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Cambrian trilobites and biostratigraphy of the Grand Canyon and vicinity, USA
Published online by Cambridge University Press: 02 December 2025
Abstract
Cambrian faunas of the Grand Canyon are neither diverse nor abundant, and several historical collections seem to have mixed taxa from several biozones. As a result, fine-scale reinterpretation of Grand Canyon biostratigraphy and broader-scale correlation of the Tonto Group and its fossils have been hampered by the lack of definitive and refined biostratigraphic indicators. Here we report new fossil occurrences from the Tonto Group, including from units that previously lacked biomineralized fossils, such as the Frenchman Mountain Dolostone. When integrated with historic collections, our data indicate that the Tonto Group ranges from the upper Dyeran to lowermost Steptoean Laurentian stages (global upper Series 2, Stage 4 to basal Furongian Series, Paibian Stage). Trilobite assemblages span the upper Dyeran (Peachella iddingsi Biozone) to lower Marjuman (Bolaspidella Biozone) stages and are distributed from the uppermost Tapeats Sandstone to lowermost Frenchman Mountain Dolostone.
Based on lithologic and paleontological data, three disconformities occur within the Cambrian of the Grand Canyon: (1) between the Dyeran and Delamaran stages in the uppermost Tapeats to lowermost Bright Angel Formation, (2) within the Delamaran Stage within the Bright Angel Formation, and (3) between the Delamaran and Topazan stages, higher in the Bright Angel Formation.
Cambrian trilobites from the Tonto Group consist of seven olenelloid, 17 corynexochid, and 33 ptychoparioid species, as well as 10 taxa that are not identifiable at generic or higher levels. New species include Kootenia barensis and Albertella nebeli. Some taxa are new to the Grand Canyon region, although they are known from the Great Basin (Nevada, California), southern Arizona, Montana, and Canada. These taxa include olenellids: Mesonacis fremonti (Walcott), Olenellus gilberti Meek in White, Olenellus terminatus Palmer, and Olenellus cf. O. fowleri Palmer; corynexochids: Anoria? bessus (Walcott), Caspimexis cf. C. radiatus (Palmer in Palmer and Halley), Kootenia aff. K. granulospinosa Palmer, 1968, and Zacanthoides cf. Z. typicalis (Walcott); and ptychopariids: Amecephalus cf. Am. normale (Resser), Am. piochensis (Walcott), Am. scapegoatensis (Deiss), Crepicephalus? upis Walcott, Ehmania weedi Resser, Eldoradia cf. E. prospectensis (Walcott), Glyphaspis tetonensis Resser, Kochina vestita Resser, Mexicella cf. Me. robusta Sundberg and McCollum, Mexicella mexicana Lochman, Modocia sp. indet., Solenopleurella? quadrata Rasetti, Trachycheilus granulosus Sundberg, and Tr. whirlwindensis Sundberg. Type specimens of Solenopleurella erosa Resser was divided into two species: the pygidia are assigned to Kootenia erosa (Resser) new combination, and the much smaller cranidia are assigned to Spencella? sp. indet.
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