EXTRAPOLATING INCOMPLETE AMMONITE SUTURES IN THE FRACTAL STEP METHOD

Fractal dimensions as indices of geometric complexity for ammonite sutures are traditionally based on hemisutures, (half-sutures, from venter to umbilicus) which are visible on moderately eroded shell surfaces. Here, we describe a revised fractal measurement method requiring only the lateral lobe and saddle (LLS), the part of a suture not distorted by shell curvature near the venter and umbilical shoulder when viewed in profile. When taking a fractal dimension using only lateral suture elements, the LLS fractal dimension must be converted to the value that would be expected for a full hemisuture, as this is the standard expression of fractal complexity for ammonites. We have circumvented this discrepancy via an additional division step inserted into the traditional fractal equation. The factors needed to perform these conversions are predictable with and specific to ammonite genera. As a result, these “Conversion Values” may have additional implications for certain unresolved taxonomic questions about ammonites. We calculated and give a reference table of the Conversion Values for a number of common ammonite genera. The original purpose of the LLS method was to circumvent loss of access to specimens during the pandemic by enabling use of photographic data for ammonites. The expansion of fractal analysis to include ammonite sutures from photographs, as well as damaged and incomplete specimens where only lateral features are available, however, will undoubtedly still be relevant beyond COVID.