Essential collectibles #1: Diacalymene trilobite

In this series of articles we’re going to be looking at those fossils many people buy rather than collect. This doesn’t mean they’re less interesting of course, but because of the way they’re acquired hobbyists often don’t know much about them beyond what they are and very broadly where they’re from.

Our first such fossil is the trilobites of the genus Diacalymene that are collected in vast quantities from Silurian sediments in the Draa Valley of Morocco and exported all around the world. They’re usually all described as Diacalymene ouzregui, but taxonomists would probably argue that name has been used a bit indiscriminately!

In any case, apart from being abundant and therefore inexpensive, these particular trilobites are enduring popular because they’re such large fossils, anything between 8-10 cm is typical. That’s a lot bigger than the average trilobite, which means its a lot easier to see all their key features. Trilobites get their name, which means ‘three-lobes’, because if you look at one from the top, you can see it’s external skeleton, or exoskeleton, can be neatly divided into three parallel regions: the left and right hand sides which lay on top of the legs and gills, and the central region that went above the other organs including its digestive tract.

Diacalymene ouzregui is a typical Diacalymene species and shows all three regions clearly. It can also be looked at in terms of body segments. Like all arthropods, trilobites had bodies consisting of a series of segments each bearing a single pair of limbs or appendages, and in the case of trilobites, most segments had a single pair of gills as well. At the front were several segments that were more or less fused together to form the head, or cephalon, protected by a large, semicircular plate.

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