A quick lesson in Trilobite anatomy

It’s back. And this time it’s badder than before. This weeks Bad Cast is one of a group of trilobite casts that were just so bad, I had to send photos of them to trilobite experts (they really do exist). The great Richard Fortey and colleagues at the Natural History Museum, London had a look at the photos for me. They actually managed to identify them  too, well not down to species level, but better than I could have done.

This bad cast belongs to an Order of trilobites called Illaenina. The cast is so bad, even world experts coudn’t get a more detailed identification.

This is a trilobite.

This is a trilobite. The head part is the bulbous section on the left. The middle, you may be able to see faint segments, and the rounded bit on the right is its bottom.


Somewhat looking like a Scandinavian artists attempt at moulding breasts, this bad cast is actually from a mould of a real fossil. These were a funny looking group of trilobites: with massively rounded heads (cephalon) and massively rounded bottoms (pygidium). The faint ‘ribs’ in the middle (the thorax) contain only a few segments. Compared to your average trilobite, this was a very strange group indeed.


A classic sketch showing trilobite anatomy. (Image from here)

A classic sketch showing trilobite anatomy. (1) Cephalon (2) Thorax (3) Pygidium (4) Right pleural lobe (5) Axial lobe (6) Left pleural lobe. (Image from here)


Trilobites have a thick exoskeleton protecting their soft, life-giving insides. In many trilobite groups, the cephalon has many clearly defined areas: the glabella (a large bulge in the middle of the head), eyes, facial sutures (to make moulting easier). Some even had elaborate spikes protruding from the head for defence. Our little trilobite has lost clear definition of these features. All edges have merged together leaving a smooth, well rounded head. The eyes are just about visible, poking up from the head; used to spot approaching predators.

Mirroring the cephalon, the arse end (pygidium) is also smooth and rounded. Most pygidiums on trilobites end in a point. This part of the trilobite has segments, but they are all fused together, forming one solid piece. This specimen lacks any clear segments.

Generally, the middle section of trilobites (the thorax) is the longest part of the body. It is divided by pleural lobes (which look a little like the ribs). These segments are different from those on the pygidium; they are not fused together. Separate pleural lobes allow the thorax to be flexible.

Our little beauty has a tiny thorax, squished in-between by a big round head and a big round ass. Such quirky features had to have some reason.  This peculiar looking trilobite reduced segments in the thorax, merged sutures in its head, and lost the segments in the bottom, all so it could roll into the most perfect ball. Many species of trilobites rolled into balls to protect their softer undersides, similar to some species of woodlice today.  But this group of trilobites evolved the ultimate way to roll; the smooth rounded cephalon and pygidium were like two halves of a ball, and the small flexible thorax folded and held these two halves together, creating the most impenetrable ball of calcium carbonate strengthened chitin.

A real fossil from teh Order Ill (Image from here)

A real fossil from the Order Illaenina. (Image from here)

As a group, Illaenina were very successful, spanning the Ordovician Period (around 490-455 million years ago) until the Devonian (around 420-355 million years ago). These funny looking trilobites were around for around 135 million years. Scurrying in the sediment in those ancient oceans, those big eyes would have spotted a predator in enough time to roll itself into a tight ball.

For such a cool little group of trilobites, it is such a shame that we have this awful thing to represent them.

This is a trilobite.

Work from a Scandinavian artist, or a cast of really cool trilobite?

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