PRELIMINARY COMPARISON OF PALEOZOIC AMMONOID SUTURE MORPHOLOGY QUANTIFIED USING A WINDOWED, SHORT-TIME FOURIER TRANSFORM TO EXTERNAL SHELL COILING

Ammonoid sutures have long been considered to be both phylogenetically and evolutionary significant due the extreme variety and swift turnover of line geometries. Despite this, these features are frequently ignored in macro-evolutionary analyses focusing on ammonoid shell morphology due to their geometric and mathematic complexity. However, recent work by the author (in press) and others (e.g. Gildner 2003) has shown that Fourier-based morphometric methods can uniquely quantify line geometries, if corrections are made to account for the irregular and non-periodic nature of sampled lines.

Here I make a preliminary comparison of suture geometry quantified by a windowed, short-time Fourier transform (STFT) to the Raupian coiling parameters (W, D, S) for ~600 Paleozoic and basal Triassic ammonoid genera. The STFT is a modified Fourier analysis which accounts for the non-periodicity and transience in suture form by 1) smoothing the sampled data by convolving it with a tapering function and 2) reducing error in spectrum estimation by calculating the Fourier transform over a moving window and averaging the results.

Suture geometries show dramatic shifts through time, with later taxa frequently avoiding previously inhabited regions of morphospace; in comparison, recent work by Saunders and others (2004) has shown that 3 modal shell geometries were shared by ~72% of the Paleozoic ammonoid genera. While this implies that on average there is little correlation between shell coiling and suture morphology (with the Prolecanitida being a well known exception), a more direct comparison of shell and suture geometry suggests otherwise. Principle components analysis of STFT-derived variables (relative component of spectrum in each of very-low, low, medium, and high frequency ranges) and Raup parameters shows nice clustering of taxonomic groups. Furthermore, more detailed comparisons indicate significant correlations exist between coiling and suture geometry within superfamilies. These data for the first time directly quantify a relationship between approximations of external shell shape and suture geometry, nicely illustrating the strength of the STFT approach as a basis for comprehensive exploration of ammonoid shell evolution.