Ammonites, subclass
Ammonoidea, and nautilids, subclass Nautiloidea are first of all both
cephalopods with external, chambered, shells. So how are they different
and how are they alike, other than for their external shells
Nautiloids
came first, having first appeared in the Late Cambrian, and gave rise
to the ammonoids in the Early Devonian. Some sticklers for cladistic
purity might say that ammoinoids are simply a type of nautilids. From a
purely evolutionary perspective this could be said to be so. However
ammonoids are sufficient distinct to warrant being put in a separate
equal taxon to the Nautilidea. Why?
Nautiloides
have concave septa dividing the chambers of the phragmocone, as viewed
from to front, looking back toward the apex of the shell. Sutures,
marking the junction of the septa with be body of the shell are
generally straight and transverse or may have shallow saddles and
lobes. A few later forms from the Late Cretaceous and early Cenozoic even have goniatitic sutures, resembling
those of the Late Paleozoic ammonoid Goniatitida. None have sutures
nearly as complex as those found in Mesozoic ammonoids
The siphuncle, the tube
that interconnects the chambers in cephalopod shells is varied and often
complex in nautiloids but is always a simple narrow tube in
ammonoids. Ammonoid siphuncles are commonly although not exclusively
found along are ventral or outer margin. Nautiloid siphuncles are
commonly ventral or central or somewhere in between. In both groups
there are forms with dorsal siphuncles found along the near the dorsal
or inner margin
In summary, one can study
ammonoids from the outside, looking at general form, ornament, and
sutures. One has to study nautiloides from the inside to get a true
picture, looking at the internal details of the shell, especially those
of the siphuncle.
