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Thanks for your kind words, all. That was really a very hard one to put together as there is so little information out there and I had to really milk every source I could for every drop of info. I really could not have contributed any more even if I had wanted to as I simply didn't have access to any research materials or articles beyond those quoted. But I'd like to say a massive 'THANKYOU!' to Dr. Joanne Kluessendorf for allowing me to use the Pohlsepia images from her research article. If you read this Joanne, I really appreciate it.
These articles are getting harder to compile as they get more specialist in nature; with ammonites it's a question of being swamped with info, with octopuses it's scraping around in the barrel.
...and I'm not even THINKING of cuttlefish!!!!
Still, we've got another article due anytime now from Neuropteris, so thankyou in advance Andy!!!!!
Looking back even further to the common ancestor of squid, cuttles, octos, ammonites etc, is there any consensus or hypothesis as to how the tentacles evolved? That is is it thought that there was one arm that split to become two, four, ten etc. in successive ancestors over time or would the 10 tentacles have evolved into tentacle-ness from some other organ, such as sensory organs (like catfish whiskers)
Thanks, Snafflehound, most kind! I must admit, I’ve wondered exactly the same thing. We really need Fujisawas to have a look at this one as I’m sure the answer lies in neural connections and complexity.
The fact that all modern coleoids (and the belemnites) have a variation of the 10 arm plan would seem to indicate that their common ancestor among the bactritids, those peculiar Devonian nautiloid offshoots, would have almost certainly have had 10 arms too. It seems likely that the ammonoids would also have had ten arms as it is believed that the bactritids were their ancestors, though as we have no soft-bodied ammonoid fossils this cannot be certain.
Yet the modern Nautilus has 80-90 arms and two pairs of gills amongst many other differences, so clearly could not be on the same nautiloid lineage that gave rise to these bactritids as it is simply too different. I suspect that the nautiloids were a diverse group of animals with a great variation in soft-bodied forms, afterall there were probably something in the region of seven or eight super-families of nautiloids when the earliest bactritids appeared in the Devonian; today there is, of course, just one, the Nautilida.
The ten arm plan was probably established way back in the Silurian or maybe even earlier amongst a particular group(s) of nautiloids. One can imagine the earliest nautiloids in the Cambrian just having a sensitive ‘mass’ on their heads and each group developing in an individual manner during the nautiloid ‘explosion’ in the Ordovician. We really need some fossils of nautiloid soft-body parts to make much sense of all this. I don’t think there are any, but if anyone knows any different…..
But as for your question of why ten arms particularly evolved, I really don’t know. The ten supple arms of the coleoids seem to be so much more complex than the basic sensory apparatus of the Nautilus; maybe there was some form of selective pressure favouring fewer arms, I think the term is ‘evolutionary streamlining’.