New Article on The Cephalopod PAge

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,


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

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.


*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,

Thanks to Phil & Dr. Wood for taking the time to create 2 great articles. Certainly filled in some yawning gaps & clarified a few points I've struggled with :read:
I feel like my "party balloon" knowledge base has been connected directly to the Zeppelin like same of these two fine cephalopodan pedagogues & I'm fillin' up ! :notworth:
After a Bank holiday weekend having brushed up on the Cambrian, gained some invaluable prepping experience & read these 2 articles I'll confidently take on a room full of Eastenders viewers at PaleoTriv !! :grad:

"Although not as glamorous as Tyrannosaurus rex or as dramatic as Velociraptor, these extinct creatures are nonetheless quite interesting animals."

Fascinating as they are in their own right, in my opinion the creatures that have & still inhabit the oceans are light years ahead in the glamour & drama dept. of any of us landlubbers i.e
Tyrannosaurus rex v. Liopleurodon ferox ? no contest :whalevsa:
Hi John, Spartacus (Hey, I'M SPARTACUS!!!),

Any new information on phylogeny of molluscs out there?

To be honest John, I really have not looked that sort of stuff up for many months so I'm afraid I couldn't really say. If I do find anything of interest, I'll let you know. Sorry, old chap!

After a Bank holiday weekend having brushed up on the Cambrian, gained some invaluable prepping experience & read these 2 articles I'll confidently take on a room full of Eastenders viewers at PaleoTriv !!

Thanks, Spartacus! Actually I was thinking about going back and changing much of that ammonite article one of these days. It was the first one of these things that I wrote and to be honest, does not really talk about the ammonites themselves that much (and the pictures are crap)...I'll get around to it one of these days. I'm trying to pull together something on prehistoric giant squids at the moment, but information is very hard to find. It will be a short one this time.

As for the Cambrian stuff, if you want to read more I would recommend:

Steven J Gould "Wonderful Life" (1989) on the Burgess Shale. A classic, even read by Jimmy Carter!

Simon Conway-Morris "Crucible of Creation"(1999). Dry reading and really a reply to Gould but written by one of the top researchers in the field so worth getting.

Richard Fortey: "Trilobite!" (2001). A brilliant little book which makes trilobites appear quite exciting and is very funny in places and full of amusing anecdotes. Fortey is a senior palaeontologist at the British Museum of Natural History in London yet somehow manages to bring up Bond villains, Australian pubs in the Outback and The Beatles. After you read this, you'll really want to find one yourself.

Also recommended about the evolution of marine life is Richard Ellis' book "Aquagenesis" (2001) which covers practically every major group of marine animals, vertebrate and invertebrate, and is very clearly written with nice illustrations. There's a good section on cephalopods too.

I'm Spartacus Artisticus, you're welcome & way too modest. Thanks for the recommended reads too, Trilobite sounds right up my street & I could read it to my pet flexi-calymene. There must be something in the water, formaldehyde possibly, at the NHM as I believe they may all be a tad eccentric. I attempted to get my name in neon lights via the entomolgy dept. with the 1st capture of otiorynchus armadillo in Norfolk. All I'd actually subdued was an otiorynchus sulcatus, in my defence they're like peas in a pod & the defining difference being something like the urine dewpoint.
BTW I've a theory (already, for one so novice you all cry) on the demise of your superbly illustrated plectronoceras which I'm surprised all you bright buttons that I now mix with haven't noticed as it IS a major design fault.
Just take a look at how close this poor creature's gills are to its anus !! :shock:
Put yourself in a plectronoceras' shoes for a minute, these creatures weren't wiped by outside influences, they just gave up ! :angel:

Is this the little critter to which you refer?

I had no idea that we were under attack by alien weevils in Southern Britain! Mind you, we get all sorts of peculiar invertebrates here in Dover, being a major port. I'm not referring to the contents of the local night club, but peculiar colonies of tiny Mediterranean scorpions infesting the old stone walls in the dock area and parts of the town. They apparantly glow under UV light, though I have not tried it myself (always wondered who first discovered that - strange). Must try and find out exactly what they are one day.

All sorts of horrendous things scuttle off the freight ships docking, especially those carrying fruit and foodstuffs. Most die quickly but not all......Another pub in the town centre that shall remain nameless, has a colony of Black Widows out the back. Had a good look at one close up once, really should have brought a camera.

Good point about the Plectronoceras, its a wonder how any animal that breathed through a cloud of its own detritus could have evolved into anything impressive... :smile:
that's the fella, grab him !! I'm still convinced I was right & Max at the NHM
is unwilling to share the limelight. He didn't even return my film pot :roll:
Seems they hide in shipments of shrubberies, courtesy of Roger the Shrubber from Italia.

Dover sounds like a bug fan's Utopia. I remember seeing Sir Dave Attenborough waving a UV lamp about down there showing off them wee critters & a few years earlier (lots) he was showing off the (native ?) scorpions of Ongar station.

always wondered who first discovered that - strange
what's stranger is which deviant found cow's milk was yummy !

Dover sounds like a bug fan's Utopia. I remember seeing Sir Dave Attenborough waving a UV lamp about down there showing off them wee critters & a few years earlier (lots) he was showing off the (native ?) scorpions of Ongar station.

always wondered who first discovered that - strange
what's stranger is which deviant found cow's milk was yummy ! :yuck:

its a wonder how any animal that breathed through a cloud of its own detritus could have evolved into anything impressive

There's a lot of people out there who talk detritus !
Time to make like George Romero and raise the dead thread! On Friday, a guest lecturer at Humboldt State University will be speaking on the Green Chiton. The lecture is titled "Green Chiton Eggs and H.A.M." and I hope to get the lowdown on some great mollusc development information and what happened to H.A.M. (Hypothetical Ancestral Mollusc) theory.

Hope to post more soon! Trying to earn my keep as a SC,

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