PALEOENVIRONMENTAL DISTRIBUTION OF THE LATE CAMBRIAN (FURONGIAN) AGNOSTOID ARTHROPOD LOTAGNOSTUS IN WESTERN LAURENTIA

In excess of 1000 specimens of agnostoid arthropods from siliceous (spiculitic) basinal carbonates of the Windfall Formation in Nevada provided the first opportunity to assess the range of morphologic variation within species of Lotagnostus in Laurentian deposits. These non-compacted and non-sheared specimens from one horizon confirm the presence of at least two non-intergrading species: a strongly scrobiculate form with conspicuous trisection of the posteroaxis, and another with faint or no scrobiculae or trisection. The lack of transitional morphs, throughout the ontogeny of both species, reinforces arguments provided by Westrop and Landing (2016) against the broad species concept advocated for Lotagnostus americanus by Peng et al. (2015), which would encompass both Windfall species. The absence of transitional morphs dismisses the claim that L. americanus is a highly variable and globally distributed species whose First Appearance Datum should define the base of Cambrian Stage 10.

Associated conodonts and trilobites allow fairly precise correlation of the Lotagnostus-dominated faunas from several horizons in the Windfall to coeval middle to upper Furongian strata elsewhere. These include shelfbreak to upper slope deposits of the Jones Ridge Formation in Alaska, and its off-platform equivalent, the Hillard Limestone. The lower slope carbonates of the Hillard have yielded only two specimens of Lotagnostus. No specimens of Lotagnostus have yet been identified within the 14 agnostoid-bearing collections of middle to upper Sunwaptan age (Proconodontus tenuiserratus through Eoconodontus Zones) recovered through over 145m of Furongian strata in the Jones Ridge Formation. The near to total absence of Lotagnostus from faunas from upper slope and platform facies severely limits the utility of that genus for correlation into the Laurentian platform and upper slope successions, making it a highly questionable choice for defining the base of Stage 10.