So, that´s my translation. I did not translate everything word by word, but more to translate the sense of the article. Sorry for the many grammatical mistakes, but my english is not best. Hopes it helps you anyway:
Nautiloids and squids have since their very early beginnings small beak-like jaws.
Ammonites in contrast had much larger "shovel-jaws" (Anaptychus and Aptychus) and it seems that even the early ammonites from the early shale deposits had such shovel-jaws.
There are different views of the acception of the aptychus which are sometimes seen as cap (opercula) instead as an enlarged lowe jaw, but the known fossils indicate more a function as seen in the models. Ammonites are often shown with predator´s tentacles, but the fossil contents of their guts show only a diet which consisted of very small animals. Furthermore strong predator´s tentacles seems to fossilize comparably often as seen in finds of coeloids, so the tentacles of ammonites were probably not very strongly built. Since 1976 it seems to be a fact that the aptychus was an enlarged lower jaw, which seems to be part of the apparatus the ammonite used to catch prey, because it was useless for a predatory lifestyle and much to big with a hypothetical buccal-musculature for a retraction in the living-chamber of the shell. Ammonites possed a radula, but their shovel-jaws can´t be compared with the much smaller jaw-apparatus of all recent cephalopods. But to see the aptchus simply as some kind of lid, similar to those of nautilus, is in contrast to the ammonite-research of the last decades.
A less known fossil of Orthoceras sp. from the swedish silurian with aptychus in closure-position. The author don´t think that it is a real "cover-position", but it could be that there were already "drawbrige-function" in the early silurian, similar to those of the models.
A Gyroceratides with lower jaw shows that already the early ammonites had another trend in jaw-development than other cephalopods.
The first sensational find of a Radula of the giniatite Eoasiantites from the
carboniferous of Uruguay which were also discovered in some jurrasic ammonites in the following years.
Isolated parts of the oral apparatus of ceratites show that ceratites had more a shovel-jaw than a beak-jaw.
As the author found this 3,4cm long Lissocerate with aptychus in clossure-position, he recognized that the aptychus could not have had a similarity with a beak-jaw.
A nautilus from the limestone of Nusplingen shows how small the beak jaws of nautilus are and that it makes no sense to compare them with the huge shovel-jaws of ammonites.
An interesting find of a aspirocerate from the limestone of Nusplingen which shows a lower shovel-jaw and a rudimentary upper "jaw". This picture shows the enormours difference in size to the jaws of nautilus.
A specimen of Eleganticeras elegantulum wit large lamellar aptychus which were the basis of the examinations of Prof. Lehmann, that the aptychus a in biological sense a large-grown lower jaw.
Prof- Lehman published this jaw-apparatus in 1976 , but it has still to much similarity to the beak of nautilus. Such a jaw with its buccal-musculature would be much to large for a retraction in the living-chamber, but many people still overlook this.
Orthaspideoceras uhlandi with Laecaptychus shows us that the aptychus had no function as some kind of jaw apparatus, but a completely other function.
The big aptychus must have had another function, and the way of live of some ammonites could indicate the it had more a protective function and was probably part of the skin between the tentacles and used to catch prey, as shown at this model.
This model shows the aptychus as part of the capture-apparatus. Reconstruction of Kosmoceras with granula-aptychus.
Model-test with Taramelliceras, whose origianal aptychus was not able to cover the living-chamber completely.
In this Distichoceras for example the Aptychus fits best in a drawbridge-function. That means that ammontites "draw their lower jaws over their heads" when they were in danger.
Model-test with colour, some ammonites show relics of colour.
The meaning of colour was probably more important for camouflage than for beauty
When ammonites did swim, the tentacles with the folded skin between them were situated on the aptychus. But it seems that the favorite swimming-orientation was forwards and not backwards.
The chalky aptychus was covered with an organic surface, which would only make sense if it was situated on the surface of the animal.
Oppened capture-funnel of a Distichocerate. Two of the (suposed) original 8 tentacles bolster the moves of the aptychus at the capture-funnel.
Ammonites had probably analogous to sepias spheric lense-eyes, because they had a similar lifestyle.
There are several isolated finds of ammonites with closed "lower-jaw protection-shield", and it seems probable that there was some kind of drawbrige-function, but the dream-fossil of an ammonite which shows all details is still not known.
Prof Lehmann:" There was a huge differenciation in ammonites, and only because some of them had proably such a function of their aptychus, not all of them must have had it. Probably a shovel-beak was used to chase swarms of ground-living epibiontes. The extraordinairy jaw-apparature had its origins in the head of the gullet."
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