Early Upper Cambrian (Marjuman) linguliformean brachiopods from the Deadwood Formation

Abstract

The Deadwood Formation is an Upper Cambrian to Lower Ordovician succession of sandstones, shales, siltstones and limestones that blanketed central western North America during the initial Phanerozoic transgression. This transgression led to a broad, shallow epeiric sea which onlapped the Transcontinental Arch to the east and was protected on its western—seaward—side by a system of carbonate platforms now exposed in the Rocky Mountains. The Deadwood Formation is mostly a subsurface unit, but several exposures exist in the northern Great Plains due to uplift by Eocene igneous intrusions. Linguliformean brachiopods were recovered from two areas: the Black Hills of South Dakota, and two subsurface cores from Alberta and Saskatchewan. Forty-five species of linguliformean brachiopods assigned to twenty-eight genera were recovered from these localities and described. Giving provisional names, one new family, Holmerellidae, one new subfamily, Neotretinae, five new genera, Amplitreta, Dianabella, Ganotoglossa, Holmerellus, and Vangaporosa, are erected and seventeen new species are described: Amplitreta cyclopis, Amplitreta elongata, aff. Anabolatreta tora, Canthylotreta parislata, Curticia pustulosa, Dianabella artemesia, Ganotoglossa leptotropis, Holmerellus convexus, Holmerellus, acuminatus, Holmerellus limbatus, Kotylotreta nupera, Linnarssonella tubicula, Opisthotreta nuda, Rhondellina albertensis, Tropidoglossa costata, Quadrisonia? sigmoidea, and Vangaporosa dakotaensis. The family Holmerellidae is distinguished by pitted larval shells and smooth postlarval shells, a feature that is unique in the Linguloidea. The composition of the new subfamily Neotretinae recognizes the evolutionary relationship of the genera Neotreta and Rhondellina, which are more closely related to each other than to any other acrotretid genera. Based on a comparison of the brachiopod assemblages with similar faunas from Australia and elsewhere in Laurentia, the sections studied are determined to be late Marjuman (early Late Cambrian) to early Sunwaptan (middle Late Cambrian) in age. The subsurface faunas provide the first biostratigraphic dates for any part of the Deadwood Formation in Canada. Faunas from South Dakota come from strata near the base of the formation and below the first trilobite occurrences, this giving a more refined age for the transgression in South Dakota. A large number of shells with perforations assumed to have been caused by predators were recovered from two localities in South Dakota, and represent the first evidence of predation of fossil lingulids. Two types of perforations were identified: round holes with sharp, non-beveled edges, and irregularly shaped holes with chipped edges. The former hole type is attributed to either steady pressure applied over time (e.g. boring) or to a swift, piercing percussive strike. The latter hole type is attributed to a smashing percussive strike with a blunt appendage. Based on criteria established by the proposed attack-mode models, various hypothetical animals are discussed as potential linguliformean predators. While the evidence for these predators is circumstantial, it indicates a more complex benthic paleoecology that had hitherto been envisaged for the Upper Cambrian.