USING BIOGEOGRAPHY TO CONSTRAIN THE TIMING OF THE CAMBRIAN RADIATION

Phylogenetic biogeographic analysis of Early Cambrian trilobites, especially those from the earliest trilobitic part of the Early Cambrian, was conducted to determine whether there is evidence for a vicariance event in these groups which may in fact precede the Cambrian radiation. Such a vicariance event would provide evidence for metazoan divergence in the Neoproterozoic. Biogeographic analysis was also used to deduce the tectonic processes that may have been operating at the time, including True Polar Wander. Biogeographic analysis incorporated phylogenetic and distributional data from the basal redlichiinids, the paraphyletic "fallotaspidoids", and the olenellinids, and used a modified version of Brooks Parsimony Analysis. Analysis of vicariant patterns suggests three major biogeographic groupings: 1) area relationship between Siberia, western Gondwana and terranes marginal to it, and southwestern Laurentia; 2) area relationship between Baltica, eastern Laurentia, and northwestern Laurentia; and 3) area relationship between East Antarctica and Australia. The first biogeographic pattern supports the notion that there was an episode of vicariance in trilobites related to the breakup of Pannotia. Such an episode of vicariance indicates there potentially was a homogenous group of trilobites distributed across Pannotia prior to its breakup, which geological evidence constrains to the interval between 600-550Ma. Based on the phylogenetic position of trilobites, which are a derived clade of metazoans, this result from biogeography may provide further support for the argument that the Cambrian radiation actually has fairly deep roots. The other biogeographic patterns accord with the presumed position of Baltica near the northeastern margin of Laurentia, and the close association of Australia and Antarctica. Patterns of geo-dispersal were also evaluated and these were poorly resolved, implying that continental fragmentation most influenced patterns of trilobite evolution and distribution in the late Neoproterozoic and Early Cambrian, rather than continental collision or sea-level rise and fall. The paucity of evidence for range expansion and the resolved patterns of vicariance suggest that if True Polar Wander occurred in the Early Cambrian, it did not leave a biogeographic signature.