NEW INSIGHTS INTO THE CAMBRIAN RADIATION: PHYLOGENETIC PATTERNS IN CAMBRIAN ARACHNOMORPHS (ARTHROPODA)

The arthropod clade Arachnomorpha contains not only trilobites and chelicerates, but also some of the more unusual soft-bodied Cambrian forms known from the Burgess Shale of British Columbia and the Chengjiang region of China. Exceptionally well-preserved arachnomorph fossils are also known from Middle Cambrian strata in Utah. Recently, two new taxa that bear characteristics suggesting arachnomorph affinities were recognized from the Wheeler and Marjum formations of Utah and are not known to occur elsewhere. Cladistic analyses were undertaken to test hypotheses of arachnomorph relationships, with the broader goal of attaining a better understanding of the phylogenetic and biogeographic context of the Cambrian radiation of arthropods.

Previously published morphological character matrices for fossil arthropods were used as a foundation for phylogenetic analyses; several characters were added and modified to facilitate inclusion of new taxa, including the genera from Utah. Phylogenetic analyses employed parsimony and made it possible to study patterns of morphological character evolution within the study group.

Phylogenetic patterns suggest that at least three major arachnomorph clades had diverged by the Middle Cambrian: the new Utah genera group separately in two of these clades. These phylogenetic patterns, in conjunction with information from biogeography, support the view that significant arthropod diversification must have preceded the Cambrian radiation. The analyses also revealed that numbers of trunk tergites in the study taxa, and other characters related to tagmosis, are of low consistency and provide little tree support: this reiterates the notion suggested by Gould and others that characters that are relatively conserved and invariant today displayed significant variation during the Cambrian. Another pattern to emerge from our study is that varying hypotheses of character homology (and consequent character coding schemes) have important influences on tree structure. This is especially the case for enigmatic aspects of morphology such as the “great appendage” structure of Alalcomenaeus, Leanchoilia, and other taxa.