The ugly ugly giant Endoceras, full of issues

I made this quick and ugly water-colour-painted drawing of the giant orthocone nautiloid Endoceras to spotlight some of the issues and problems about the life-reconstruction of nautiloids like ammonites and orthocones. There are several huge problems with the reconstruction of the life-appearance with this animals, because we do not only lack any soft-tissue impressions of them (unlike belemnites and some other fossil cephalopods with nearly perfect body impressions), but also because nautiloids were just very very different in certain aspects of their anatomy compared with modern cephalopods.
For example, despite the apparant similarity to modern species of Nautilus, they were very different. Not only in aspects of shell construction, but also in their feeding apparatus. Modern Nautilus (as their fossil relatives which lived alongside ammonites) have real beaks like other "normal" cephalopods, but nautiloids had no beaks at all. Instead they had apparantly some sort of filter-feeding apparatus, and based on stomach contents preserved in some shells, they fed probably on very small prey items. Futhermore they had two other strange traits which are not found in any modern cephalopods, the aptychus and anaptychus. This strange structures were possibly developed from the original beak, but it´s neither known for sure for what they were actually used, nor (and more importantly) where they were actually located. From some fossils it seems obvious that the aptychus, which consisted of two segments which formed a somewhat round-to oval shape, was used to seal the opening on the shell. However, it was very different from the "hood" of Nautilus, which consists just of soft tissue, which has the same embryonic origin as the multiple tentacles.
If the aptychus was in fact a modified beak, it seems also problematical to place it at the same location as the hood of Nautilus. So where the hell was it? I don´t know, and there are several possibilities. In this reconstruction I placed it on the ventral side below the arms and in front of the siphon, because the aptychus seems to be a derived lower jaw, so it would be quite hard to end up at the top of the head. Furthermore there are some exceptionally well preserved fossils which show the aptychus in situ, and apparantly at the ventral side of the head.
Ammonites are comparably often depicted with multiple thin tentacles like in Nautilus, but we have to keep in mind that Nautilus is an evolutionary line of its own, and the multiple tentacles are no ancestral trait. In fact those arms form during embryonic development from a much lower number of arms which "split" into those thin tentacles. The ancestral cephalopods, including ammonites and other nautiloids, had probably only 10 arms. If one pair was already modified into elongated tentacles is however fully unknown.

Giant orthocones and also giant ammonites are often depicted as huge apex predators, but there are several problems again. Keeping in mind that they lacked a beak to really eat larger prey items, and the feeding apparatus and stomach contents which indicates a diet consisting of tiny prey, there is also a very different possibility. Perhaps they were just giant planctivores, like nearly most other giant marine animals. For this reason I depicted Endoceras similar to Spirula, a small planctivorous modern cephalopod.

We have also no evidence if ammonites had suckers as modern squid or octopi. Some ammonite fossils are extraordinarily well fossilized, and we can assume that chinitous parts of suckers or hooks would have been preserved - if they had existed. But that´s not the case. I depicted this version with arms and tentacles which have pads of numerous tiny suckers, similar to those seen in Spirula.
Another possibility would be slime-secreting ciliae and enlarged membranes between the arms as in the modern detritus-eating Vampyroteuthis

I don´t say it looked surely like this, I really don´t know. But given the many issues about the real life appearance and behavior of nautiloids, we have to consider the possibility that they looked very different from most depictions.

Some more information about the controversies and problems (including some really great model reconstructions):
www.steinkern.de/vermischtes/s…
w8z1yvo38.homepage.t-online.de…