Biogeography and the Cambrian radiation of arachnomorph arthropods

ONE of John Shergold’s abiding research interests, and an area in which he made fundamental contributions to the fields of palaeontology and geology, was the study of Cambrian arthropods in general, and trilobites in particular (e.g., Shergold 1977, 1988, 1991; Shergold et al. 1990; Shergold & Laurie 1997). Here we focus on what Cambrian arthropods, including trilobites, can tell us about the nature of evolutionary and biogeographic patterns, and their relationship to geological changes, during a key episode in the history of life, the Cambrian radiation. The first appearance of animal life in Cambrian strata has stimulated the curiosity of naturalists ever since Buckland (1836). A growing body of evidence, from studies of molecular sequence evolution to trilobite biogeography, supports the hypothesis that bilaterian lineages originated and began to diverge at the end of the Neoproterozoic, perhaps a few tens of millions of years before their first appearance in the fossil record (e.g., Meert & Lieberman 2004; Peterson et al. 2004). Much recent attention has focused on the roles that extrinsic environmental or geological factors may have played in the Cambrian radiation (Hoffman 1991; Knoll 1996; Dalziel 1997; Lieberman 1997; Veevers et al. 1997; Hoffman et al. 1998). The late Neoproterozoic to early Cambrian was a period of substantial tectonic activity and Meert & Lieberman (2004; also see references therein) provided a recent review of these events, which are briefly recounted here. The supercontinent Rodinia broke apart ca. 750 Ma, with major rifting occurring between western Laurentia and Gondwana (Australia, East Antarctica and South China). Around 600 Ma, the ephemeral supercontinent Pannotia formed. Pannotia rifted apart during the subsequent 50 million years (550-600 Ma), resulting in four distinct land masses by the beginning of the Cambrian: Laurentia; Baltica (Scandinavia and eastern Europe); Siberia; and Gondwana (including South America, Africa, Antarctica, Australia, India, North and South China, Avalonia, and western and central Europe). Of interest here are the roles that tectonic factors may have played in the early evolution and radiation of arachnomorph arthropods, which HENDRICKS, J.R. & LIEBERMAN, B.S., 2007:12:21. Biogeography and the Cambrian radiation of arachnomorph arthropods. Memoirs of the Association of Australasian Palaeontologists 34, 461-471. ISSN 0810-8889.