Friday, 18 February 2011

Roll up! Roll up!

I have a couple of nice enrolled trilobites at home already, but bought this one in Tucson. It's a phacopid from Morocco, near Western Sahara. Think its name is Gravlops. Trilobites had a diverse array of body shapes and ornamentation, but most trilobites could roll up to some extent to protect their soft undersides with their exoskeleton. Some into balls like a hedgehog, some tucking into their broad headshield, some, like the tiny Agnostus, folding flat like a... like a... pastie? Phacops and its close relatives became masters of the ball defence, developing specialised grooves to enable a precise enrollment.

Many trilobites are preserved with a common sort of  bend, their cephalon at a ninety degree angle to their bodies. It's known that many were burrowers and these bent trilobites may have died while poking out of their burrows, head resting on the surface, the rest tucked safely away. What would they have been hiding from? Trilobites were around for about 280 million years, so what they were eating and what was eating them changed a fair bit over that time. To begin with, in the early Cambrian 520 million years ago, they would have had to hide from nautiloids, eurypterids and the star of the Burgess Shale, Anomolocaris. With the rise of the fish in the Devonian, trilobites had something else to worry about. The final curtain, though, was the Great Dying, the extinction event at the end of the Permian, 250 million years ago, when 96% of marine species kicked the bucket. Rolling up into a ball doesn't save you from everything.

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