Laurentian lingulate brachiopods on the outer shelf were affected by a series of three extinction events coinciding with trilobite extinctions (biomere boundaries). There was rapid turnover of the brachiopod faunas during each event. These extinctions are seen at the Steptoean/Sunwaptan, Sunwaptan/Skullrockian, and Skullrockian/Stairsian North American Upper Cambrian–lowest Ordovician stage boundaries. Lingulate brachiopods were examined from the Orr, Notch Peak, House, and Fillmore formations of western Utah and from the Catlin Formation and Hales Limestone of Nevada. Associated with each extinction event, brachiopod genera/species appeared that are also found in coeval strata in Kazakhstan and/or western Gondwana (e.g., Quadrisonia minor, Zhanatella rotunda, and species of Eurytreta). Some of these taxa may have appeared in the deeper water/higher latitude environments of Kazakhstan or Gondwana before appearing on the Laurentian shelf. Eurytreta may have appeared in deep-water slope environment at Tybo Canyon, Nevada before appearing in shelf environments in Utah.
The Steptoean/Sunwaptan and Sunwaptan/Skullrockian boundaries were also examined in the Wilberns and Tanyard formations of central Texas. Latest Steptoean strata yield taxa widely distributed in Laurentia, although endemic to it, such as Linnarssonella girtyi. There is abrupt faunal change at the base of the Sunwaptan, followed by a fauna that is also endemic to Laurentia and is virtually identical to a fauna described from the Snowy Range Formation of Wyoming and Montana. This fauna is not present in coeval strata in Utah and Nevada. Mid-Sunwaptan strata in Texas yield a fauna with strong affinities to coeval strata in Utah; all species are endemic to Laurentia. The Sunwaptan/Skullrockian boundary is also characterized by complete turnover, this time with a replacement fauna having affinities both to the Laurentian outer shelf and to Gondwana and Kazakhstan.
This pattern of events suggests that Late Cambrian–earliest Ordovician extinctions in Laurentia were caused by environmental changes that altered conditions on the Laurentian shelf to conditions more similar to those in deeper water (Kazakhstan) or higher latitudes (Gondwana), and these changes facilitated migration of globally distributed lingulate brachiopods.
2011. 543-543 p.