02371nas a2200217   4500008004100000022001400041245015700055210006900212300001400281490000700295520161600302100001701918700001601935700001601951700001601967700001601983700001301999700001802012700002002030856010302050       2025                        eng d  a2205-887700aTrilobite biostratigraphy of the upper Skullrockian Stage (Lower Ordovician; lower Tremadocian) on the northern Laurentian margin, western United States0 aTrilobite biostratigraphy of the upper Skullrockian Stage Lower   a397–4320 v573 aOngoing field collecting has permitted the development of a high resolution trilobite biostratigraphy of the upper part of the Skullrockian Stage (lower Tremadocian; Lower Ordovician) in shallow water facies along the (Ordovician) northern Laurentian margin. This follows previous work documenting new schemes for the Tulean and Blackhillsian stages (upper Tremadocian and Floian) and Stairsian Stage (mid-Tremadocian). Relevant sections are in the Red Canyon Member of the House Formation in the Ibex area, Millard County, western Utah, and in the Garden City Formation, Bear River Range, Franklin County, southeast Idaho. The interval under study was previously assigned to the genus-based “Bellefontia-Xenostegium Zone.” This is replaced with seven formally proposed species level zones, all of which are new in concept. Each zone is characterised by a set of species mostly exclusive to it, and turnover between zones is extensive, with only a limited number of species occurring in more than one zone. Turnover rates are high, as in the most fossiliferous section (Lava Dam North in the southern House Range, western Utah) the seven zones occur through less than 75 m of section. The new zones in ascending order are the Praepatokephalus housensis Zone, Skullrockicurus plummeri Zone, Parabellefontia concinna Zone, Politicurus edwardsi Zone, Bellefontia ibexensis Zone, Politicurus politus Zone, and Rossicurus lepidus Zone. Atop the R. lepidus Zone is the Paraplethopeltis genacurva Zone, the lowest zone of the overlying Stairsian Stage. Three of the name-bearing species of the new z​o​n​e​1 aAdrian, J.D.1 aWestrop, J.1 aLanding, E.1 aKarim, T.S.1 aLosso, S.R.1 aNg, R.Y.1 aBradley, A.B.1 aPerez-Peris, F.  uhttps://nysm.nysed.gov/staff-publications/trilobite-biostratigraphy-upper-skullrockian-stage-lower