Cretaceous Ammonites

[Hajar]

A friend just sent me a link to this beautifully presented page on Cretaceous ammonites.

 

[Architeuthoceras]

Beautiful fossils and photos, thanks for the link Hajar

 

[OB]

That is a VERY well done presentation, the lighting deserves special mention.

 

[DWhatley]

The one (actually there are two but one shows more of it) called Placenticeras Meeki (13th from the bottom) has patterns on the shell that look lik lichen but seem to be too regulary to be a fungus. If it is patterened, what is the conjecture as to why it would have these grooves?

 

[OB]

It is a natural fractal, Denise. The genetic base is fairly simple, but, like with a bird's feather, the end result rather beautiful.

 

[DWhatley]

I kind of got that part when I decided it was unlikely to be something eroding the shell (but actually could be because some of that stuff would leave repeating patterns but the coverage seems too uniform and complete to be a parasite infestation) but is there a theory on why it would have shallow patterned grooves?

 

[OB]

Well, from the evolutionary perspective little more effort and energy goes into making convoluted sutures as would go into making smooth ones, so it's a trait which would not be selected against. An unwarranted feedback loop in the genetic code, dictating "make bend halfway and continue this until three levels down" would not hinder the organism in finding food, avoiding trauma/predation or procreation. According to Oloriz and Palmqvist in their 2007 paper, suture complexity seems more closely related to shell structural types than to the average depth at which the ammonites lived; it used to be argued that these fractal sutures represented adaptation to high water pressure in combination with gas filled chambers, I think this has now been laid to rest.

For another interesting read on the origins of fractal sutures, you might wish to consider this 1990 paper by Carzia-Ruiz et al.

I realize you'll need a(n institutional) subscription for both of these

 

[Architeuthoceras]

The suture pattern of Placenticeras, like all true ammonites, is very complex. That figured from the chart in the Morphology article matches the photo in question fairly good.



They can be misidentified as dendrites (pseudofossils) or vice versa.

The shell has been removed from the fossil of P. meeki in Hajars link just so the sutures will show. Most of the other fossils on that site still have most of their shell, so the sutures don't show as well.
The fossil above P. meeki (14th from the bottom), P. intercalare, shows the sutures only where the shell is missing. It also shows a fan like pattern on the shell which is common to Placenticeras and has to do with the way the shell was deposited.

 

[DWhatley]

So these patterns are on the INSIDE? Boy can you tell I am a greenhorn at looking at these. It makes it more interesting to wonder why though as it rules out the cosmetic answer that I think is over used in general. It just seems it would give a hint on what the softbodied thing that lived in there would look like. Are there living creature that have internal shell patterns like this? It would seem that a rough shell and even the spiked ones would give the animal a hold fast when it was pulling something like an octopus uses the substrate. Perhaps we should be looking that the most successful patterns for designing antislip mats. Where are our grad students, this would be such a fun project to test.

I realize you'll need a(n institutional) subscription for both of these

I may need to be institutionalized but sadly have no access.

 

[Architeuthoceras]

Suture lines are the point at which the septa are sutured to the inner shell wall. Usually the chambers are filled with either matrix or some form of precipitated mineral holding everything together. In the fossil below the septa are preserved with some form of replacement mineral (probably Calcite), but the chambers (Camerae) remain empty. You can see that as the septa approach the inner shell wall they get a lot more complex. If you were to fill these empty chambers with plaster, then remove all the rock and shell down to the hardened plaster, you would end up with something that looked like that P. meeki fossil.



More photos of this fossil on This old Thread, some showing the suture pattern. It is partially an internal mold, where some of the chambers were filled with matrix, and partially a replacement, where the original shell has been replaced with calcite.

 

[DWhatley]

Thanks Kevin,
That picture and the older thread helped a lot in correcting my misconception. I expected very shallow grooves rather than deep pockets (relatively speaking).