New Article on The Cephalopod PAge

[ceph]

I'm please to announce the lattest The Cephalopod Page contribution. A Broad Brush History of the Cephalopoda is located at: http://is.dal.ca/~ceph/TCP/evolution.html This article is on the evolution of cephalopods and contains images of both fossil and living cephs. Enjoy.

 

[Architeuthoceras]

Thanks James, and thank Dr. Monks, what a great article, I thoroughly enjoyed reading that while I was supposed to be working.

 

[Phil]

Very good article. Clearly written and informative.

Thanks!

 

[tonmo]

Yes, thanks Dr. James... related to this, as has been stated on the TONMO.com ticker for some time now, we'll have our own Ammonite article to post soon... Phil provided a basic overview for the TONMO.com community, which I greatly appreciate! I just have to get around to posting it... This week Phil, I promise...

 

[tonmo]

Posted it!

Ammonites: A General Overview

 

[Architeuthoceras]

Two Great articles about fossil cephs in one day now I am in heaven. I got to read Phils' at home without my boss looking over my shoulder GREAT job Phil!

 

[Fujisawas Sake]

Outstanding article! Must study more...

Sushi and Sake,

John

 

[Fujisawas Sake]

Dr. Wood,

Great article! I noticed though that it seems to hint that the cephalopod lineage is somewhat polyphyletic. I see three distinct lines, but is there any idea or theory on what the "proto-cephalopoda" may have been? Also my invert zoo instructor liked to nickname cephs "siphonopoda" because he believed that the siphon evolved from the foot. Any thoughts?

Years ago there was a concept known as the "Hypothetical Ancestral Mollusc" or H.A.M. theory. I posted a subject last year called "H.A.M. and Legs" which asked about the development of the cephalopod arms from the bauplan of the "general" mollusc. I know that H.A.M. theory has gone the way of the of the Ammonites, but is there any thought on what the "proto-cephs" may have actually looked like, and from what branch these creatures may have sprouted?

Thanks for the great article!

Sushi and Sake,

John

 

[GeoffC]

Cool article Phil, looks like a lot of work into went into that. I havent got much time but I wanted to let you know, cheers!

Geoff

 

[Phil]

Years ago there was a concept known as the "Hypothetical Ancestral Mollusc" or H.A.M. theory. I posted a subject last year called "H.A.M. and Legs" which asked about the development of the cephalopod arms from the bauplan of the "general" mollusc. I know that H.A.M. theory has gone the way of the of the Ammonites, but is there any thought on what the "proto-cephs" may have actually looked like, and from what branch these creatures may have sprouted?

Yo John,

Why has the HAM theory been disregarded these days? Is it because it implies the existence of an animal for which we have no evidence for, or has it gone the way of the dodo due to modern cladistic analyses? I suppose the HAM would imply that all the forms of mollusc originated at a single point in time from a single ancestor. I know that there has been much debate about the origin of molluscs recently with some researchers placing the molluscs in early lineages with ancestral annelids. Much of this work comes from recent advances in molecular biology examining RNA sequences in annelids and molluscs. (Just don’t ask me to explain it).

What on earth was going on in the late Precambrian to the Cambrian ‘explosion’ is totally obscure, relying on trace fossils and enigmatic faunas such as the famous Australian Ediacaran fauna. Even the animals that we know of from this mysterious dark age often defy easy description, weird quilted mattresses that may have been animal or vegetable or something in between and tiny impressions of worms and segmented enigmas that seem to have left no clear descendents.

I think it is generally accepted that the earliest molluscs may well have resembled aplacophora such as the ‘living fossil’ Neopilina that was dredged from the ocean floor in the 1950’s. That there were such creatures in evidence in the late Precambrian is highly likely. Scrape marks on rocks are recorded from the late Precambrian period that may well have caused by such primitive molluscs and tiny cap-like shells (a couple of millimetres across in most cases) have been recorded from the Tommotian faunas of Siberia, datable to about 550mya, at the beginning of the Cambrian. Some of these resemble monoplacophora and some bear a similarity to the earliest cephalopods in the late Cambrian such as Palaeoceras and Plectronoceras, though it is clear that they were not cephalopods as they do not have chambered shells containing a siphuncle.

I always imagined that the arms/tentacles on the head of these late Cambrian primitive cephalopods developed from sensitive patches on the head of the primitive proto-cephalopods somewhere in the late Precambrian or early Cambrian, these must have been devices used to feel the immediate local environment, possibly used as a survival aid. Indeed, I imagine they may well have predated the evolution of eyes in molluscs. Perhaps these feelers later developed the secondary function of ensnaring prey when the primitive arms were developed enough to grasp whatever they came across in the environment and push it towards the radula. Thus the path towards a predaceous life amongst cephalopods was set at the earliest days of their ancestry.

I know this is all speculation of mine, and I have never studied zoology, but it sort of makes sense to me!

Cheers,

Phil

(Almost glad he’s off sick for the day; it gave me time to write this!)


BTW, Cheers, GeoffC!

 

[Fujisawas Sake]

Phil,

That's EXACTLY what they thought the H.A.M. might have looked like! Aplacophoran design is pretty ancestral for molluscs, and it would make sense. My opininon is that they scrapped the theory because there was no solid proof on the H.A.M and not a lot of fossil evidence either way. I still think it bears a second look.

I think it has to do with the idea that scientific theories have to be based on evidence. H.A.M. probably fell by the wayside due to this concept. The problem is, evolution is not always cut and dry, and sometimes a well-educated guess is a good place to start. I think that, given current knowledge of malacological evolution, we can build a backwards model of ceph evolution and be pretty cloes to the truth.

For someone who never studied zoology, you can sure hold your own! 8)

Sushi and Sake,

John

 

[Fujisawas Sake]

Phil,

I think that there's something brewing as far as deep ocean ecology is concerned... Kat seemed to indicate that there are more species of giant squid out there, and that got my attention. Any thoughts on the ecology of the nautiloids? I wonder how the soft-bodied cephs did during that time?

John

 

[Phil]

John,

Yes, I think you maybe right.....if I remember correctly, wasn't there a hint on these pages a few weeks ago that there may be another giant out there apart from Archi and Messie? Or perhaps I just misinterpreted a comment (as I often do).

Well, as to your other point, you would need someone much wiser than me to explain the intricacies of ecology of the Palaeozoic oceans. However, if you are referring to 'soft-bodied cephs' as squid, octopi and cuttlefish then there was no real overlap with the nautiloids. The nautiloids really came to dominance way back in the Ordovician period where they certainly assumed the position of top predator. There was an evolutionary explosion of nautiloids at this time with no less than nine orders, most of these had cone shaped or even straight shells and were really quite diverse in their morphologies. Most of these nautiloid groups became extinct by the end of the Permian excepting just two groups, the Orthoceratina which lingered on to the end of the Triassic and, of course, the nautilida from which our modern nautilus is descended. The modern Nautilus really is the sole surviving twig on what was once quite an exotic bush.

The gradual extinction of the nautiloids at the end of the Devonian extending in the case of some orders into the Carboniferous can (probably) partly be explained by the emergence of the ammonoidea in the Devonian, especially the goniatites. Why these should have come to dominate is a very good question, perhaps the ammonoidea were specialists and more adaptable or perhaps they had a faster growth rate and a shorter lifespan so that they could evolve quicker than the nautiloids. If anyone has any good theories, I'd love to hear them here!

Some of the nautiloids were adapted to deep water, indeed, one researcher, Westermann (1985), has established crush depths for these ancient creatures. It is estimated that one nautiloid, the Carboniferous Michelinoceras, could probably withstand depths of 1125m. It seems that direct competition with the shallow-water ammonoids could not be sole reason responsible for explaining the demise of the nautiloids, there must have been other factors as well.

Anyway, going back to your origin point about soft-bodied cephs, the squid and octopi did not appear until the Jurassic era, by which time the only surviving nautiloid order, the Nautilida, had adopted the familiar form we have today and were probably already established in the deep-water niche they occupy today. So it seems likely that there really would not have been that much competition between the two groups. As an aside, I think the cuttlefish are first known from the Cretaceous and as shallow water creatures would certainly not have made much of a direct impact on the Nautilida.

Anyone still awake? :sleeping:

 

[Fujisawas Sake]

I wonder if their specialization led to their downfall? I mean, the oceans during that period went through large scale changes in relatively short periods of time. Changes would have occoured in salinty, prey distrubution, and in the nature and size of their predators. Even then, I wonder why more of them still aren't around.

As for what I was thinking about giant squid, Kat gave me a resounding "yes" when I asked if she thought there were any more massive squid species out there. *sigh* Better start thinking up more imaginative common names, I guess....

Thanks for the reply again! As always, a pleasure to read.

Sushi and Guiness,

John

 

[ceph]

Posted it!

Ammonites: A General Overview

Well done! I'll have to link to that when I have some time to work on TCP.

I think we date ourselves with these theories. I was also taught the HAM theory, that cephalopods evolved from an aplacophora/monoplacophoran ancestor. One of the advantages of this theory was it’s use in teaching as it makes intuitive sense. I’m not sure what happened to it, perhaps it is another “just so” story. Looking at my old invertebrate text book, there was little evidence presented. Modern molecular evidence should tease out the relationships between the various mollusk groups which would help to support or refute this theory. I focus on living cephalopods and am honestly not current in this area.

Dr. Neale Monks wrote the The Cephalopod Page article that kicked off this thread. He is much more knowledgeable about the cephalopod fossil record and their evolution than I am. I’m sure he would be happy to answer questions. I'll send him a copy of this.