CAMBRIAN TRANSGRESSION AND 'EXPLOSION' OF METAZOAN LIFE LINKED TO OPENING OF IAPETUS-PACIFIC OCEANIC CONNECTION

The abrupt appearance of almost all animal phyla in the fossil record is often colloquially referred to as the Cambrian ‘explosion’ of life on Earth. It is also named ‘Darwin’s dilemma,’ as he appreciated that this seemingly mysterious event posed a major problem for his theory of evolution by natural selection. It coincided with a time of major marine transgression over all the continents. Although the metazoan ‘explosion’ is now seen as more protracted than formerly recognized, it is still regarded one of the most critical events in the history of the biosphere. One of the most striking aspects of the earliest Cambrian fossils is geographic differentiation. In particular, the first benthic trilobite faunas on Laurentia, ancestral North America, and the newly amalgamated southern supercontinent of Gondwana are distinctly different. This has led to the suggestion of an unknown vicariant event intervening between an ancestral trilobite clade and higher members that are represented in the fossil record, possibly one related to the breakup of a supercontinent. Laurentia is bordered by latest Precambrian-Cambrian rifted margins and must therefore have been located within a Precambrian supercontinent. Geochronologic and geochemical evidence indicates that it was attached to part of the East Antarctic craton in the late Mesoproterozoic. Igneous rocks on both continents indicate that rifting to sever that link occurred just prior to the first appearance of trilobites in the fossil record. This event would have separated the Olenellid trilobite fauna of Laurentia from the Redlichiid fauna of Gondwana by opening a major oceanic connection between the developing Iapetus and pre-existing Pacific ocean basins with profound global environmental effects at the time of the Cambrian ‘explosion,’ including expansion of continental shelves. The paleogeographic settings of the two great transgressions of the Phanerozoic, the Cambrian and Cretaceous, are remarkably similar. Both seem to have involved comparatively rapid increase in ridge crest length within the ocean basins of the time.