Small shelly fossils and trace fossils near the Precambrian‐Cambrian boundary in the Yukon Territory, Canada

Abstract

In the past an ‘explosion’ in diversity and abundance of small shelly fossils and of trace fossils has served to mark the base of the Cambrian. However, no evidence has been presented to prove that the ‘explosions’ of the two groups were synchronous. We describe small shelly fossils and trace fossils from the same phosphatic limestone beds that indicate that the two events were separate in time. The small shelly fossils are Anabarites trisulcatus, Hyolithellus cf. H. isiticus, Microcornus? sp., Protohertzina anabarica, P. unguliformis, P. sp. A, Pseudorthotheca sp. A, Rushtonia? sp. A, four types of tuberculate plates and one type of reticulate plate. These fossils represent a restricted, ‘pre‐explosion’ fauna and are assigned to the Anabarites‐Circotheca‐Protohertzina Assemblage Zone, an uppermost Precambrian zone in the Meishucun Stage, Yunnan Province, China. A point at the top of this zone has received strong international endorsement for future designation as the base of the Cambrian. Associated with the small shelly fossils are the trace fossils Cruziana sp. A, Cruziana? sp. B, Rusophycus sp. A, Palaeophycus rubdark and arthropod scratch marks. If found in isolation, this trace fossil assemblage would be considered as post‐Precambrian because it includes large, highly organized arthropod traces that are traditionally accepted as occurring above the trace fossil ‘explosion’. We therefore conclude that the trace fossil ‘explosion’ predates the small shelly fossil ‘explosion’. If the proposed location of the base of the Cambrian in Yunnan is accepted, the small shelly fossil ‘explosion’ concept and its relationship to the boundary would not be greatly modified. The trace fossil ‘explosion’, however, would no longer indicate the base of the Cambrian and the ranges of some trace fossils would be extended into the Precambrian.

Integrated Ediacaran chronostratigraphy, Wernecke Mountains, northwestern Canada

“…A slope setting is also interpreted for the lithologically similar Sheepbed Formation in the Stratigraphic levels of fossil occurrences based on the present study and reported occurrences from Hofmann et al (1983) Nowlan et al (1985) and Narbonne and Hofmann (1987). …”

“…A terminal Proterozoic age is indicated by: (1) the occurrence of Ediacaran fossils and simple trace fossils in the Blueflower Formation (Hofmann et al, 1983;Narbonne and Hofmann, 1987); (2) the assemblage of small shelly fossils of Early Cambrian age in the Ingta Formation (Nowlan et al, 1985); and (3) the association of complex trace fossils reported from the overlying Vampire Formation (Fig. 4).…”

“…Abundant small shelly fossils, including Anabarites trisulcatus and Protohertzina anabarica, that define the base of the Cambrian in the Ingta Formation were documented by Nowlan et al (1985). Complex trace fossils, including Treptichnus pedum, Rusophycus and Cruziana, indicative of a sub-trilobite Cambrian age, are also present at the base of the Vampire Formation .…”

The Lower Cambrian Fossil Record of the Soviet Union

“…11, panel 56), which occurs at the base of the Nemakit-Daldyn Horizon (Missarzhevsky, 1973). Protohertzina was widespread over the entire Siberian Plat- form, and in Kazakhstan, Mongolia, China, the Lesser Himalayas, and western North America (Conway Morris and Fritz, 1980;Missarzhevsky and Mambetov, 1981;Luo et al, 1984;Nowlan et at, 1985;Brasier and Singh, 1987;Hamdi, 1989). In the Maly Karatau , Mountain Shoria, and Mongolia, Proto hertz ina cultrata Missarzhevsky occurs up to the Botomian.…”

Commentary

“…Many and varied potential criteria for their distinction were suggested, but it was not until the exhaustive review of type specimens by Pemberton and Frey (1982) that the presence or absence of wall linings emerged as the primary ichnotaxobase. This ichnotaxoDownloaded by [North Carolina State University] at 10:43 23 September 2012 base has since been consistently adopted by other authors for distinction between the two (e.g., Ekdale et al, 1984;Nowlan et al, 1985;Clausen and Vilhjalmsson, 1986;Narbonne and Hofmann, 1987;Martino, 1989;GierlowskiKordesch, 1991;Pickerill, 1992;Leszczynski and Uchman, 1993;Miller, 1993;McCann, 1993;Stanley and Pickerill, 1994) thus providing nomenclatural stability. Hence, the normal provisions of the I.C.Z.N.…”