The functional signi 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 (W h ), 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 (D f ) value of the suture line, is positively correlated with W h , and that the sutures of oceanic shells tend to provide, for a given W h value, lower D f 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 D f mean value in neritic species than in epioceanic ones. Signi cant differences in sutural complexity were detected for groups of ammonites classi ed according to shell shapes (WD, S). Oxycones and discocones, streamlined potential swimmers, show the highest D f 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. & Teatinos, 29071 Ma´laga, Spain; Juan A. Pérez Claros [Juan. Perez@urz.uni-heidelberg.de], Geologisch Paläontologisches Institut, Universitaet Heidelberg, Im Neuenheimer Feld 234, D-69120 Heidelberg, Germany; 29th June 2000, revised 4th March 2002 The functional signi cance of uted septa and complex sutures in ammonoids has stirred debate during recent decades (for recent interpretations and controversy, see review and references in