WESTERMANN MORPHOSPACE METRICS APPLIED TO COEXISTING UPPER CRETACEOUS NAUTILIDS AND AMMONITES

As an alternative to previous techniques for analyzing shell shapes of fossil cephalopods, such as principle component analysis of multiple shell measurements, geometric morphometric Procrustes analyses, and elliptical Fourier analysis of cross sections, we used scaled ternary diagrams or Westermann Morphospace Metrics (WMM). WMM were previously tested for planispiral ammonites. We extended the technique to nautilids (Eutrephoceras). We compare WMM results for nautilids and ammonites (Hoploscaphites, Rhaeboceras) coexisting, and indeed sometimes found in the same fossil concretions, from the Upper Cretaceous (lower Maastrichtian) Pierre Shale on the Cedar Creek Anticline in east-central Montana. We discuss the possible implications of the Westermann Morphospace positions of these related but different cephalopods in terms of their possible swimming abilities, habitat, and potential ecological competition.