That don't look like no shoe I ever seed on a horse!

That is, of course, a Horseshoe Crab.

Why would I start with a picture of a modern day crab when this post is about ancient critters?

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Well, the Horseshoe crab is the closest living relative of todays subject: Trilobites.

Trilobites were one of the planets most successful animals. They are in the fossil record continuously from the Cambrian explosion, ~550 million yeas ago, to the Permian Extinction Event, ~250 million years ago. For 300 million years, trilobites lived and propagated happily. They survived several extinction events that killed off many creatures that shared the same habitat.

No one knows for sure why they were so resilient, nor why they finally died.

What is known, however, is trilobites were about as diverse as dinosaurs were. So far, there are over 5,000 identified trilobite species, with more being found everyday.

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So, what is a trilobite? It was an arthropod. You could almost consider it to be like an underwater cockroach. (not that they were pests, but that they were everywhere)

The name trilobite refers to the fact that they are all made up of three lobes, the center Axial lobe and the Left and Right Pleural lobes. Their bodys were also segmented into three distinct parts, the Cephalon, (head), Thorax, (body), and Pygidium, (tail). All trilobites, from the smallest to the largest, all followed this body plan.

Trilobites mostly scurried on the sea floor grazing for food. Some would grab food and shove it up into their mouths, while others were filter feeders with large, circular filters at the tip of the cephalon.

**NOTE** By now, you're probably wondering where the pictures are. Well, in yesterdays post, I had issues with the pictures and text formatting correctly. Also, I could spend all year putting pictures of trilobites in this post. Instead, I'm saving the pictures for the end of the post.

Where were we? Ah yes, I remember now. Some trilobites would eat anything. They were scavengers taking what they got when they got it. Others were actually predators. Ichnofossils, (trace fossils that don't include the creature), have been found that show trilobite tracks homing in on burrowing worms, and buried hiding places where they would lie in ambush.

Trilobites were also prey. There were a lot of creatures that ate trilobites. One of the main predators was the Anomalocaris. Trilobite fossils have been found they bite marks that match the patterns of good old Anomalocaris.

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When it felt it was in danger, the trilobite would enroll. Basically, it would roll itself into a ball, tucking it's tail under it's head. This meant it's exoskeleton surrounded the entire trilobite, making it harder for predators to eat it. Plus, many trilobites developed spines, so when they enrolled, they were little spiky balls of mouth pain.

Trilobite fossils don't actually consist of the trilobite, but only it's exoskeleton. Like some modern day arthropods, trilobites had a hard exoskeleton that they would outgrow. They would shed their shell about a dozen times in their lives. You can find trilobite fossils that are shed fossils or you can find examples of the shell that the trilobite was wearing when it died.

There are a couple of easy ways to tell a shed apart from a death fossil. A shed will usually be missing the Free Cheeks. The free cheeks are two segments, one on each side of the head, that break off and allow the trilobite to crawl out from under it's shell.

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The other easy way is to see if your fossil has a Hypostome. The hypostome was a small piece of skeleton that sat under the head. It protected the brain, mouth and partial digestive tract. It wasn't affixed to the rest of the exoskeleton, so if your fossil has one, the trilobite died in that shell.

Occasionally, when the conditions are just right, some soft flesh will be fossilized. This happens with all creatures, not just trilobites. I posted a wonderful picture of a trilobite with legs and antenna preserved in the comments to Mondays post. In the case of trilobites, it has allowed us to discover that they breathed with their legs. Their legs had wispy gills on them that filtered oxygen out of the water and supplied it directly to the blood stream.

For a long time, one of the biggest arguments against evolution, in fact, it was the thing that gave Darwin nightmares, was the eye. How does the eye evolve? Half an eye would be useless, right? Well, trilobites provided the answer.

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Trilobites eyes are preserved as part of the exoskeleton. They are made up of calcite rods that were connected to optical sensors on the trilobites head. Trilobite eyes were multi lensed, and circular in shape. Some go almost 280 degrees.

But, the make up of the eyes tell us that, while a trilobite could see, it would be blind compared to us. All it could see was light. Picture seeing nothing but a flat, grey background. As you walk, a tall, thin darker object appears. It doesn't move, so it's probably a plant. Another slightly dark spot moves across your field of vision. It's small, so not likely a threat. Suddenly, everything gets darker. Uh oh! That's bad. *Boom!* You're lunch.

So, trilobites could see, but they couldn't see much. As the eye evolved, things like sharpness, clarity, focus and color started making an appearance.

But, without the trilobite, the eye may still be stumping us.

I suppose I should shut up and just show you the pictures, huh? Well, all the pictures included here, except for the one at the beginning, all come from my personal collection of trilobites. I apologize for the quality. My camera is my phone, and I am by no means a photographer. I am including only a small part of my collection. To get a better idea of how odd a trilobite can be, do an image search for them. I am unfortunately too poor to be able to afford the truly spectacular specimens in my collection.

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I have numbered the pictures, and short explanations follow the set. Oh, and I intentionally held some of them to give a sense of scale.

I will also break from my modus operandi and I will include a link to a wonderful source of information about trilobites if you're interested. There are some nice pictures, and the site is run by one of the leading experts on trilobites today.

And now, on to the pictures!

1 - Paralejurus

2 - Paralejurus with hypostome exposed

3 - Koneprusia. A spiny mouthful of ouch!

4 - Koneprusia, again, because I like it.

5 - Leanchoilia. A trilobiteomorph. Not technically a trilobite, but very closely related.

7 - Phacops, showing detail of eye.

8 - Hard to see, but it's missing it's free cheeks.

9 - Crotalocephalus

10 - Crotalocephalus, showing the hypostome.

11 - Microscopic trilobite larvae.

12 - Oops. This is a Keichousaurus, not a trilobite. It lived durring the time of the dinosaurs, but is an amphibian, not a dinosaur.

13 - Andalusiana. One of the larger species.

14 - Sometimes the small ones are left in the original stone matrix.

15 - Elrathia. The teenage years.

17 - Peronopsis. A blind trilobite.

18 - Enrolled Flexicalymene. Like a blowpop! Crunchy on the outside, chewy on the inside!

19 - Tiny and Spiny!

20 - One of my cabinets with just a few of my fossils. Crappy pic, but I'm too lazy to take a better one.

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