I posted the photos of my model in the thread Phil opened "Sordes amazing nautiloid models".
But to come back to the main topic, the aptychus. Two days ago I was once again in the paleontological institute in Tübingen and discovered some nice nautiloid models with relics of the aptychus.
The largest one was neraly the size of a car tyre and showed a real large aptychus, from which each part was about 10cm or so in length. Sadly the photo isn´t very sharp.
The "Orthoceras with aptychus" is an oddity to me... but it does indeed appear to be a nautiloid with an aptychus. Of course the ammonoids may have inherited the aptychus from their ancestors; the more modern nautiloids do not have one. I think, however, that this specimen may be a mislocalized baculitid ammonoid or else that rather than being an aptychus, this is a calcified hood. It does appear to have 3 components, which aptychi do not.
Earlier ammonoids (eg ceratites and phylloceratoids) had an anaptychus, which is a unitary lower jaw. These can be locally common, often with no signs of shells, as I have seen in the field not too far from my home. Anaptychi are deeply convex. They are about the right size to seal the aperture, with the tip of the beak laid against the previous whorl of the shell and somewhat resembling the hood of Nautilus. The aptychus was a solid object, and appears to have been a fully functional biting jaw. Phylloceratoidea, the only superfamily to survive the Triassic-Jurassic extinction event, had an anaptychus. At least the middle Triassic members of Ussuritidae did. However the lower jaw, aptychus, of their descendants was hinged in the middle, and the preserved parts are flat; when the animal was fully retracted the aptychus neatly closed the aperture. The aptychus would appear to have lost its biting function; perhaps, as well as functioning as an operculum, it also was used as a scoop, either to feed off the bottom or to remove captured (filtered??) food from the tentacles.
I don't know when the anaptychus evolved into the aptychus. Perhaps it happened within Phylloceratoidea. Psiloceratidae appear to belong in the Phylloceratoidea, but I don't know whether thay had an anaptychus or aptychus.
Nautiloid lower jaws are very solid and not so small. I have compared those of Nautilus with those of the Norian Proclydonautilus mandevillei, and both are almost identical in structure and shape. Fossil lower jaws are known as rhyncholites and would be easy to sculpt.
Complete Nautilus beaks are offered for sale on as "peck of Nautilus". This is where I got mine to study.