Mar 7, 2009
Here's a fascinating thing which I'd never seen "in the flesh" before.

Just 10 mm across and labelled Gyroceratites cf gracilis BRONN, from the early to mid Devonian, near Erfoud Morocco.

Does anybody else have similar examples?


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Excellent pictures, Hajar. Resemble Mimagoniaties zorgensis (Roemer), fig 19, p L31 in Part L of the Treatise.

Are these your specimens and do you have working access to any like them? If so, have you thought about cutting and polishing any through the protoconch so see what's inside. Or have you done this already.

Shigeta, Zakharov, and Mapes published a paper on: Origin of the Ceratitida (Ammonidea) inferred from the early internal shell features; Paleontological Research, vol 5,no3, pp201-213, Sept 28, 2001, by the Paleontological Society of Japan. Others of you may know of it. In it are photos of median sections through the ammonitella of Permian proletcanitds and goniatitids, Caboniferous ammonoids, and Permian and Triassic ceratitids.

Best regards
Thanks very much for the reference.

That specimen is here with me, but I wouldn't want to polish it (there's just this one example). I also wonder how much internal detail is preserved in these hematite-replaced fossils. Given more material it would be worth a look.
Some pictures of tiny, 1 mm, initial chambers of Devonian orthocones. These are from a specimen labelled "Baculites sp, Hatch Formation, Late Devonian. Ontario County, New York, USA." I can't see if they have marginal siphuncles though. There's a mass of these tiny individuals surrounding a larger 56 mm shell.

These look close to b and f in Spath's Figure 1.


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Just a question here, recent octopods have different size eggs (small egg species and large egg species), lately they have been grouping orthocones according to the size of their protoconch, is this really a good diagnostic feature? Is it possible that some orthocones could be closely related and still have large or small protoconchs?
Kevin, I would tend to agree with you -- it seems that protoconch size would be a very plastic feature. I bet someone has done some work on estimating the relative developmental/genetic plasticity of various shell traits, given the extremely detailed record we have, and the extensive work that people have done on ornamentation. Do you know of any?
The venerable Spath said (a long time ago):

"Variations in the size of the protoconch, however, still occurred in the Upper Cretaceous, as they occurred in the Devonian, without any obvious explanation, except perhaps the extremely loose coiling in Anarcestes as in Gaudryceras, with their large protoconchs. Otherwise the protoconch in Ammonoidea is dependent on the shape and coiling of the later whorls as much as in Nautiloidea and varies too much in goniatites, for example, to be of systematic importance. Even if not going so far as Schindewolf and insisting on the omission, henceforth, of the initial chamber from the diagnoses of the two orders Ammonoidea and Nautiloidea, it must be admitted that it has ceased to be an element of decisive importance."

Kevin, I've had another close look at the specimen with the tiny orthocones and cannot convince myself of lobes in sutures, or even clearly see any sutures at all. I remember certain layers in the Early Devonian Hunsruckschiefer having masses of little dacryoconarids - do some of those have smooth shells and protoconchs? On the other hand, I've just found some illustrations from Bandel et al. (1983) showing Devonian teuthids, with hatchling Protaulacoceras being a very good match for my examples.
Hajar, I do see dark bands on a few of the pics that suggest septa, but tentaculitids were quite common in rocks of that age. :-/

Hallucigenia, I did a small search with little luck, most papers on protoconchs only deal with a few well preserved specimens of single genera. :cry: Looks like Hajar may be on to something though.

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