around 50 million years, so we have a lot of catching up to do.  We've found trilobite fossils on every continent and described over 15,000 species.  

One of the weird reasons why trilobites are so well-studied and why we find so many of them is that most trilobite fossils aren't actually trilobites.  It's nice being an organism whose skin grows with them, but that is not the case for arthropods.  If an arthropod wants to grow, it has to shed its exoskeleton and then move out of that old house that was too small for it, and when it does that, it leaves behind that shed exoskeleton which then can fossilize, and that happened a lot obviously with trilobites.  So the majority of trilobite fossils aren't the organism that died, they're the shed exoskeleton, which of course, we can still use to date the trilobite and then to figure out what kind it was and learn more about the species.

There are a bunch of reasons trilobites probably were so successful.  They had strong exoskeletons, they had jointed legs, but they also, this is a pretty big deal, had the first complex eyes, and trilobite eyes are freaking awesome.  Some of them have thousands of individual lenses and the lenses are made of calcite.  They're a crystal.  Like, the trilobite grew crystals in its eyes.  Our eye lenses, they can flex and change size, so if I look at something close up, I focus on it, I look at something far away, I focus on that, the lenses in my eyes are actually changing shape.  If your lens is made of rock, that's hard to do.  You also end up with a problem that astronomers had in the 17th century which is that you get weird distortions unless you use very specific lens shapes.

In the 1600s, two astronomers came up with two different lens shapes that helped to fix this problem.  Here's the one that (?~3:01) Huygens came up with and here's the one that Rene Descartes came up with.  Yes, that Rene Descartes.  He did a lot of stuff.  And here are the eye lens shapes for the trilobite Crozonapsis and the trilobite Dalmanitina.  This is so dang cool, right?  These lens shapes allowed for objects to be in focus when they were pretty close up or pretty far away and also got rid of some of that lens distortion.  It took humans 300 million years to catch up with trilobite evolution.  I just love it, and if you wanna learn more about trilobites, our first episode is up, youtube.com/eons where me, Blake, and Callie will be taking you through the entire history of life on Earth.  It's gonna be fun.  Join us there.  John, I'll see you on Tuesday.