2000 GSA Annual Meeting -- Reno, Nevada
Author(s): BUTTERFIELD, Nicholas J., Dept. of Earth Sciences, Univ. of Cambridge, Cambridge, UK CB2 3EQ, email@example.com
Keywords: Cambrian, exceptional-preservation, arthropods, worms, problematica
Burgess Shale-type fossils provide an unusually accurate view of ancient life by documenting the occurrence of non-mineralizing organisms. Their characteristic preservation as organic-carbon films appears to have been enhanced during the Lower and Middle Cambrian, particularly along the margins of Laurentia. In Jasper National Park, Alberta, Burgess-type fossils are found in both the Lower Cambrian Mural Formation and Middle Cambrian Pika Formation. Fossiliferous shales of the Mural Formation straddle the Nevadella Bonnia-Olenellus boundary, and conformably overlie archaeocyath limestones. Burgess Shale-type fossils, both in situ and from locally derived scree, include a variety of arthropod carapaces, anomalocarid claws, worms, and specimens of an undescribed arthropod. Most significant are numerous specimens of the enigmatic ?arthropod Vetulicola, and a single occurrence of the 'walking appendages' of an anomalocarid, possibly Parapeytoia. Previous reports of both these forms have been limited to the Lower Cambrian Chengjiang fauna of China; their appearance in the Mural suggests a possible temporal correlation. The Pika Formation falls within the upper Middle Cambrian Bolaspidella zone and is broadly correlative to the Wheeler and Marjum formations of Utah. HF maceration of a mudcracked shale horizon near the top of the formation has yielded a range of organic-walled fossils, including Wiwaxia sclerites/chaetae, simple chaetae, and an assortment of bizarre toothed structures, apparently arthropodan. This co-occurrence of middle Lower Cambrian and upper Middle Cambrian Burgess Shale-type fossils spans most of the known time range for such preservation. Despite the marked difference in palaeo-water depth between the two assemblages, there are indications of evolutionary turnover: Wiwaxia, for example, has proven to be both geographically and environmentally widespread in younger Burgess-type assemblages, but has yet to be detected below the middle Bonnia-Olenellus zone.
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