PUTTING AMMONITES ON THE MAP: GIS MAPPING OF CONSTRAINTS AND ASYMMETRY IN AMMONITE SUTURE SHAPE

Geographic Information Systems (GIS) provide powerful tools for the spatial analysis of geographic features, but only a few studies have applied GIS techniques to other spatially interesting phenomena. In particular, many anatomical features of fossil organisms, such as ammonite sutures, are amenable to spatial analysis. A new method for inputting and analyzing ammonite suture patterns within a GIS has been developed. This method provides a user-friendly way to compare ammonite suture patterns visually and quantitatively as an aid to classification and the study of septal construction and function.

Exactly how suture patterns were formed as the ammonite constructed each septum remains unclear. Many researchers have argued that septal form was constrained by the position of fixed tie points, located at the tips of sutural lobes, at which the protoseptal membrane was attached to the shell. Most workers assume that these tie points, and therefore folds in the septum, were symmetrical across the animal’s midline. To investigate the existence and symmetry of these purported tie points, suture patterns in the Middle Turonian acanthoceratacean ammonite species Coilopoceras springeri were analyzed with the GIS method. If the “lobe-tip tie point” idea is correct, we expect the suture lines to be more tightly constrained at lobes than at saddles. However, in this species, each specimen’s suture line largely matches those of other conspecific individuals, both at lobes and saddles, suggesting that the entire line is narrowly constrained. Symmetry in suture patterns was assessed by comparing sutures from the right and left sides of specimens of C. springeri. Right and left opposing sutures from the same specimen do not match precisely, although the difference is less than that seen between two right sutures from different specimens. In particular, the right suture is consistently shortened relative to the left suture, even in specimens from different localities, a phenomenon also seen in other members of the Coilopoceratidae. In addition, left suture patterns appear more tightly constrained than right suture patterns. This asymmetry may reflect aspects of the ammonite’s soft part anatomy.