COLONISING THE CAMBRIAN WATER COLUMN WITH BRADORIID ARTHROPODS

Bradoriids were small (sub-centimetric) bivalved arthropods that possessed a global distribution for about 15 million years from the beginning of Cambrian Epoch 2 (at about 521 Ma) through the earlier part of Epoch 3. Most bradoriids were epibenthic, as signaled by their lithofacies, faunal associates, and strongly provincial signal. Evidence for an integumental circulatory system that resembles those of modern myodocopid ostracods, suggests that many bradoriids had an active lifestyle, and they clearly favoured oxygenated seas, with an environmental range essentially exclusive of the dysoxic facies occupied by, for example, phosphatocopid arthropods. Most bradoriids were extinct by the end of the Drumian Age (mid Epoch 3), and the post-Drumian interval is characterized by dysoxic shelf facies and the dominance of phosphatocopids. Co-occurring with these phosphatocopids in black mudstone lithofacies is the bradoriid Anabarochilina primordialis, which possessed a latitudinal range from the tropics to high southern latitude. Its wide environmental and geographical range, coupled with a carapace morphology that indicates possession of an integumental circulatory system, and an active mode of life in well-oxygenated waters, indicates that it was truly pelagic. The earliest Anabarochilina species (from Cambrian Epoch 2) appear to have been shelf marine and epibenthic. Given a benthic antecedent, what anatomical, physiological and environmental feedbacks facilitated the passage of Anabarochilina into the water column? Further, does it display a pattern of zooplanton colonization that is replicated in colonization events by other plankton in the Phanerozoic, most notably by ostracods in the Silurian?