Morphostructural constraints and phylogenetic overprint on sutural frilling in Late Jurassic ammonites

The functional signiŽ cance of frilled septa and complex sutures in ammonoids has generated ongoing debate. The ‘classic’ hypothesis envisages ammonoid shells and septa as designed for resisting ambient hydrostatic pressure, complex sutures being the evidence of strength in shells for colonization of deep habitats. Here we address the ‘suture problem’, focusing on the analysis and interpretation of variables in our database of Late Jurassic ammonites not included in previous studies, such as whorl height (Wh), whorl shape (S), shell coiling (WD), taxonomic grouping and basic planispiral shell shape. The results indicate that sutural complexity, as measured by the fractal dimension (Df) value of the suture line, is positively correlated with Wh, and that the sutures of oceanic shells tend to provide, for a given Wh value, lower Df estimates than do those of neritic shells. No general trend of increase in sutural complexity was noted for specimens recovered from swell areas belonging to oceanic fringes with respect to those that inhabited epicontinental shelves. In fact, Perisphinctoidea, the clade best represented in the database analysed, shows a higher Df mean value in neritic species than in epioceanic ones. SigniŽ cant differences in sutural complexity were detected for groups of ammonites classiŽ ed according to shell shapes (WD, S). Oxycones and discocones, streamlined potential swimmers, show the highest Df mean values, while spherocones and cadicones, which were presumably vertical vagrants, present the lowest ones. This indicates that sutural complexity was more related to shell geometry than to bathymetry. & Ammonoids, fractal analysis, Late Jurassic, shell shape, sutural complexity.