Here are some pictures of the Muramotoceras, 6 cm long, with at least three other ammonite species accompanying. Okamoto (Palaeontology 31) has some fine reconstructions of how life orientation changed during growth.
Here's Okamoto's reconstruction of life orientation changes in Muramotoceras with the initial straight section pointing upwards, then a flip over with corresponding change in obliquity of ribs and finally the growth of the spiral enveloping the early straight section, now pointing downwards in a new stable orientation. He shows the same pattern for Eubostrychoceras.
The paper shows one turnover event, but it looks like there was another from looking at your specimen. The straight part on yours looks like it turned over and assumed the same orientation as the first straight part. Is there any indication that the siphuncle moved inside the shell? Was ventral always ventral? Did this animal live upside down for awhile?
Just a little sketch to show the last turnover of the shell shown in the first fig. below (or above depending on your thread setup), fig B, this would be sometime after stage 70 on the stereograph.
Thanks Kevin! Good observations and good questions.
Yes, Okamoto’s “growing tube” model stopped before the turnover to this last straight section seen in the specimen. It’s not a full turnover, but rather orientated at about 45 degrees to the coiling axis. The stereograph plots the vector from computed centre of buoyancy to computed centre of gravity relative to the axis of rotation (fixed in the centre of the stereograph). I can only guess what that would have looked like for this last stage, but here below is an attempt to sketch how that might plot. Filled dots represent the apex of the first straight section pointed upwards and the open circles represent it pointing downwards.
I don’t see any evidence for how the siphuncle runs – but that would be a good question for someone like Neale. Neale?
I just noticed that my specimen is dextral, whereas the one in the reconstruction is sinistral (if I have the convention right).
Here is the inferred life orientation of Allocrioceras given by Wippich & Lehman (2004) after consideration of the soft part remains in their Lebanese examples.
and this 4 cm Allocrioceras from Lebanon arrived today, sent by a French friend. It seems to show internal soft parts and stomach contents. More photos later.
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