Shell features, main colonized environments, and fractal analysis of sutures in Late Jurassic ammonites

Abstract

A precise approach to the quantification of relationships between suture complexity, as measured by fractal analysis (step-line procedure), the architecture of shells, and the main colonized environments, has been made in a set of Late Jurassic ammonites (N=507). Statistically significant differences between fractal-dimension (Df) mean values of evolute and involute shells are interpreted as caused by differences in the surface:volume (S:V) ratio. Suture complexity is also related to the shape of whorl section. The lowest Df values correspond to subcircular whorl sections (low S:V ratio) and the highest ones to acute sections (high S:V ratio). The shape of flanks shows correlation with suture complexity. The highest values of Df are found in planulate shells and the lowest ones in whorl cross sections with convex flanks. Highly significant differences appear between Df mean values from unsculptured shells and those from ammonites with ribs and/or tubercles of medium to large size. Multivariate analysis shows a combined variation of shell features and suture complexity, resulting in a heterogeneous distribution of Df values within the ammonite morphospace, mainly according to structural (shell architecture) and ornamental (sculpture strength rather than density) factors. Finally, the data obtained on relationships between suture complexity and the colonized environments (epicontinental vs. epioceanic inhabitants) suggest that suture complexity is not primarily related to bathymetry, and/or that no major differences in habitat depths existed between epicontinental and epioceanic ammonites.