FAUNAL DYNAMICS AND SYSTEMATICS OF DEPAUPERATE POST-EXTINCTION TRILOBITE FAUNAS, TERMINAL SKULLROCKIAN STAGE (LOWER ORDOVICIAN), GREAT BASIN

The depauperate Paraplethopeltis Zone trilobite fauna was established following a cratonic mass extinction near the top of the Lower Ordovician (Ibexian) Skullrockian Stage. The extinction horizon at sections in the Ibex area (Millard County, Utah) occurs immediately above an unusual, orange-weathering dolomitic siltstone. The overall stratigraphy of the section suggests a sea-level fall coincident with the extinction, followed by a general deepening trend. Fossil packstones occurring within the Paraplethopeltis Zone in upper House Formation beds yield abundant silicified trilobite sclerites and articulate brachiopod shells. Fossils are in excellent condition and show little evidence of sorting. The packstones are probably shallow-shelf storm deposits.

The depauperate post-extinction trilobite fauna, sampled in both the Ibex area and the Bear River Range of southern Idaho, includes three species of the hystricurid Paraplethopeltis (one new), and previously undescribed tuberculate hystricurids and an agnostoid arthropod. A more diverse but still species-depauperate fauna 6.5 m higher includes the earliest known Laurentian cheiruroidean, a leiostegiid, two hystricurids related to Paraplethopeltis, a remopleuridid, and an agnostoid arthropod. Diversity does not return to pre-extinction levels until a horizon some 19 m above the extinction event.