Rhaeboceras - a wonderful beast

Steve O'Shea

TONMO Supporter
Staff member
Nov 19, 2002
Thought I'd post a few images of this wonderful ammonite Rhaeboceras. Neil Landman of the American Museum of Natural History has sent us a few of these amazing beasts. In some images you will see these truly fantastic structures that have been identified as 'radula teeth', in others the aptychus in situ (in a fragment of the body chamber), and then an image of the entire shell. They date to the Upper Cretaceous, Montana.

We could have some great debate online about these ammonites, and whether the aptychus is a beak and these radular teeth are in fact homologous with a true radula (or whether they are gizzard teeth and gizzard plates). Who knows.
And these images are of the aptychus; sorry that they are so dark, but I'm having a battle with this camera trying to dim the flash (otherwise there's complete whiteout).

These 'radula teeth' are they analagous to any structure in any modern ceph you have examined?

Never seen anything like them in any extant cephalopod Phil. I'll have images somewhere of the buccal bulb, odontophore and radula of a typical squid somewhere (just have to track them down) and will post online asap. I have a great deal of trouble envisaging the musculature associated with such enormous teeth being accommodated inside that body chamber.

Have you ever seen anything like this?
Um, is it possible those teeth belong to something else? Something that bit into the Rhaeboceras?

Maybe it's just me, but those teeth look...foreign. Though the fracturing of the shell near the teeth's concretion might have been post-mortem, it is a bit suggestive.

Phil? I'm scared, Phil.


Steve O'Shea said:
Have you ever seen anything like this?

I'm afraid not. I think the discovery of aptychi/anaptychi is a rare discovery in itself. Unfortunately I have never been so lucky. On the other hand, many of the ammonites that I have found have been coated in the original aragonite so I suppose you can't have everything!

This function of the aptychus is most peculiar. In some examples it almost fills the whole of the living chamber leaving little room for the fleshy parts. In some examples it takes up to a fifth of the whole bulk of the creature requiring massive musculature indeed.

Why can't those researchers sitting on this soft bodied ammonite image publish? Or if they have let us mortals see the results by printing it in the popular press? An image would be great to speculate about. How is the aptychus orientated? Clam shell doors or beak? How were the muscles attached?

I'm going to have to some reading!

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