TERMINAL SKULLROCKIAN AND TERMINAL STAIRSIAN (LOWER ORDOVICIAN) LAURENTIAN MASS EXTINCTIONS: THE LAST "BIOMERES"

Laurentian cratonic benthic faunas were affected by a series of three significant mass extinction events during Late Cambrian time. Palmer termed these diversification/extinction cycles "biomeres," and they later became the basis for the upper three stratigraphic stages of the Laurentian Cambrian. Although the Cambrian events have received considerable attention, less well known is that the pattern continued well into the Ordovician. An earliest Ordovician "Symphysurinid Biomere" has been recognized, though the nature and extent of its terminal Skullrockian Stage mass extinction remains poorly documented.

Extensive meter scale field collections of rich silicified trilobite faunas in the Great Basin provide details of the terminal Skullrockian event. Further, they reveal an equally striking mass extinction at the top of the succeeding Stairsian Stage. The terminal Skullrockian event removed most of the major trilobite clades common in western Laurentia: hintzecurine hystricurids, Bellefontia-group asaphids, and the widespread genus Clelandia all became extinct. They were replaced in the Stairsian by faunas dominated by the hystricurid subfamilies Hystricurinae and Hillyardininae. Both became extinct in the end-Stairsian event, which no established trilobite genera survived.

These Ordovician events were equal in scope and severity to the better known Cambrian extinctions. The end-Stairsian event was the last of five major cratonic mass extinctions in the Cambrian-Ordovician of Laurentia. The generic composition of post-Stairsian trilobite faunas changed in much more gradual fashion until the great end-Ordovician extinction.