THE ORDOVICIAN RADIATION: A MACROEVOLUTIONARY CROSSROADS

The Ordovician Period has long been a focal point for integrative research on the history of global biodiversity. Because it closely followed the Cambrian Explosion, researchers are seeking to understand whether the three-to-four fold increase in genus- and family-level diversity recognized during the Ordovician Radiation was an inevitable outcome of macroevolutionary transitions set in motion during the Cambrian, or whether it was contingent on physical or biological events unique to the Ordovician Period, or both.

Here, we consider three important themes related to the dissection of the Ordovician Radiation:

1) The nature and timing of the abrupt global transition from what was apparently a “dead interval” with little diversification during the Late Cambrian to the remarkable radiation that followed shortly thereafter. Continued evolution of the geologic timescale may affect significantly our interpretation of evolutionary rates and transitions during this critical interval, as will improved assessments of Late Cambrian paleobiogeography.

2) The relationship among diversification at the alpha (within-community), beta (between-environment and between-region), and global levels. Preliminary analyses suggest that, whereas alpha diversity increased in conjunction with the global increase, beta diversity may have decreased.

3) The relationship between biological and physical transitions at local and regional scales. The Ordovician Period was marked by significant temporal variations in carbon, oxygen, and sulfur isotopes, but the extent to which these relate to lithologic and tectonic transitions in a geographic context, as well as to macroevolutionary patterns, remains to be determined.

Given the remarkable research progress of the past two decades and the continued integration of data from new sources such as molecular biology, the Ordovician Radiation offers an unparalleled opportunity to understand the causes of a major global diversification, but a multidisciplinary effort will be required to achieve this goal.