Ammonoid internal shells?


TONMO Supporter
Nov 19, 2002
I had a chat to some chap last night who was of the opinion that hoplitid ammonites may have had internal shells based on external scarring he may have identified. Not all ammonites mind, but just that rather wonderful keeled, ridged and spined family.

I'm not convinced but open minded. Any opinions anyone?
Hoplitids are the ancestors of Placenticeras, the shells that the Ammolite in canada comes from, so the group obviously has a tendancy for nacreous shells. Some placenticeras shells are very large and I just cant see them being internal. I have seen some shell remains from the large placenticeras I find around here and I am quite sure the inside is alot smoother than the outside, seems an internal shell would be smooth inside and out. Still, I will keep an open mind, as many a so called truth has been shot down with the gain of new evidence.

Any chance of a pic of this external scarring?
From my place of gross ignorance, I'm going to suggest that a smooth outside is more likely to be for streamlining as an external shell, and ornamentation might be more likely to serve the purpose of keeping an internal shell from sliding with respect to the animal flesh surrounding it.

Of course, I just made that up, so even calling it a hypothesis is rather bogus, I'm mostly just trying to stir up the discussion!
Smooth shells may be useful streamlining, but then some of the hoplitids, and other families, had keels which I suppose may have channeled waterflow and possibly aided directional control. The ramshorn squid, Spirula, has a utterly smooth internal shell that still retains the phragmocone and siphuncle. I'm sure that if hoplitid ammonoids had tracing of muscle scarring on the outside of the shell, then someone would have noticed by now.

I suppose there is also the problem of spines; it's hard to imagine spines growing through an enveloping fleshy mantle from the shell surface. Are there any other molluscs that do this does one think?

That leads to another question, how exactly does Nautilus repair shell damage away from the secreting edge of the head?
I dont have access to the following paper, but it may have some evidence for the mystery at hand.

Doguzhaeva, L. A. and Mutvei, H., 1993, Structural features in Cretaceous ammonoids indicative of semi-internal or internal shells, in: The Ammonoidea: Environment, Ecology, and Evolutionary Change, Systematics Association Special Volume 47 (M. R. House, ed.), Clarendon Press, London, pp. 99-104.
Thanks Kevin!

I've found the abstract but the article does not seem to be online unfortunately. The authors seem to conclude that there is some evidence that the shell was (at least) partly internal in at least three genera.

Structural features indicative of a semi-internal or internal position for the shells are reviewed in representatives of three Cretaceous genera: Gaudryceras and Ptychoceras, both belonging to Lytoceratida, and Aconeceras which is a member of Ammonitida. In Gaudryceras, the shells are covered by thick coating layers with lamellar structure, similar to that of the nacreous layer. Very probably these layers were secreted by the mantle epithelium which covered the shell surface. In Ptychoceras, the semi-internal or internal position of the shell is indicated by the following features: (1) the mode of truncation of the initial portion of the shell in three species; (2) the frequent alterations of the ultrastructure of the shell wall during ontogeny; and (3) the formation of a thin coating layer with nacreous strncture on the shell surface. Also, in Aconeceras, the shell surface was quite probably covered by the mantle, at least at the apertural region. This is indicated by: (1) the secretion of an additional porous spherulitic layer on the outer surface of the keel; (2) the formation of a thin coating layer on the lateral sides of fully grown shells; and (3) the structural changes of the shell wall during ontogeny.

Shop Amazon

Shop Amazon
Shop Amazon; support TONMO!
Shop Amazon
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.