Recent discussions on a GSSP for the base of the Middle Ordovician Series have centered on the proposal of Finney and Ethington (2000) to use the lowest occurrence of Tripodus laevis in Whiterock Canyon, Nevada. Among the criticisms of this section was the conjecture that an unconformity must exist within the Ninemile Formation at or below the base of T. laevis. Detailed collection from fossiliferous limestones within the Whiterock Canyon section has yielded 14 taxa distributed through 24 trilobite-bearing collections (including 12 below the base of T. laevis). Significantly, the stratigraphic ranges of Raymondaspis vespertina, Nileus hesperaffinis, and Acidiphorus ? lineotuberculatus extend from within the overlying Antelope Valley Limestone, past the base of T. laevis, downward 53m (163 feet) to the base of the section. Through the lower 37m (112 feet) of the section these species overlap with the Ibexian trilobite Lachnostoma latucelsum. Absence of a resolvable discontinuity in species ranges should end speculation on the possibility of a significant unconformity within the section.

A second criticism of this GSSP proposal has been the suggestion that the base of T. laevis is younger than Castlemainian. R. vespertina, Nileus svalbardensis, and Cybelurus halo have been recovered from the upper Ninemile and the Profilbekken Member of the Valhallfonna Formation of Spitzbergen. Fortey (1976) has previously correlated the Profilbekken with the late Castlemainian; a correlation affirmed by Cooper and Lindholm (1990). This suggests that the upper Ninemile, including the boundary point, are also Castlemainian.

Elimination of these concerns about the Whiterock Canyon section should reinvigorate its consideration as a GSSP for the Middle Ordovician Series.