EVIDENCE OF GLOBAL SIGNAL FOR LATE CAMBRIAN AND EARLY ORDOVICIAN CYCLICAL MASS EXTINCTIONS

A series of closely spaced (3-5 my) mass extinctions have long been recognized on the Laurentian paleocontinent. Three terminate Laurentian Cambrian stages, a fourth, early Tremadocian, event has a sparse literature history but little significant study, and a fifth, late Tremadocian, extinction was documented only recently.

Data derived from taxonomic compilation and revision of Trilobita suggest that these extinction events were either global or pan-tropical in character. Stage-level sampling bins bounded by mass extinctions should yield obvious taxic patterns: the proportion of genera unique to such bins ought to be high and the proportion of genera crossing the boundaries between bins ought to be low. This is exactly what Laurentian data record: boundary crosser diversity through the interval of mass extinctions hovers between 9 percent and 24 percent, but soars to 65-71% for the remainder of the Ordovician. Conversely, the proportion of genera unique to bins (singletons) ranges from 62-81% during the interval of cyclical extinction then drops to less than 15% for the remainder of the Ordovician.

A similar pattern can be demonstrated in the summed global record: proportion of singletons during the extinctions ranges from 64-75% then drops to 14-23%. The Early Ordovician events show intermediate signal, with singleton proportions of 34% and 39% (late Tremadocian event). There is a clear distinction in these data between low-latitude continents where the Laurentian pattern obtains (and there is hence strong evidence of mass extinctions) and high latitude continents where the data record no obvious extinction effects.

Depth may play a role, with continents where sampling is weighted toward deep water faunas showing little signal. Tremadocian data from Baltica, for example, are strongly weighted toward deep water olenid faunas and have singleton proportions of only 27% and 11% (early and late Tremadocian). In the present state of knowledge, however, it is difficult to test numerically for this effect.

In sum, the singleton/boundary crosser signal of the end-Marjuman, end-Steptoean, and end-Sunwaptan extinctions appears to global, and reflected in both low and high latitude data. The signal of the early and late Tremadocian events appears to be pan-tropical, but not present in temperate regions.