must
have enjoyed free reign of the oceans, preying most likely on the numerous
trilobites. By the end of the Cambrian, this subclass of cephalopods had
diversified into many different shapes and sizes. Some shells had partially
uncoiled to straight shapes and ranged from a few centimeters to 3 meters
across. There are even fragments that suggest sizes up to 10m! For 100
million years the nautiloids must have been the rulers of the oceans, until
the development of a similar but more successful creature: ammonoids.
At first glance, it is amazing that a different classification was given to these other shelled cephalopods. There are however, several subtle, but significant differences. Two major contrasting features stand out once the inside of the phragmacone is observed. The siphuncle, which in nautiloids is well within the interior of the shell, is instead placed next to the outer surface. More significantly, the sutures
(the
area where the septa meet the inner wall of the shell) are not straight
as in the Nautilus. They instead trace complex, waving patterns. This is
the main characteristic that divides the two classifications, though the
actual function of complex sutures is still far from resolved and an area
of debate among invertebrate paleontologists.
