Siphonal notches in fossil cephalopods

Neale Monks

Jun 29, 2003

I must confess I only just noticed your question about siphonal notches in Ordovician nautiloids and their relevance to jet propulsion in ammonites.

The bottom line is that while siphonal notches, that is, embayments in the anterior margin to allow the siphon to protrude freely, do exist in various fossil (and living) nautiluses, they aren't known from ammonites. Quite the reverse. Many ammonites have modified apertures sometimes with constrictions and collars (very common with heteromorph ammonites) or rostra (many Jurassic and Cretaceous true ammonites). The rostra are rather like long spikes sticking out the front. These are distinct from the well known "lappets" that form on the sides of the aperture, rather like ears.

In some species the rostra points forwards, curling slightly in keeping with coil of the shell (e.g. Diploceras) while in others it curves backwards and underneath like a mini-spiral hanging under the shell (e.g. Mortoniceras). Either way, if there was a siphon it might have laid across the rostrum but couldn't have curled underneath the animal to allow it to swim forwards (as Nautilus does today). One solution is that these animals had two siphons, one on either side of the rostrum. The alternative is that the animal extended far further from the shell than the nautilus does, and so the rostrum could have been a support for the mantle, with the siphon well ahead of it.

Among the heteromorphs, constrictions that narrow the aperture are frequent, while among some like Sciponoceras the thing develops a curved hood that folds over the aperture so that if the head stuck out it would point at ninety degrees from the long axis of the shell. Clearly, such a thing couldn't have swum by jet propulsion.


This is interesting, I'm almost understanding it after several slow reads... :biggrin2:

How heavy were the shells of ammonites compared to their bodies? For today's nautiluses, I believe I remember reading that their shells are light... (hence the name "paper nautilus", right?)... So, I can imagine them being able to get around fairly well by making use of the siphonal notches.

From the looks of some of the illustrations I've seen of ammonites and early nautiloids (from Phil and others), it would seem their shells were relatively solid. Before reading your post, I never really considered how these things may have gotten around.... But now that I think about it, most shelled creatures are benthic, right? Must be hard work for a shelled creature to get around a water column.
One solution is that these animals had two siphons, one on either side of the rostrum.
That's an interesting idea... is there any "precedent" for this? I mean, are there any known species (throughout any time in history) of cephalopod with two siphons?

Ammonite shells strike me as being made of a thinner layer of shell than nautiluses, but with the strength being maintained by more bracing being offered by the more complex septal walls. My guess is this is how they were able to grow faster than nautiluses (as seems the case), i.e. by using less shell material but keeping the same sort of strength through clever design.

In both cases the shell was neutrally buoyant or close to it, so that the animal could float in the water column with minimal effort, working in the same way as a swim bladder in a bony fish. On the other hand, Klaus Ebel thinks that some ammonites, perhaps most, were benthic and that the shell didn't work this way at all, and the thing was more like a snail. I don't quite buy into this, but I think he has a point that ecologically at least ammonites were much less "swimmers" than fish, despite their portrayal in "Walking with Dinosaurs" type things as being the Mesozoic equivalent of bony fish. Personally, I think they're much more like the various neogastropods like cone shells and whelks that are important, if not particularly dramatic, predators today.

The "paper nautilus" is something else again, an _octopus_ that happens to produce an egg case with a spiral, hence nautilus-like, shape. Some people think this is a clue to a closer relationship between the octopuses and ammonites than has usually been considered (see stuff by Zeev Lewy).

I've only seen the two-siphon hypothesis described once, in a paper by Geczy I think but I'm not sure. His ideas were interesting on the whole, for example he hypothesised that cephalopods evolved not as hopping gastropods on the sea floor as is commonly suggested, but as planktonic organisms, more like pteropods. The floating shell allowed them to be neotenic, i.e. literally hang around in the plankton for longer.

To this day there is great uncertainty as to where cephalopods came from and how they acquired their unique shell design.


[I must confess I only just noticed your question about siphonal notches in Ordovician nautiloids and their relevance to jet propulsion in ammonites]

I have noticed that the nautiloid Aturia has a monstrous hyponomic sinus.

What does this imply for the position of the eyes in this beast? The sides of the aperture become in effect huge lappets... are the eyes ahead of or behind the lappets, or on long stalks and projection over them? Shape of the hood?

Shop Amazon

Shop Amazon
Shop Amazon; support TONMO!
Shop Amazon
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.