Sutural complexity in Late Jurassic ammonites and its relationship with phragmocone size and shape:: a multidimensional approach using fractal analysis

The evolution of intricate septa and complex sutural patterns in cephalopod ammonoids is one of the best documented trends in the fossil record towards increased levels of complexity. Functional interpretation of septal folding is still, however, a matter of controversy. Tentative explanations have been linked to the structural reinforcement of phragmocones, mantle area increase, buoyancy control and even metabolic functions concerning respiration or cameral liquid transport. Here we use fractal analysis in order to estimate suture complexity in a large set ( N = 524) of Late Jurassic ammonites, and its covariation with phragmocone size, shape and ornamentation. Sutural complexity, estimated by fractal dimension (D-f), is closely related to phragmocone whorl height and the degree of shell involution, while this trend is reversed for tubercle size. On average, specimens from epioceanic habitats display lower D-f values than those inhabiting epicontinental waters. Our results reveal a complex relationship between sutural complexity and morphometric descriptors of phragmocones, indicating that septal folding was more closely related to shell geometry than to bathymetry. In addition, these results fit predictions of a recent model relating sutural complexity to energetic demands of ammonoid metabolism. However, future research should not neglect the implications of phylogenetic legacy as an important source of variability in fractal dimensions.