Comparative assessment of outline-based vs. virtual modeling-based methods to analyze the ammonoid whorl profile

Abstract

In this report we compare two geometric morphometric methods (Outline-basedvs. Virtual Modeling-based) aiming to analyze the ammonoid whorl profile shape (designatedas whorl cross-section shape in previous contributions). A dataset including 50ammonoid whorl profiles is used to evaluate the two approaches, and the resultingmorphospaces are compared. The morphological variations depicted by PC1 and PC2are consistent across methods and are congruent with previously reported patterns(the predicted morphologies are similar). PC1 captures from 66% to 70% of the totalvariance; along this axis the morphological transformation is mainly associated with thedegree of conch compression. For PC2 the variation is associated with the elongation/compression of the whorl profile, and with the relative height of the imprint zone(degree of whorl overlap). The variation summarized across the first two principal componentsis about 88% for the Virtual Modeling Method (VMM) and about 94% for theOutline-based Method (OBM). The principal differences between the two approacheslie in the way they quantify the imprint zone. The transformations associated with theshape of this region are related to PC3 and are more specifically linked to the shapeand lateral dimensions of the umbilical walls. VMM allows a more accurate quantificationof the imprint zone shape, while OBM shows more limited performances for quantifyingthis specific zone of the whorl profile.