I have a number of questions concerning ammonites, some of which I am not sure are answerable. I am mostly interested in whether they were more nautiloid-like or coleoid-like in their characteristics.

• How many arms did they have?
• Did their arms have suckers? Or could they have had hooks like belemnite arms?
• Might they have had extendable feeding tentacles like squid and cuttlefish?
• What did ammonites eat, anyway? (Particularly the pelagic varieties.)
• Did their eyes more likely have the "pinhole camera" design of a nautilus's or the complex structure of a squid's?
• Did they all have a nacreous shell coating in life?
• Were their siphons flap-like (nautiloids) or tube-like (coleoids)?
• What were their lifespans? Did they live long enough to breed year after year, or did they just live a couple years, then breed once and die?
• Could they change color? (I know this is probably the least answerable question of all.)

I apologize for the long list. Take your time responding if you need to.

Dear Bryan,
                        My Ph. D. was on ammonoids, so I will try to answer your questions. I have to preface my remarks with the important information that we have very little actual evidence of ammonoid soft-parts, but we do know about the structure of their shells, the size of their eggs and the anatomy of their jaws. On the grounds of 16 or so characters that ammonoids share with belemnites, squid and other living cephalopods we think that ammonoids are more closely related to these animals than Nautilus. The similarity of the external shell between ammonoids and Nautilus is only one character, which is apparently the result of convergence.

Arms
On the basis of the similarity between squid, belemnites etc we think ammonoids probably had ten arms. Two long-grabbing arms and eight sensory/prey-handling arms. Ammonoids probably lacked suckers and hooks. There is no evidence for finding hooks that cannot be assigned to other taxa that I know of.

Eyes
Due to their closer relationship to squid, ammonoids are more likely to have had a complex eye

Breeding
Due to close relationship to squid, and also beds in the fossil record where apparent 'mass mortality' events are recorded it is thought that ammonoid probably only breed once then died.

Ammonoid diet
Another real unknown. An ostracod where we would expect the ammonoid crop to be is the only evidence for diet. There has been much speculation on ammonoid diet, including cannibalism of older ammonoids on younger ones.

Nacre
Nacre is widespread through the history of ammonoids, although it is more common in Cretaceous specimens since they have had less time to have their nacre stripped by geological processes.

Siphon
The siphon could have varied among taxa

Colour change
Their shells certainly had colour-banding, as in Nautilus. This alone can be surprisingly effective. The point of camouflage is to disrupt outlines and the 'search image' as much as to blend in with the background. From my own observations there may have been structures in some ammonoids that allowed colour change, but this would involve having parts of the body exposed close to the outside of the shell. We still don't fully understand how colour-matching works in living cephalopods and it may be that ammonoids had a passive system that used mirror-like cells to help them match their background, rather than a system under active nervous control.

"Hope is a duty from which palaeontologists are exempt."
David Quammen