Palaeoporn 15

You spin my head right round, right round, . . .

Carrying on the theme of Palaeoporn 14, namely moulting in trilobites, here’s one to make your head spin.

Literally!

The trilobite at the top of the post is a big Redlichia takooensis (around 12-14 cm in length), common in the Lower Cambrian Emu Bay Shale, except that it's head is on backwards and inside out!

First a little background. The trilobite exoskeleton is rigid, and so to grow they need to shed this outer covering, expand, and mineralize another exoskeleton around the expanded body (crabs do this today, with soft crabs – those that have shed their exoskeleton and are awaiting the new exoskeleton to harden - prized as bait.)

Since the exoskeleton is rigid, there needs to be an exit strategy so that the body can get out of the old exoskeleton. This is usually achieved in trilobites by having lines of weakness – or sutures – at strategic places on the body, which preferentially break. These are usually placed on the head and around the eyes, and separate the central part of the head - or fixigena - from the outer part of the head - or librigena - which is also called the free cheeks 'cos they represent the cheeks of the head and they get freed up during moulting.

When the trilobite starts moulting, usually the suture lines break, the exoskeleton around the head separates into fragments (fixigena and librigena), allowing the body to exit through the head, leaving the exoskeleton intact, and the head fragments to fall back into place.

Usually.

Sometimes it doesn’t go to plan.

The free cheeks can be displaced, but that is usually the trilobite being careless on the way out. But sometimes things go wrong.

For comparison here is a proper R. takooensis (right, trilobite length 5 cm without spine) with its head on straight, fixigena and librigena all facing front and orientated correctly, even if the free cheeks are slightly displaced indicating that it is a moult.

So what happened to our backward friend up front?

Well, I'm pretty sure it wasn't born that way (no offense Jake), so it looks like a moulting accident.

Also the librigena isn't.

Liberated that is.

Although we don't have the whole body, the portion of the head outside of the eye (the librigena) is still in place indicating that the suture did not split.

What probably happened was that once the facial sutures failed to split, the exoskeleton broke behind the head. The animal then exited the old exoskeleton, pushing the head exoskeleton into the vertical and then beyond, which forced the head exoskeleton upside down and the front margin to point backwards.

In other words, imagine the head is an upside down bowl as in the diagram below, where F = front of the head, and B = back of the head.


Instead of splitting along the sutures, the whole head exoskeleton comes detached from the body exoskeleton. The trilobite then pushes its way out by forcing the head exoskeleton to tip 90 degrees onto the front margin, and then 180 degrees to lie upside down, with the front margin now pointing backwards and exposing the internal surface of the head exoskeleton.

The trilobite then escapes the old exoskeleton and is free to go.

Scary stuff perhaps, but not the worst example.

Next time - "Ultimate Moulting - when moulting REALLY goes bad"