for extra chaos, I just re-read
Spirula spirula
4 rows of arm suckers (how many rows of hooks did belemnites have?) and tentacle clubs covered in just a "lawn" of suckers. No corneas (
Sepia have corneas) Both arms IV hectocotylized.
And I'd point out that another view is that, because of the lack of fossil arms, comparison to the extant arms may be the only way to fit the coleoids in with the shelled fossils.
And the shell: coiled opposite ammonites (and we know that from cameral fluid, of course) and
Nautilus. Siphuncle marginal, not central. Simple sutures. No ornamentation. No living chamber. No aptychi.
"Spirula is able to withdraw its head and arms completely within the mantle; the mantle opening can then be closed by folding over the large dorsal and ventrolateral extensions (= flaps) of the mantle margin"
Switching to Nixon & Young:
Giant fiber system like the other decabranchia (lacking in octopods and
Vampyroteuthis). Arm suckers have peduncles like other decabranchians with "an inner ring which has blunt teeth of almost uniform height and size." Tentacle suckers similar but smaller. Beaks: lower rostrum "protrudes and is sharp." Radula vestigial, lateral buccal flaps with stout teeth. Eye muscles resemble
Sepia and
Sepiola. "The central nervous system follows the general plan found in other decabranchians, but is without special similarity to Sepiolidae or Sepiidae." "Discriminant analysis of several parameters of the statocyst place
Spirula amongst the buoyant squids."
Which brings up an unrelated question for Kevin and the other fossil hunters: does anyone ever sift through the matrix of ammonites looking for the statolith? Obviously, it's a needle-in-a-haystack proposition, but there are a lot of ammonite beds out there, one might think that the statolith is occasionally preserved-- it's pretty much a rock, after all. Assuming ammonites even had them (quite likely, both given their presumptive lifestyle and because
Nautilus and all coloeoids have statocysts, although
Nautilus lacks a statolith ad rather has "statoconia," which are crystals that are ovoid or spherical.) Knowing what ammonite statoliths looked like would be interesting in light of the extensive studies of how coleoid statocysts match their lifestyles.
We need laboratory cultures of
Spirula and a
Spirula genome project and several squid, octopus, vampyrotetuhis, and nautilus genome projects for comparison. And make sure to do an argonaut genome as well, for comparison. Any takers?