Dr Neale Monks (UK).
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.
If you’re looking at a your Diacalymene ouzregui specimen head-on, you’ll see the cephalon is large, quite chunky, and with the middle region much more swollen in appearance than the left and right hand sides. This is entirely typical of trilobites, and this swollen region, called the glabella, is thought to have been where food was stored and digested.
On either side of the glabella there are often several pairs of obvious indentations or furrows. Take care not to confuse these with the eyes, which on most Diacalymene fossils are not obvious because the lenses aren’t usually get preserved. The eyes are the two round, almost spherical swellings on either side of the glabella. Trilobites were among the first animals to evolve sophisticated eyes, but they weren’t lensed eyes like ours but compound eyes like those used today by crustaceans and insects. Diacalymene had what are called holochroal eyes, the standard type of eye among trilobites. This had thousands of individual calcite lenses packed together, each with its own light-sensitive cells underneath. Although not especially good at image forming, this type of eye is excellent for detecting movement, and probably worked to help Diacalymene avoid predators.
Like most trilobites, Diacalymene depended upon their exoskeletons for defence. Looking at the segments immediately behind the head you can see they’re similar in shape, individually quite short, and even as a fossil the way they’re arranged gives the definite impression of being connected together in a flexible way. This part of the body is called the thorax, and there are about a dozen or so individual segments in the thorax of an adult Diacalymene. The legs and gills were underneath these segments, and as the trilobite moved over (or even through) the sediment these segments would have been able to move as required to provide just the right amount of flexibility, much as you can see today with things like millipedes and woodlice.
Underneath the head was a structure called the hypostome, and though it’s sometimes referred to as the animal’s mouth, it’s perhaps better to think of the hypostome as the structure upon which the mandibles (or mouthparts) were attached. Relatively unspecialised trilobites like Diacalymene were probably deposit feeders, meaning they shovelled sediment into their mouthparts, sifted away sand and silt, and extracted tiny invertebrates, algae, and organic detritus.
Of course trilobites were sometimes food items for larger animals too. Their sensitive eyes were an ‘early warning system’ of sorts, and if alarmed, they’d roll themselves up into a ball that protected their delicate legs and gills. The final piece in the defensive puzzle was the pygidium, the several tail-end segments fused together to form a robust plate that helped protect the back end of the animal. Again, this is a feature that is easily seen on Diacalymene, though only in a relatively unspecialised state where the individual parts of the pygidium still resemble the segments that make up the thorax. In other trilobites the segments that make up the pygidium are so extensively modified that the entire thing looks like a single large shield.
Having looked over the upper surface of our Diacalymene, we might turn it over to look at its undercarriage, but alas, there won’t be much to see. For a start, most trilobite fossils are what remained each time they moulted. Like crabs, they had to do this to grow, and while the top half of the exoskeleton was tough enough to fossilise relatively frequently, the more delicate limbs and gills hardly ever did. But in the case of the Diacalymene fossils exported from Morocco, most are casts and moulds of the exoskeleton rather than the exoskeleton itself, so it’s a bit like looking at a plaster cast of a footprint rather than at the foot itself.
While Diacalymene might not be the most exotic trilobite, it’s one of the best for learning about the way these animals worked. Good specimens are not expensive, but do look out for those that have been tweaked a little with filler or careful chisel-work! Even if you end up collecting your own trilobites from Wales or wherever, they’re unlikely to have the sheer presence of the average Diacalymene, and that alone guarantees their place on our list of essential collectibles.