New Article on The Cephalopod PAge

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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
 
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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

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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.

:goofysca: Anyone still awake? :sleeping:
 
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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.... :lol:

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

Sushi and Guiness, :beer:

John
 

ceph

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tonmo said:

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.
 
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Dr. Wood,

See, I don't think H.A.M. is completely wrong, its just that its only a guess, albeit an educated one. I think that the relationship between ancestral forms of molluscs and the cephs is a less direct one, given the overall extreme modification of the body plan.

Think about it like this: take your average rorqual (I went whale-watching today, so I'm still stoked about seeing a gray whale up close) and a human. The skull bones are pretty much the same, but its like they've been re-shaped and pushed and ... well, I'm sure you've taken mammalogy or nat. history of the vertebrates, so you get the idea. We can see the similarities, but overall the body plan of the ceph is so different that I don't see a very direct H.A.M - ceph link. ALTHOUGH... Maybe a pteropod-ceph link may be forseeable. *shrug*

Sorry, that's the cladist in me speaking... :oops:

How accurate do you think that molecular phylogeny will be on such an old phylum? I mean, are they scanning mitochondrial DNA or HOX genes or something else? Taking 500 million years of evolution and mutation, I wonder how exact we could actually be?

Thanks for the reply!

Sushi and Sake,

John
 

Phil

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John,

I thought you might be interested to see this.

A 425 million year old creature known as Acaenoplax has been reconstructed in 3D. It comes from a Hertfordshire in the UK and comes from a quite remarkable collection of microfossils that were preserved in Silurian-age nodules. This creature, Acaenoplax is thought to lie somewhere between the aplacophora and polyplacophora. As such it is thought to be a primitive mollusc and therefore related to the earliest ancestors of the cephalopods. (Though, of course, cephalopods were already established as a group in their own right by that date).

Strangely it lacks the classic molluscan foot but looks like a spiny caterpillar.

Acaenoplax; an early mollusc
 
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Phil,

Dude, that was brilliant! Thanks for the heads up.

I was thinking about this subject the other day, and I think the discovery of Acaenoplax lends some credence to the annelid-mollusc link. It looks like a polychaete, sans really noticeable evidence of metamerism in its bauplan (body plan). I don’t think this is accidental though. A little background on why:

In the book Invertebrates, by Steve and Gary Brusca (1990), four individual theories of molluscan evolution are presented. They are:

The Turbellarian Theory - This theory states that molluscs may have evolved from acoelomate turbellarians, and it based on the homologous (or analogous) mucociliary gliding surface. They don’t like this theory, and I think it’s a long shot at best. I'm sorry, but I'm just not that much of a cladist.

The Modified-Turbellarian Theory – This theory states that molluscs evolved alongside annelids from a turbellarian ancestral group. Hmm… not sure there either. This seems to be dependent on the idea that the aceolmate design "branched" into the annelid coelom and molluscan hemocoel. I think this is also a weak theory, and I go into a bit more detail below.

The Coelomate Theory – This theory states that molluscs evolved from a coelomate ancestor alongside the annelids. Given the idea that molluscs and annelids both show embryonic spiral cleavage, 4d mesentoblast development, and trochophore larvae, I think this is a good theory.

And last, but not least, The Annelid Theory – This theory states that molluscs are direct descendants of annelids. The Bruscas go on to say that the major problem with this theory is the apparent “de-evolution” of segmentation and coelomic development that would have had to occur in the annelida. Given the aplacophorans and caudofoveatans, and their apparent modifications of the molluscan bauplan, I’m not entirely sold that this isn’t the case. I’ll explain below.

For one thing, “de-evolution” is not an exact term; indeed, body plans are highly modified to suit new patterns in genetic expressions, environmental adaptations, etc. Good examples are arthropod parasites, most notably parasitic crustaceans. Remember the rhizocephalan? At first glance, it appears to be a fungal endo-parasite, assuming hormonal and developmental control of its unlucky crab host. But its larval form betrays its true ancestry. It has a cyprid larva; it is a cirripedian… a barnacle. The modifications on that body plan are insane. I think it was on a neotenic path, and then somehow took a parasitic turn to the dark side. A “simple” parasitic body plan is the result of millions of years of evolutionary specialization, and therefore is not “simple” at all.

The caudofoveatans and aplacophorans are given the “ancestral”, or “primitive” moniker due to their shell-less bodies. Cladists tend to put them low on the molluscan cladogram. Remember H.A.M.? Well, these two groups are enigmatic because, if these cladists are right, the molluscan shell would have vanished from a shelled H.A.M. ancestor, then reappeared. The book says that this is unlikely. HOWEVER, a recent finding in the insecta (Published in Scientific American) seems to indicate that body parts “lost” in the evolutionary process can return (in the given case, it was wings). Darn, I need to find that article… If this is true, then the gene or genes for shell formation would still be found in these two classes, and the lack of shell would be due to extensive modification of their molluscan bauplan.

Imagine evolution reversing its own engineering! Stranger things have happened…

Annelids are segmented, and eucoelomate, but each body part has subtle variations that make it unique. Given their protostome heritage, it is not entirely inconceivable that one or more groups would gradually modify the existing body plan in such a way that the coelom would also be affected. Over time, an ancestral form of annelid (shelled polychaete maybe) could have begun a move toward a less segmented, more molluscan, open-hemocoel form. This is not truly a “de-evolution”, rather it’s a shift in development due to selective pressures and expression of changes in linked genes.

And given the developmental difference, yet structural similarity between coeloms in protostomes and deuterostomes, I think that a lot of genetic analysis will need to be done before we can even consider the coelom as an evolutionary link between groups. The coelom may just be a great idea run through the convergent evolution copy machine.

Hmm... I do play the polyphyly card a lot, don’t I? :heee: Hell, maybe its just my way of copping out, but I can't shake the feeling that similar characteristics do not homologues make.

If the annelid-mollusc links are true, Acaenoplax could be the “missing link” between molluscs and annelids. This is definitely worth a second look.

AUGH!! WHERE IS THE PROOF?!?! @$#%&$*

*sigh* The truth is out there.... oops, wait... that's the X-Files...

Sorry about being so long winded… I think I would have been a lot more coherent if it weren’t for the insomnia.

Sushi and Sake,

John
 

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