THE MEEK SHELL INHERITS THE EARTH: PHYSIOLOGICAL AND ENVIRONMENTAL CONTROLS ON EARLY PALEOZOIC EXTINCTION AND RADIATION

Biological innovation and extinction during Cambro-Ordovician time likely reflect complex emerging ecological interactions and environmental fluctuations. For example, many paleontologists argue that the Cambrian Explosion in some way reflects increased oxygenation of the marine realm as well as intensifying ecological interactions. However, the environmental conditions that followed the Cambrian Explosion and presaged the Ordovician Radiation are less well constrained, as are the evolutionary and ecological effects of these conditions. Here, we discuss potential connections between environment and evolution during this interval. Recent work on the Terreneuvian Cambrian extinction indicates that it was similar in physiological selectivity to other Phanerozoic calcification crises, many of which also greatly impacted reef ecosystems. We discuss potential extinction kill mechanisms. Quantitative analysis of carbonates from Cambrian through Ordovician strata in western Newfoundland and across Laurentia suggests that calcifying organisms did not greatly increase in abundance or as major contributors to carbonate production until Middle Ordovician time. The Ordovician radiation thus represents not just a major evolutionary and ecological milestone, but also a critical turning point in biogeochemical cycling. The delay between the appearance of body plans in the Cambrian Explosion and the expansion of shelly taxa in the Ordovician may be the consequence of long-term anoxia and diminished surface water saturation state with respect to carbonate. The radiation of calcifying organisms during the Ordovician may thus reflect the appearance of a ventilated marine realm conducive to widespread calcification.