March 20. Ordovician trilobites

Ordovician trilobite

Are you familiar with pill bugs? Roll-up bugs? I haven’t seen one in decades – maybe they don’t live here in the arid west, or heaven forbid, maybe I’ve lost my curiosity at what might be under a rock. If that’s it I’ll have to remedy that. But I recall well as a child in Michigan finding these little gray multi-legged critters in the garden. If you touched them, they’d roll up into a tight little ball. Their soft underbellies were protected by their relatively hard carapaces.

Pill bugs are arthropods like insects, centipedes, and spiders, but they aren’t closely related to them. They are actually crustaceans, isopods, more closely related to shrimp and lobsters. And of course, trilobites were arthropods too, and they shared with pill bugs the ability to roll themselves up into a defensive posture.

Flexicalymene, enrolled

Trilobites may have actually peaked during the Cambrian Period, but they certainly participated in the Ordovician diversification, with lots of new species appearing. They seem to get a little fancier in their ornamentation and development of spines and they clearly became adept at enrolling. Trilobites could definitely roll themselves into tight defensive balls early in the Cambrian, but for some time it was thought that the earliest trilobites couldn’t do it. But in 2013, a team from the University of Cambridge described some olenellids – early trilobites – from about 510 or so million years ago that could and did enroll themselves. So this is not strictly an Ordovician trilobite thing. But some Ordovician trilobites, such as Felexicalymene from the Ordovician of Ohio, Kentucky, and Indiana, made themselves famous by doing it.

—Richard I. Gibson

Ordovician trilobite photo by Vassil, under GNU free documentation license. Flexicalymene photo by Steve Henderson, used by permission.

Further reading:
http://www.trilobites.info/enrollment.htm
http://www.sci-news.com/paleontology/science-enrollment-trilobites-01420.html
http://www.livescience.com/39920-trilobites-curled-like-pill-bugs.html