A morphometric analysis of mygalomorph spider carapace shape and its efficacy as a phylogenetic character (Araneae)

Abstract

Shape features are often used as characters in cladistic analyses, yet states of these characters are seldom delineated in an objective, repeatable fashion. Carapace shape is one such character that is often used in analyses of mygalomorph spider relationships. For example, most analyses have used cephalic region morphology (e.g. steeply arched v. flat or sloping) as a key feature that delineates some major clades. In practice, carapace shapes at the extremes are relatively easy to identify; however, intermediate carapace shapes have proven to be much more difficult to objectively score in one of three shape categories. In this study, carapace shape is used as an exemplar characteristic to evaluate the utility of shape features in phylogenetic analyses and to evaluate our ability to effectively score discrete character states. Carapaces from 178 spider specimens sampled across 13 of the 15 nominal mygalomorph families were digitally photographed and their outlines were traced and pseudo-landmarks plotted. An elliptical Fourier analysis was then employed in an attempt to both delineate and assess character states. It is concluded that landmark analyses are inappropriate for carapace shape delineation as a result of difficulties in scoring precision and, thus, outline analyses are the preferred method given the morphology. Carapace shape is not a very useful character for broad-level systematic studies of mygalomorph relationships owing to the variable nature of the feature and our inability to detect discrete discernable shapes.