HUGGING THE SHORE: THE EMERGENCE OF MARGINAL MARINE BIOTAS

The biotas of Ordovician marginal marine Konservat-Lagerstätten in North America include very long-lived clades that are still found in similar environments today (xiphosurids, linguloid brachiopods, hydrozoan medusae, and others), occurring with members of long-extinct groups (eurypterids, conodonts). These biotas are particularly significant as they lived in environments from which organisms were beginning to move landward. Their overall familiar taxonomic makeup may make them seem more “modern” than coeval mineralized assemblages dominated by exclusively Paleozoic brachiopods, trilobites, and corals.

It should, nevertheless, be recognized that the Ordovician world in which these nearshore biotas evolved was strikingly different from our modern one. These ecosystems developed as the result of dynamic events: the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE). The former saw the advent of widespread biomineralization, the marine substrate revolution, increased biotic interaction, and the establishment of all major eumetazoan body plans. The GOBE encompassed even greater changes in global climate, geography, and marine environments, setting the stage for a second extraordinary burst of evolutionary innovation. Starting 488 MYA, and ending in devastating mass extinctions about 45 MY later, the GOBE saw rapid expansion within groups that first appeared in the Cambrian Explosion.

What was the global context for the GOBE and the emergence of marginal marine biotas? For much of its duration, the Ordovician Period experienced “greenhouse” conditions. Mean surface temperature was about 2° C higher than present, partly a consequence of a mean atmospheric CO2 approximately 15 times greater than pre-industrial levels. With little or no water in polar ice caps, mean sea level was up to 220 meters higher than today. Atmospheric O2 volume was 68% of the modern value, there was no significant land life, and no vertebrate predators in the seas. Rapid sea-floor spreading drove maximal dispersal of tectonic plates, and tropical shelf area was more extensive than at any other time. Lower solar luminosity, shorter day length, and vastly different tidal regimes contributed to an Ordovician world in which seemingly familiar organisms evolved to cope with very unfamiliar conditions.