The septa meets the inner wall of the cephalopod shell to form suture lines , a useful feature in distinguishing cephalopods. Nautiloids have simple suture lines. Earliest nautoloids had conical shells which later evolved into animals with planispirally coiled shells. Ammonoids of the Mesozoic era are sometimes referred to as ammonites . Most ammonoids, like nautiloids, are planispirally coiled.
Ammonoids differ from the nautiloids in some aspects.
The most important being the suture lines which are not as simple as in nautiloids. Further, the septa are convex towards the body cavity and the septal necks point toward the aperture. One of the reasons for complex suture lines in cephalopods is that strongly zigzag septa is needed to support the shell which is thinner compared to nautiloid shells. Another reason may be that complex septa hold the muscles tightly to prevent the body from being torn by predatory attacks. Another difference is the strong external ornamentation in the form of growth lines in ammonoids.