OK, here's the "Scientific American" version.

Cephalopods, including ammonoids and actinoceratoids, are mollusks. Living
cephalopods include squid, octopus, and nautilus. Extinct representatives
include the lovely spiral shells of the ammonites.

Early cephalopods had relatively straight shells, like long cones with
chambers inside. The chambers were separated by walls ("septa") and
connected by a long tube ("siphuncle"). In some groups, the shell became
coiled into a flat, symmetrical spiral (e.g., modern Nautilus). The septa
also became very complexly and beautifully shaped in some groups (e.g.,
ammonites). But in the Devonian, there were still plenty of straight,
conical ("orthocone") cephalopods swimming in the sea.

Oddly, some of the ammonites became UNcoiled, though they retained the
complex septa. These relatively late forms are common in Cretaceous rocks
and include genus Baculites.

Andrew K. Rindsberg
Geological Survey of Alabama