Questions on fossil cephalopods and ceph evolution

[Cephkid]

I am now doing a research paper for school on ceph evolution, and I'm kind of confused about the current data on belemnites and coleoids, and was hoping you could clear some stuff up for me. First of all, I kind of don't understand how it was determined that belemnites split off and are unrelated to modern Coleoids. Meaning no disrespect, I simply don't understand, and an explanation, and/or image of evidence would help immensely. Also, I was wondering about if you could explain why Architeuthis, Mesonychoteuthis, and most teuthids in general are so similar (not counting size, of course) to belemnites? (i.e. 10 tentacles/arms, each with-typically-hooks, body shape, etc.)
TIA,
CK

 

[Architeuthoceras]

Cephkid,

Belemnites are Coleoids, all Coleoids are probably descended from Bactritids which are ammonoids, that were probably descended from orthocerids, which are nautiloids. The main difference between Belemnites and modern coleoids is the internal shell of Belemnites is different than squids, cuttle fish, and spirula.

Belemnites: A Quick Look

 

[Cephkid]

I understand that Belemnites are Coleoids, (That article was one of my sources) but I dont understand how/why it was determined that they were split off from Aulacocerida and the other main branch forming two branches approx. 330 mya. Also I was asking why they resembled Teuthids so greatly.

 

[Phil]

Hi Cephkid,

Information on the Aulacocerida seems to be hard to come by and I'm afraid I don't really have any definitive answers for you. However, I have found out a few snippets from ENK Clarksons 4th ed. of "Invertebrate Palaeontology and Evolution" (1998).

Clarkson states that little is known about the early stages of belemnite evolution in the Triassic and the Aulacocerida, which really seem to represent a half-way house between the orthocones and the belemnoids. He records that the Aulacocerida derived from orthocone nautiloid cephalopods simply by the expansion of the thin covering of the phragmocone to form the massive guard from which we know belemnites so well.

As an example, he mentions the primitive Triassic belemnoid Ausseites known from Austria (Induan-Rhaetian 248-205mya) which has a small guard and very long smooth phragmocone but very much resembles an orthocone. Aulacoceras (Norian 220-209mya), on the other hand, has a larger ribbed phragmocone though the guard resembles that of Ausseites. Clarkson assumes a gradual progression from an orthocone via external thickening and the gradual development of the belemnite guard via this slow process. I suppose why exactly this happened is anyone's guess!

I have always imagined that the ancestral Bactritids had ten arms, hence the 'primitive' condition of all the coleoids, i/e teuthids, squid, belemnites, and the earliest octopods. Most probably the particular branch of the nautiloids that the Bactritids arose from had ten arms too ; Nautilus probably descended from a different nautiloid lineage altogether.

I attach a photo and cross section of Aulacoceras for your interest. Beyond the details above, I'm afraid I do not have any more in-depth detail. I hope it's a start for you!

 

[Cephkid]

Thanks!

A few more questions; do you know anything about where I might find more data on early Aulacocerids? (i.e. from 350-310--hopefully 330--mya) I'm also trying to understand-and thereby make it possible to incorporate in my paper-the, er... "big split" I guess you could call it (when the aulacocerids and ancient spirula diverged from each other).

 

[Phil]

Hi Cephkid,

Origins of the Aulacocerida....hmmm.......

Sorry for a complete cop-out reply but I'm afraid I don't have any materials that cover this as it is quite specialist, that's really a research-paper area. Believe me, I tried to find out!

I can only direct you websites such as Theo Engesers Fossil Coleoid Page which I'm sure you've already seen. I would be cautious that the graph he has provided as a timeline is not as 'set in stone' (pardon the pun) as it appears, there are a couple of differing theories that date the origin of the teuthids to much earlier than the early Tertiary than he has depicted and link the cuttlefish to the belemnites much more closely.

I can only suggest that you put 'Aulacocerida' and 'Aulacoceras' into various search engines to obtain some references, maybe with university resources you could search some out? If I do come across any sites or information I will certainly let you know.

I must admit, I'd love to know what happened in the Late Devonian too, with the origin of the coleoids and more advanced ammonoids from the Bactritids. That really was a pivotal period in cephalopod history.

Best of luck, if you find out any interesting facts and theories about coleoid origins, please let us know.

Phil