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’.
Help, FJ!
(just kidding 
