February 7. Trilobites

Let’s talk today about some of the most common life forms that lived in Cambrian seas. Trilobites were arthropods, invertebrates with segmented bodies, jointed legs, and relatively hard exoskeletons on the outside of their bodies. The group includes insects, spiders, centipedes, and crustaceans like shrimp, lobsters, and crabs. And trilobites.

Trilobite is a simple name meaning three lobes, which reflects their basic body plan – a central lobe flanked by two more that cover the legs. They appeared during the Cambrian explosion, around 540 to 521 million years ago, and were incredibly diverse and successful – all told, there are more than 17,000 species of trilobites, and they survived for about 270 million years.

Trilobites are a kind of holy grail for geology students, at least those of us who lived in places where they were not common. Because of their many segments and legs and other parts, they tended to break apart when they died, so they are fragile and can present a challenge to collectors to find and retrieve, but when you do, it’s a treasure. Finding your first trilobite was a big deal – at least for me. I even have a trilobite etched on my mug down at Quarry Brewing.

They range in size from tiny – around a millimeter – to huge, at 70 centimeters or 28 inches long. Their exoskeletons were not hard calcium carbonate, like so many shells are, but were a material called chitin, more like humans’ fingernails, and containing calcium, phosphorous, and organic material. This adaptation certainly offered some protection compared to soft-bodied animals like jellyfish, and allowed them to be mobile as well.

Specific subdivisions of Cambrian time around the world are usually related to the kinds of trilobites found in a particular time interval. So you have the Olenellus Zone, or the Bathyuriscus Zone, or whatever. Trilobites are the yardsticks of Cambrian time.

Trilobites had compound eyes, much like many insects, with lenses made of calcite, calcium carbonate.   As we discussed yesterday, the development of eyes and light sensitivity may have created evolutionary pressure that drove, or even initiated, the Cambrian explosion. Trilobites seem to have hung out in mud a lot, and that’s good news, because the really fine grained sediment in mud lends itself to preserving details in fossils, even the eyes. And it might help that muddy sea floors might tend to be anoxic, stagnant areas, with less oxygen to attack and decompose the animals when they died.

We’ll talk about trilobites from time to time again over the next few months, to point out a particularly weird or interesting variety.

The Wikipedia page provides a reasonable overview about trilobites. If you’re geeky enough to like cool names for trilobite parts like the cephalon and pygidium, start there and move on to the Trilobites.info site. If not, just go with head and tail.

When Trilobites Ruled the World - NY Times, March 2014

Blog extra: Feb. 7, 1812, was the date of the fourth and strongest in the series of earthquakes that shook southern Missouri around the town of New Madrid. The Feb. 7 quake damaged buildings in St. Louis and rerouted the Mississippi River, creating Reelfoot Lake in what is now northwest Tennessee.

—Richard I. Gibson