Hi Andrew,
Do you study ammonites or cephalopods? My husband attended a talk last
week at our Paleo Society and the presentation was on ammonites and
modern cephalopods.  (I wish I'd gone). The presenter expressed the same
thing about the argonaut egg case looking like an ammonite.  He also
said that the argonaut egg case is rather "spongy".  I've never seen
one, other than in pictures.
James Wood, in the Cephalopod link I've pointed out shows argonauta as
an octopus, and not closely related to modern nautilus.  I think that is
old news to those who study cephalopods, but I was surprised to learn
this.  Paleontologists also believe that ammonites and nautilus are not
closely related.
 
Kay Lavalier wrote:
 
>  Andrew Vik
> Tampa, FL., USA
> [log in to unmask]
>
> Nora:
>
> I have found the recent discussions of Argonuata interesting. The
> shell (eggcase)  of the argonaut seems to me to resemble an ammonoid
> shell rather than that of a nautiloid. Both the nautiloid and the
> extinct ammonoid belong to the primitive tetrabranch, or four-gilled
> wing of the cephalopoda. The argonauts belong to the more advanced
> dibranchiate, two-gilled cephalopods. The dibranchiata tend to have
> internal, non-chambered and non-coiled shells (Spirula being the
> modern exception). How did the argonaut come to emulate the shell of
> it's more primitive relatives? I think that the ancient argonauts,
> back during the reign of the ammonites, used empty ammonite shells as
> brood chambers for their offspring. As these natural nurseries
> gradually became scarce, due to the decline of the ammonites, the
> argonauts adapted by manufacturing their own facsimiles. Thus, the
> only non-fossil ammonite shells to be found today are counterfeits
> created by the argonaut with it's genetic memory of what an ammonite
> shell looked like all those many millions of years ago. That's my
> guess, anyway. Any other thoughts on this subject?
>
> Andrew
>
> NORA BRYAN wrote:
>
>> Here's a nice picture of an argonaut with shell case.
>> http://is.dal.ca/~ceph/TCP/Argo.html
>> By the way, for those interested in Cephalopods, this is a great
>> site.  I found it
>> as a link on the Jacksonville shell club page.
>>
>> Thomas E. Eichhorst wrote:
>>
>> > Nora,
>> >
>> > The argonaut shell is indeed an egg case developed by the female
>> > argonaut.  The males are much smaller than the females and without
>>
>> > shell.
>> >
>> > Tom
>> >
>> > NORA BRYAN wrote:
>> > >
>> > > Thanks David, I forgot to acknowledge you in my earlier Reply
>> (somehow your
>> > > reply wasn't connected to the thread).  I thought that the
>> Argonaut shell was
>> > > an egg case, and therefore solely female.  I need to do some
>> more reading I
>> > > guess.
>> > > Nora Bryan
>> > > Calgary, Alberta
>> > > Canada
>