Cambrian trilobite biostratigraphy and its role in developing an integrated history of the Earth system

Abstract

One of the principal biostratigraphical tools used in the lower Palaeozoic, and especially the Cambrian System, is trilobites. Historically, both polymerids and agnostoids have commonly been included as ‘trilobite’ groups, although currently the question of whether they comprise a monophyletic group or are polyphyletic is unsettled. Beginning in the late 1800s, the base of the Cambrian System was marked by the lowest occurrence of olenelline trilobites. Discovery of a rich pre‐trilobite metazoan record in the mid‐1900s led to significant modification of that concept, but polymerids and especially agnostoids still rank among the leading biostratigraphical and chronostratigraphical guides through much of the Cambrian. Chronostratigraphical sub‐division of the trilobite‐bearing part of the Cambrian System derives largely from biostratigraphical, ecological and evolutionary concepts about agnostoids and polymerids introduced in the 1940s to 1970s by A.H. Westergård, A.R. Palmer and R.A. Robison. Westergård introduced a zonation for Scandinavia that was largely based on agnostoids. Palmer explained the distribution of Cambrian trilobite faunas in terms of restricted and unrestricted access to open oceans. Together, these ideas coalesced in Robison's recognition of separate zonation schemes for restricted‐shelf polymerids, open‐shelf polymerids and open‐shelf (cosmopolitan) agnostoids. Palmer also introduced the concept of biomeres, which placed sharp limits on biostratigraphical intervals recognizable from trilobites. Global correlation in the upper half of the Cambrian today depends to a large extent on the ranges of agnostoids and some polymerids characteristic of open‐shelf to slope areas, ones that facilitate identification of precise intercontinental tie points. Agnostoid and polymerid biostratigraphy is now being integrated with information about coastal onlap and eustatic sea‐level history, geochemical cycling and other data to provide a more complete understanding of the early Palaeozoic biosphere and its complex physico‐chemical context.

Global Standard Stratotype-Section and Point (GSSP) for the conterminous base of the Miaolingian Series and Wuliuan Stage (Cambrian) at Balang, Jianhe, Guizhou, China

“…5), and in eastern Guizhou and western Hunan it is overlain by the agnostoid Ptychagnostus gibbus Zone of the Huaqiao Formation (Peng, 2009(Peng, , 2018. The Peronopsis taijiangensis Zone is correlatable with the Ptychagnotus praecurrens Zone of Laurentia, where the agnostoid Ptychagnotus praecurrens Zone, together with the overlying Ptychagnotus gibbus Zone, correlates with the upper half of the polymerid Oryctocephalus Zone (Robison and Babcock, 2011;Babcock et al, 2017). Ptychagnotus praecurrens is a widespread agnostoid trilobite, known from Sweden (Westergård, 1946;Weidner and Ebbestad, 2014;Ahlberg et al, 2019), England (Rushton, 1966), Russia (Siberia) (Egorova et al, 1976;Naimark, 2008;Shabanov et al,2008), Australia (Laurie, 2004;Kruse et al, 2009), the USA (Utah and Nevada) (Robison, 1982;Sundberg, 2007, Sundberg, 2011), and probably from Kazakhstan (Ergaliev and Ergaliev, 2008).…”

The Contribution of Fossils to Chronostratigraphy, 150 Years after Albert Oppel

“…Babcock et al . (this issue) review the global use of trilobites in biostratigraphical and chronostratigraphical studies that have led to the development of the global series and stage nomenclature for the Cambrian System. Of the two series and five stages now ratified, apart from the Terreneuvian Series/Fortunian Stage base defined on a trace fossil, each of the other defined stage bases is identifiable primarily by the first appearance of a cosmopolitan agnostoid species.…”

Agnostus pisiformis — a half a billion-year old pea-shaped enigma

“…Seemingly insignificant and contrasting with the polymerid trilobites, agnostoids are characterized by a head region (cephalon) and a tail region (pygidium) of nearly equal size (e.g., Robison, 1975;Kaesler 1997;Babcock et al, 2017). These are, in turn, separated by two thoracic segments with tergites.…”

“…Whilst agnostoids could grow to a maximum length exceeding a centimeter, they usually are less than 10 millimeters long (e.g., Robison, 1975;Kaesler, 1997;Babcock et al, 2017). Adult specimens of A. pisiformis may reach approximately 10 mm in total length (c. 4.5 mm for each shield).…”

“…However, the 'soft tissue' (appendage) structures discovered from the 'Orsten' Lagerstätte rather suggested a crustacean (sensu lato) affinity (Müller and Walossek, 1987). Although this conclusion was drawn from empirical evidence from the fossil records and was based on uniquely well-preserved material, contrasting opinions with regard to the evolutionary relationship between polymerid trilobites and agnostoids do still persist (for a recent summary, see Babcock et al, 2017, and references therein). However, as noted by Haug et al 2010, a sister group relationship of the Agnostina and Crustacea does not exclude a close relationship to the Trilobita.…”