CAPTURING THE FUNCTIONAL VARIETY OF EARLY GNATHOSTOMES: A CASE STUDY USING CLADISTIC CHARACTERS AND BIOMECHANICAL TRAITS

An increasing number of paleobiological studies use pre-existing cladistic datasets to estimate shifting levels of morphological disparity over time and investigate the relative positions of taxa within a multivariate morphospace. Although these patterns are derived from characters selected to reconstruct phylogenetic relationships, it is sometimes suggested that they also reflect functional or ecological variation. We tested this assertion by comparing disparity metrics calculated from 24 published cladistic matrices targeting early jawed vertebrates with disparity measured using a purpose-built matrix of functional mandibular traits. The functional mandibular traits reflect a series of biomechanically relevant morphological metrics collected from the lower jaws of 198 Silurian-Devonian gnathostome genera. Distance matrices calculated from functional and cladistic data are significantly correlated, but substantial variation in functional dissimilarity goes unexplained by variation in cladistic dissimilarity. When cladistic characters are limited to those pertaining to the skull and then mandible, agreement between distances matrices sequentially improves, but not substantially. When multivariate morphospaces are created by subjecting the dissimilarity matrices to principal coordinates analysis, there is no clear relationship between phylogenetically independent contrasts for scores on lower functional and cladistic coordinate axes. However, even though the distance matrices underlying the ordinations are phenetic, we do find that phylogeny explains the majority of variation on the first axis of the ordinations based on cladistic characters. A resampling procedure reveals a positive, but weak, correlation between cladistic and functional disparity measures for equivalent sets of taxa. Although our results show non-random relationships between disparity patterns extracted from biomechanical and cladistic character sets, we conclude that these relationships are not sufficiently strong to allow clear inferences about functional variation to be drawn from data assembled for phylogenetic reconstruction.