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The topology of the septa, especially around the rim, results in the various suture patterns found. [5] The septal curvature in nautiloids and ammonoids also differ in that the septa curves towards the opening in nautiloids, and away from the opening in ammоnoids. [6] Regions of the suture line and variants in suture patterns Ammonite clean cut
Nor are goniatitic sutures limited to the Goniatidia. The sutures of nautiloids are by comparison somewhat simpler, being either straight or slightly curved, whereas later ammonoids showed suture patterns of increasing complexity. One explanation for this increasing extravagancy in suture pattern is that it leads to a higher strength of the shell.
Suture patterns in Prolecanitida evolved differently than in the Goniatitda, by increasing the number of umbilical lobes rather than by subdivision of the lateral saddle. Moreover, the body chamber in Prolecanitida was comparatively short, taking up only about half of the largest whorl.
Where the septum meets the shell a suture line forms; in some ammonoids these lines became extremely complex and elaborate, providing strength without the necessity of added weight. Elaborate sutures allowed for thinner shells, and hence less time needed for shell growth and less time spent in the vulnerable juvenile stage.
Daraelitids are probably ancestral to Ceratitida, the dominant order of Triassic ammonoids. A third proposed family, the Mississippian-age Prodromitidae, is occasionally also placed within the order Prolecanitida. [1] The affinities or monophyly of this family are uncertain due to drastic changes in their suture patterns through ontogeny.
At the same time, shell material is added around the shell opening , enlarging the body chamber and providing more room for the growing animal. Sutures (or suture lines) appear where each septum contacts the wall of the outer shell. In life, they are visible as a series of narrow wavy lines on the outer surface of the shell.
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Ceratitida is an order that contains almost all ammonoid cephalopod genera from the Triassic as well as ancestral forms from the Upper Permian, the exception being the phylloceratids which gave rise to the great diversity of post-Triassic ammonites.