T274 Evolution of Marine Life in the Cambrian II: Biotic, Biogeochemical, and Stratigraphic Contexts

Wednesday, 25 September 2024
1:30 PM - 5:30 PM
204C

RECURRENCE OF BURGESS SHALE–TYPE PRESERVATION IN THE CAMBRIAN STAGE 4‒WULIUAN PIOCHE FORMATION IN NEVADA, USA

Paper number: T274-2
Program: Technical Sessions
Time: 1:35 PM – 1:50 PM

Authors

Abstract

Some 20 exceptionally preserved (EP) fossil assemblages are known from the Cambrian of the Great Basin region in the western USA, but these biotas are not equal in terms of fossil abundance and taxonomic diversity. For example, two EP faunas were found in the Pioche Formation (Cambrian Stage 4–Wuliuan) in eastern Nevada – one in the topmost beds of the Combined Metals Member (Nephrolenellus multinodus Zone) and the other in the overlying lower Comet Shale Member (Eokochaspis nodosa Zone) – but none exceeds half a dozen EP taxa, whereas close to a hundred soft-bodied species are known from the middle Marjum Formation in western Utah. The reasons for this disparity are multiple and may include the difficulty to precisely identify the stratigraphic interval associated with exceptional preservation, even when a specific lithology is targeted (e.g., clay-rich shale).

We report the discoveries of new EP fossils in two stratigraphic levels of the Cambrian Stage 4 Comet Shale Member of the Pioche Formation in eastern Nevada. The taxonomic diversity of the lower Comet biota is enriched by the finding of three arthropods (including the Burgess Shale genus Molaria), a possible lobopodian, a worm, and four sponges (three reticulosans and one ascospongian), the first sponges ever reported in this formation. A new EP assemblage – the third from the Pioche Formation – was discovered in the topmost strata of the Comet Shale (Amecephalus arrojosensis Zone), hitherto thought to solely contain a hyolithid (Haplophrentis carinatus) and two trilobites (Mexicella robusta, Tonopahella walcotti). The radiodont Houcaris magnabasis, a highly spinose species of the arthropod Tuzoia, a possible lobopodian, a scalidophoran, three reticulosan sponges (e.g., Diagoniella sp.), and the brachiopod Nisusia were found in these beds.

The three Pioche EP biotas are all found in strata interpreted as representative of times of maximum flooding of the shelf. We hypothesized that these transgressive peaks coincided with the invasion of the depositional environment by oxygen-depleted waters, which facilitated the preservation of soft-bodied species living in the waters above or the benthic habitats nearby.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-403008

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