GSA GEOPHYSICS DIVISION GEORGE P. WOOLLARD LECTURE: WAS THE CAMBRIAN EXPLOSION AN ARTIFACT OF TRUE POLAR WANDER?

Charles Darwin suspected that the Cambrian "explosion" might be an environmental artifact. A more recent, initially controversial hypothesis that repeated true polar wander (TPW; reorientation of the solid Earth to spin around the maximum moment of inertia of the planet) triggered the Ediacaran-Cambrian explosion of animal life has been supported by numerous paleomagnetic and geochronologic refinements. These data imply TPW about an equatorial axis of ~75° between 535 and 515 Ma, coinciding with “exploding” metazoan diversity and disparity. Here we emphasize that contingency of Cambrian paleogeography may have been crucial in determining the dramatic "explosion" pattern relative to earlier ages of synchronous bio-innovation and polar wander.

During early-middle Cambrian TPW, ecosystems should have experienced changes in standing diversity consistent with the latitudinal diversity gradient (LDG). Because biodiversity increases toward the equator for ecological reasons, equatorward-rotating areas during TPW should increase in biodiversity, while poleward-rotating areas will experience contraction. Rapid TPW also predicts a quadrential pattern of relative sea-level change: parts of the globe rotating equatorward experience transgressions; those rotating poleward experience regressions. Cambrian TPW thus explains the intensity of the Laurentian Sauk transgression relative to its apparent absence on the Baltic platform. During TPW, this association of LDG biodiversification with transgression, and the corresponding diminished biodiversity associated with regression will yield a net fossil record of diversification both for ecological (LDG) and preservational (stratigraphic, geographic) reasons.

Although diversity might thus be considered partly artifactual, enhanced rates of origination and extinction also could increase disparity, especially if Early Cambrian TPW occurs during a time interval when genetic regulatory networks were critically poised for expansion and exaptation.

Phylogenetic and physiological innovation among metazoa has become widely recognized as much as 100 Myr prior to the Cambrian explosion. We suggest that the iconic early Cambrian phylum-level diversification strongly reflects the contingency of TPW acting upon the paleogeography of that age.