Hemichordates and algae from the Cambrian (series 3) Wheeler and Marjum Formations (Utah) and Burgess Shale (British Columbia)

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Geography and Geology


The Wheeler and Marjum Formations of western Utah provide a particularly rich archive of Mid-Cambrian biodiversity, rivaling the Burgess Shale in this regard and indeed containing many of the same nonmineralized taxa. This report provides an overview of newly discovered specimens of dithecoid graptolites from the Wheeler and Marjum Formations, the first examples of this group of primitive graptolites to be recorded from these units. Study of this material was aided by the use of backscattered electron (BSE) imaging, which revealed otherwise hidden constructional details. Dithecoid graptolite specimens from the Wheeler Shale are considered to represent Archaeolafoea Chapman, 1919 and Tarnagraptus Sdzuy, 1974, genera previously known only from Australia, Siberia, and Spain. BSE images of these specimens show clear fusellar structure, including oblique sutures, confirming a graptolite affinity for Archaeolafoea. This material occurs in association with specimens that resemble Yuknessia simplex Walcott, 1919, an algal taxon erected on the basis of material from the Burgess Shale but also previously reported from the Cambrian of Utah, including the Wheeler Shale. The Wheeler Shale specimens recovered in this study, however, show fusellar structure in BSE images and are identified as cephalodiscid hemichordates, previously known from this unit on the basis of a single specimen. Specimens of Y. simplex from the Burgess Shale, including the holotype, on the other hand, do not show fusellar structure in BSE images, supporting their original description as algae. Type material of the algal taxon Dalyia racemata Walcott, 1919 from the Burgess Shale also appears essentially featureless in BSE images with the exception of one specimen. This syntype, which is conspicuously different in gross morphology from the rest and from a different locality, shows a pattern resembling fusellar structure, suggesting that it represents a hemichordate taxon previously undocumented in this key Konservat-Lagerstatten. The newly documented Utah occurrences of dithecoid graptolites significantly increase the known species-level diversity of this group in Laurentia during the Cambrian and markedly extend paleogeographical distributions for this early group of hemichordates.

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