Ammonite vs. fossil paper nautilus vs. fossil spirula vs. calcerous tube of polychaetes...


Feb 28, 2015
How does one tell the difference between all the whorl-y things?

I don't understand why Sphenodiscus lenticularis is an ammonite and not a fossil paper nautilus. I mean it has bizarre suture lines and the shell radius doesn't increase quite as quickly as a modern paper nautilus's (though it gives the illusion of doing so by overlapping) but it still seems more morphologically similar to the paper nautilus's egg case/shell thing than to other ammonites.

Why is Spiroceras an ammonite - which in my understanding scientist think lived in their shells - as opposed to an animals similar to Spirula which have an internal shell?

Or how do we know the nipponites were ammonites rather than polychaete worms forming calcarous shells or something else?

Or how do we know that helicoprion teeth whorls are teeth whorls? They don't seem to be found near any other teeth as one would expect in a jaw (though maybe there are other teeth records in the same matrix and people just don't talk about it). Sure Onychodontiformes have been found with a similar teeth whorl but it's nowhere near as massive and bizarre. All the teeth whorls seem like they could just be imprints of some ammonite (or a relative) shell

I'm sorry if all of this is answered somewhere I haven't read yet...
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I poked @Architeuthoceras to look at your questions but the first one intrigued me enough to make a guess and then look to see if I could validate the idea. My thought (from some of Kevin and @Terri 's fossil posts) was that an ammonite, like the nautilus is physically connected to its shell with its buoyancy control tube (siphuncle) so if Sphenodiscus lenticularis has a siphuncle, it would be nautilus like vs octopus (knowing that the paper nautilus is an octopus and not physically connected to its egg casing - the male not producing any kind of shell) like. I found this fossil (for sale) post that mentions including traces of a siphuncle (and even mentions that it is part of the ammonite consideration :grin:) so I am hoping Kevin will approve my thought as at least part of the answer to question 1.
Photos on the computer screen just don't show everything that needs to be seen ;) If you look at actual specimens in a museum you can see a lot more, especially the things that separate one thing from another. Sphenodiscus is an ammonite because it has septa separating chambers in the shell connected with a siphuncle (like D says), the egg cases of argonauta do not have either. Spiroceras has septa with fluted sutures and a ventral siphuncle, so it is placed in with the ammonoids, Spirula has septa without fluted sutures, a dorsal(?) siphuncle, and if I'm not mistaken, inner type shell material on the outside showing it as an internal shell, there is still some debate as to whether some ammonoids had internal shells, but the fluted sutures would still separate the two. Nipponites have septa with fluted sutures and a siphuncle, polychaete worms forming calcareous shells do not. The teeth in Helicoprion teeth whorls are made of the stuff that most teeth are made of and are found inside the remains of a jaw, while they have the coiled look of some cephalopods, they have nothing else in common. Have you noticed some photos of octopus have their arms coiled to look like ammonoid shells? There is a coil (gene?) in almost all cephalopods and they all seem to express a coil, or two, or eight, one way or another.
It is kind of fun to try to apply what I have learned about nauts this last year when looking at ammonites. I thought about the shell divisions (septa) where the animal moves forward to a new chamber as they grow but did not/could not tell if Sphenodiscus lenticularis (and did not know for sure if this is a requirement for ammonites) had these divisions.

@Architeuthoceras Are/were there animals that used the full shell without the septa separated chambers?
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