The trilobites arse

One of the commonest fossil parts of a trilobite which can be found is it’s arse. This bit of the trilobite, called the pygidium, is one whole piece with all the segments fused together. Like other animals with a hard exoskeleton, trilobites shed their outer shell as they grew larger. The head part, the cephalon, may  break up after shedding because it has lines of weakness which help with an otherwise complex shedding. The middle part, the thorax, sheds into many different sections as this part is made up of lots of individual segments. But the arse, it will shed whole, and if lucky, it will be buried and preserved in one piece.


A simple diagram illustrating the basic anatomy of a trilobite. (Image from here)

A simple diagram illustrating the basic anatomy of a trilobite. (Image from here)


In my time I have seen many a trilobites arse. I remember getting a little too exciting in the beautiful bleak wilderness of southern Spain, as my hammer struck a lucky bedding plane: the sun beating on my naked back, the beads of sweat dripping from my forehead, as I opened the rock to reveal the most perfect trilobites arse I have ever seen. Another time, as the rains lashed down in wet and windy Pembrokeshire, every strike of my trusty geological hammer exposed groups of arses.

Nothing beats that moment when the hammer strikes the right bedding plane, opening a rock that has been sealed for over 400 million years and being the first eyes to see the remains of an animal that scurried along an ancient ocean that no longer exists.

This weeks bad cast is a trilobites arse. I have seen many different types of trilobites butts in my time, from the perfectly round to the elaborately frilly. Unfortunately I am no connoisseur in this area. So I sent an image to a few people who really know their arses. Experts at the Natural History Museum, London were able to identify this trilobite.


Just a goddamn fine looking trilobites arse.

Just a god-damn fine looking trilobites arse.


This is actually a pretty interesting bad cast. It is two casts; one of the part and one of the counterpart. The part is the rock which has the fossil (the cast on the left). The counterpart is the section on the right and has the imprint of the fossils. The counterpart is important because sometimes it has some detail of the original fossil preserved. An interesting example is the counterpart slab of the dino-bird that never was, Archaeoraptor, which showed that this ‘fossil’ was a fake. Sometimes on very finely preserved fossils, feathers, hair or other fragile parts may be on the counterpart.

Our part and counterpart has clearly been put back together several times, chipping off the crude grey paint in the process. The experts identified this arse as belonging to a Genus of trilobites called Dalmanites. The pygidium of the genus has an elongated spine at its tip. These are really beautiful trilobites, with gorgeous headshields, perfectly streamlined bodies, and cute asses that end with a long spine.


A beautifully, perfect, Dalmanites trilobite. (Image from here)

A beautifully, perfect, Dalmanites limulurus. (Image from here)


The Genus of our bad cast is identified. But there were a number of different species within this genus. I found the card since the experts identified it. Scribbled in terrible old school handwriting is ‘Dalmanites rasutis‘. But I cant find this species. At least we have he Genus of a pretty awesome group of trilobites with a elongated spine at the tip of its arse. As with all trilobites, Dalmanites were marine creatures – no fossils have been found in freshwater environments. Its beautiful streamlined body indicates it was a swimmer, but stayed close to the sediment, as its large, curved eyes were stuck out from its head giving a great view for approaching predators.

We don’t know what the spine was used for, perhaps defence, or even digging in the sediment to lay eggs. What I do know is that this arse is amongst the finest in the trilobite kingdom.

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