COMPARING CATEGORICAL AND GEOMETRIC MORPHOMETRIC METHODS IN QUANTIFYING MORPHOLOGY WITHIN OSTEOSTRACI

The quantification of morphology allows the exploration of evolutionary dynamics that are not captured by studies of biodiversity or phylogenetic reconstruction alone. However, the methodologies utilized in assessing trends in disparity are extremely variable depending on the taxa in question, the material available, and the hypotheses being addressed. The two most common approaches are geometric morphometrics and discrete characters. Previous studies have found congruence in the results using these two methods, which is expected given their relationship to the underlying phylogenetic structure. However, it is plausible for these methods to differ in ways of particular interest such as the anatomical distinctiveness of clades, the timing of morphological expansions and contractions, and the relative disparity of groups.

To explore how the different approaches in capturing morphology influenced patterns of disparity, we examined the Early Paleozoic stem-gnathostome clade Osteostraci. The two methods produced similar clustering of clades within morphospace and timing of morphological diversifications. However, comparing the relative disparity between clades is more complicated. The relative disparity of the two largest clades within the dataset (Benneviaspidida and Thyestida) is in disagreement depending on the method used (categorical verse geometric morphometric). To explore this disagreement, we subsampled the empirical data by randomly selecting groups of the size of the largest clades and calculated the ratio of disparity using both the categorical and geometric morphometric data. In addition, we simulated geometric and categorical traits based on the phylogentic tree and calculated the ratio of disparity of the modeled Benneviaspidida and Thyestida. In both cases, there is a strong correlation in the disparity ratio between the two methods. However, the two methodologies often disagree (~20-50% of subsamples) on which of the two groups is most disparate.

Overall, both geometric morphometrics and discrete characters are capturing morphologic trends generated by the underlying phylogenetic framework. Nonetheless, these two methods still capture different aspects of morphology such that discrepancies can arise even within a broad agreement in morphological trends.