LIFE ON THE EDGE IN THE CAMBRIAN: LESSONS FROM TAPHONOMY, REDOX, AND EXTINCTION

The Cambrian Explosion and Ordovician Radiation are two of the greatest evolutionary events of the Phanerozoic, connected by an interval of elevated extinction rates, pervasive low oxygen, and ecosystem stress. Geochemical, sedimentological, and paleontological records provide evidence that the middle and later Cambrian and Early Ordovician marine realm experienced spatiotemporally low and varying oxygen levels that had profound biological consequences. Low-oxygen marine settings impacted not only the ecology of benthic animals but also the taphonomy of their fossil records; modes of soft-tissue and skeletal preservation linked to redox-sensitive authigenic mineral phases are particularly common during this interval. Biomeres, trilobite extinction events that span this interval, often co-occur with major perturbations to the carbon cycle and evidence for low-oxygen conditions. Skeletal abundance of animals, both within and outside of reefs, remained low until Middle Ordovician time, suggesting that unstable environmental conditions persisted until well into the Paleozoic. The persistence of relatively low bioturbation levels, as well as the prevalence, in other shallow marine settings, of wrinkle structures and other microbialites, further point to the protracted development and expansion of classic Phanerozoic animal ecologies. Thus, the Cambrian and Early Ordovician in many ways represent a transitional interval–new evolutionary advances with clear holdovers from earlier intervals of Earth history, as organisms and ecosystems evolved in the face of persistent stress. Here we review myriad lines of evidence that point to a Cambrian world on the edge, with biological innovations occurring in the face of sustained low and variable oxygen levels.