HOW DO THOSE CRINKLES FORM?: SUTURE VARIATIONS IN THE EXTREMELY PLASTIC CRETACEOUS AMMONITE NEOGASTROPLITES AND PROCESSES OF SEPTAL FORMATION

Suture patterns are a staple character for ammonoid systematists, but controversy prevails over the processes by which these complexly folded shell structures formed. Proposals of closely regulated genetic controls on attachment points or growth rates have vied with models of hydrostatic forces acting on fluids of differing material properties. No clear winner in this debate has emerged. Investigating patterns of suture variation in a well-defined, morphologically variable clade may shed new light on the processes of septal formation. The Early Cenomanian hoplitid ammonite Neogastroplites displays pronounced variability in shell shape and ornamentation. Suture patterns from specimens of three stratigraphically consecutive species of Neogastroplites were analyzed with Geographical Information Systems software to produce detailed, quantitative comparisons among sutures from different shell shapes, sizes, and stratigraphic positions.

Suture patterns are remarkably similar across all specimens, regardless of shell shape, although subtle, consistent differences in the placement of sutural elements exist among the three Neogastroplites species. Hence, suture patterns can potentially distinguish among these taxa, even though shell shape varies widely and overlaps across the three species. The earliest species, N. cornutus, shows more variable suture patterns than the later two species. Sutural complexity does not vary over time or with increasing relative shell width or umbilical diameter; rather, shell size is the primary determinant of suture complexity. Suture line lengths increase linearly with shell diameter in all three species, while the number of folds present along the suture increases logarithmically, at least during early ontogeny (i.e., shells less than 50 mm diameter). These results do not support the idea of genetically-controlled attachment points, but cannot rule out some claims of the viscous fingering model. Variable growth rates along the septal membrane may have produced sutural complexity. The observation that shell size, rather than shell shape, is the key control on sutural complexity also supports the contention that sutural folding was functionally related to a metabolic process, rather than acting as architectural support for the shell walls.