The quest for an Australian Cambrian stage scale

Abstract

The chronostratigraphical scale is a hierarchical scheme that is subdivided into increasingly smaller units, from eonothem down to the level of the substage and beyond. Boundaries of chronostratigraphic intervals typically coincide with individual bioevents. As these intervals become smaller, their geographic utility tends to shrink. Typically, where the original interval is inapplicable, the next highest interval in the scale is used for communication. Where appropriate intervals are unavailable, confusion reigns. A stadial subdivision of the Australian mid–late Cambrian (equivalent to the international Miaolingian and Furongian series) was completed in 1993 with the definition of the Furongian Iverian Stage. One stage, the Ordian, was initially suggested as the lowest stage for what is now the Miaolingian, but should be considered to belong to upper Series 2 of the Cambrian. No older Cambrian stages have been proposed in Australia. Indeed, the base of the Ordian has not been defined, due in part to the incompleteness of Series 2 successions in central and northern Australia. A longstanding impediment to the establishment of lower Cambrian stages in Australia arises from the fact that the entire Australian stadial scheme for the Miaolingian and Furongian series was established in the cratonic basins of central-northern Australia, whereas the lower Cambrian is best developed in separate South Australian basins. With the rapid increase in knowledge of the biostratigraphic successions in the South Australian lower Cambrian (Terreneuvian and Series 2) over the last three decades, the time seems ripe for the establishment of such a stadial subdivision. This will require careful correlation between the mostly Terreneuvian and Series 2 succession in South Australia and the mostly Miaolingian–Furongian succession in central and northern Australia. Taxa that can be used for such a stadial subdivision include trilobites, organophosphatic brachiopods, archaeocyaths, small shelly fossils, molluscs and acritarchs, as each has provided the basis of zonations in the South Australian successions.

John R. Laurie [[email protected]], Geoscience Australia, Symonston Australian Capital Territory 2601, Australia, and School of Natural Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia. Peter D. Kruse [[email protected]], South Australian Museum, Adelaide South Australia 5000, Australia. Glen A. Brock [[email protected]], School of Natural Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia. James D. Holmes [[email protected]], Palaeoscience Research Centre, University of New England, Armidale New South Wales 2351, Australia. James B. Jago [[email protected]], University of South Australia-STEM, Mawson Lakes, SA 5095, Australia, and South Australian Museum, Adelaide, South Australia 5000, Australia. Marissa J. Betts [[email protected]], Palaeoscience Research Centre/LLUNE, University of New England, Armidale New South Wales 2351, Australia. John R. Paterson [[email protected]], Palaeoscience Research Centre, University of New England, Armidale New South Wales 2351, Australia. Patrick M. Smith [[email protected]], Australian Museum Research Institute, Sydney New South Wales 2010, Australia, and School of Natural Sciences, Macquarie University, North Ryde New South Wales 2109, Australia.

Keywords:

• Cambrian
• stages
• Australia

THE DEVELOPMENT of the concept of the Cambrian System has been detailed by Cowie et al. (1972), and the early history of the discovery and study of rocks from that system in the various parts of Australia has been presented by Shergold (1989, 1995) and Cooper & Jago (2007). In many cases, the Cambrian age of these rocks was suspected before it was known, and it was often Etheridge (1890, 1896, 1897, 1902, 1905) who initially determined their age.

Cambrian rocks are widespread in Australia, being present in every state and the Northern Territory (Fig. 1). However, most of the successions that have been intensively studied palaeontologically are in basins within the middle third of the continent. These include the Stansbury, eastern Officer and Arrowie basins in South Australia; the eastern Amadeus, central and western Georgina, Wiso, Daly and eastern Ord basins in the Northern Territory; the eastern Georgina Basin in western Queensland; the entirely subsurface Warburton Basin at the borders of South Australia, Northern Territory and Queensland; the subsurface western Officer Basin and the Bonaparte and western Ord basins in northern Western Australia; the Koonenberry Belt in northwestern New South Wales; and the Dundas, Adamsfield and Dial Range troughs and Smithton Basin in western Tasmania.

Fig. 1. Map of Australian basins and provinces containing Cambrian sedimentary rocks.

In Australia, rocks of possible Cambrian age were first reported by Burr (1846) in South Australia (fide Cooper 1984); Selwyn (in Fairfax 1859) in Victoria; Gould (1867) in Tasmania; Hardman (1884, 1885) in Western Australia; Brown (1895) in the Northern Territory; Saint-Smith (1924) in Queensland; and Warner & Harrison (1961) in New South Wales. Despite many of these discoveries being made well over a century ago, a comprehensive biostratigraphic framework for the Cambrian has been long in the making (Shergold 1995, p. 6). Activity has been intermittent; it was initially exploratory and descriptive, and was subsequently followed by research conducted largely in support of regional mapping programs. It is only in the last few decades that much more detailed work has been undertaken, building upon the seminal work of Etheridge (1880–1919), Whitehouse (1927–1945) and Öpik (1956–1982).

Much remains to be done; specifically, the completion of an Australian stadial subdivision of the Terreneuvian Series and unnamed Series 2 of the Cambrian. Previously, correlations in the Australian lower Cambrian have often resorted to potentially imprecise correlations to the Siberian stadial scheme. A regional stadial scheme is needed to allow more precise correlations within Australia.

Towards the current stadial subdivision

Etheridge’s (1890, 1896, 1897, 1902, 1905) work was largely descriptive and occurred during a period of initial discoveries of Cambrian rocks as exploration of the continent expanded and a gross stratigraphy developed. At this time, a distinction between Cambrian and younger systems was more important than establishing an accurate position within the Cambrian (Shergold 1995).

Whitehouse (1927, 1930) was the first to begin the subdivision of the Cambrian System in Australia, using fossils to identify the middle and upper Cambrian rocks of the Georgina Basin in northwestern Queensland. Whitehouse (1930) created a Templeton Series, which was initially deemed to contain two faunal stages: the Dinesus Stage and the Redlichia Stage (Fig. 2). The following year, Whitehouse (1931) added the Leiagnostus Stage to the middle Cambrian, and additionally recognized two upper Cambrian stages based on Pagodia Walcott, 1905 and Proceratopyge Wallerius, 1895, separated by the occurrence of Glyptagnostus reticulatus (Angelin, 1851). All of these names were used by Whitehouse (in David 1932) in an attempt to provide a framework within which to integrate successions in South Australia, Victoria, Tasmania and Queensland. The lower Cambrian was divided into three stages: Archaeocyathus, Protolenoid and Redlichia; the middle Cambrian into three: Obolella, Dinesus and Leiagnostus; and the upper Cambrian into four: the Pagodia and Proceratopyge stages of Queensland overlain by the Florentine Valley and Caroline Creek faunas of Tasmania (both of which are now known to be Ordovician). Subsequently, Whitehouse (1936, 1939) recognized the Georgina, Pituri and Ninmaroo series succeeding the Templeton Series in the Georgina Basin. At this time, the Templeton Series was expanded to contain the Redlichia, Amphoton, Inouyella, Dinesus, Phoidagnostus, Anomocare and Solenopleura stages in the middle Cambrian, whereas the upper Cambrian contained the Anorina, Glyptagnostus, Pagodia and Elrathiella stages (Shergold 1995, p. 6).

Fig. 2. Development of current stadial scheme. The columns showing the schemes devised by Whitehouse (1936, 1939), Öpik (1956, 1961, 1963, 1967, 1968), Jones et al. (1971), Öpik (1979) and Shergold (1993) are calibrated against the ‘traditional’ tripartite division of the Cambrian in the left-hand column, and are plotted where the various authors suggested they belonged at the time of publication. The current scheme, which is in the column second from right, is calibrated against the most recent global scheme of Peng et al. (2020), which is shown in the right-hand column. Zone abbreviations: A. seminula = Agnostus seminula; L. laevigata = Lejopyge laeviagata; G. nathorsti = Goniagnostus nathorsti; P. punctuosus = Ptychagnostus punctuosus; H. parvifrons = Hypagnostus parvifrons; P. atavus = Ptychagnostus atavus (= Acidusus atavus); P. gibbus = Ptychagnostus gibbus.

In 1939, Whitehouse proposed further modification of his stratigraphic scheme for the Georgina Basin. In this, the Redlichia Stage was retained in the lower Cambrian; the middle Cambrian contained, in ascending order, the Amphoton, Eurostina, Dinesus, Agnostus seminula, Phoidagnostus, Papyriaspis and Anomocare stages; and in the upper Cambrian, the Eugonocare, Glyptagnostus, Rhodonaspis and Elrathiella stages (Fig. 2). Subsequently, David & Browne (1950) combined the first three of Whitehouse’s middle Cambrian stages into a Xystridura Stage, retaining the Redlichia Stage in the lower Cambrian (Shergold 1995, p. 7).

Regional mapping, initially (1947–1948) by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and subsequently (1949–1980) by the Bureau of Mineral Resources, Geology and Geophysics (BMR), indicated that there were problems with Whitehouse’s (1939; in David & Browne 1950) understanding of the Cambrian succession in the Georgina Basin. These problems included the inability of some of the stages to be identified with certainty, and the unclear or erroneous position of some relative to others (Öpik 1956, p. 6). For example, the Amphoton Stage was considered by Whitehouse (1939) to be immediately above the Redlichia Stage, whereas Öpik (1956, p. 7) determined that it belonged in the uppermost middle Cambrian; the Phoidagnostus Stage was the ‘top of the Middle Cambrian’ (Öpik 1956, p. 7); and the position of the Anomocare Stage in the sequence of stages was ‘not yet identifiable’, but was not the topmost division of the middle Cambrian (Öpik 1956, p. 8).

An indication of where Öpik was likely to proceed is given in the left column of his middle Cambrian correlation chart (Öpik 1956, fig. 3). In this ‘time scale’ column, he mostly lists agnostides above the Redlichia and Xystridura–Dinesus stages, and in the caption to the figure notes that it is based on Acado–Baltic agnostides, and ‘reflects the sequence of the species as constructed from observations of about 300 field stations’ in the eastern Georgina Basin (Öpik 1956, p. 16) (Fig. 2).

Fig. 3. Development of South Australian Cambrian biozonations and suggested First Appearance Datum points. Columns from left to right are: global series and stages; trilobite zonation modified from Jell (in Bengtson et al. 1990); archaeocyath zonation of Zhuravlev et al. (1994) based on Gravestock (1984), modified after Kruse & Debrenne (2020); small shelly fossil (SSF) and mollusc zonations of Gravestock et al. (2001), which have been shown to be poorly characterized and to lack usable definitions (see Jacquet et al. 2016, p. 6–7); acritarch zonation of Zang et al. (2007); small shelly fossil zonation (SSF+) of Betts et al. (2016b). Suggested First Appearance Datum points are: 1, Parabadiella huoi Zhang, 1966; 2, Pararaia tatei (Woodward, 1884); 3, Pararaia bunyerooensis Jell in Bengtson et al., 1990; 4, Pararaia janeae Jell in Bengtson et al., 1990; 5, Syringocnema favus Taylor, 1910; 6, ichnotaxa Treptichnus pedum (Seilacher, 1955) and Sabellidites isp.; 7, Watsonella crosbyi Grabau, 1900; 8, ichnotaxa Skolithos isp. and Diplocraterion isp.; 9, Sunnaginia imbricata Missarzhevskiy in Rozanov et al., 1969; 10, Kulparina rostrata Conway Morris & Bengtson in Bengtson et al., 1990 and brachiopod Askepasma saproconcha Topper, Holmer, Skovsted, Brock, Balthasar, Larsson, Petterson Stolk, & Harper, 2013; 11, Micrina etheridgei (Tate, 1892) and brachiopod Askepasma toddense Laurie, 1986; 12, Dailyatia odyssei Evans & Rowell, 1990 and Stoibostrombus crenulatus Conway Morris & Bengtson in Bengtson et al., 1990; 13, brachiopods Kostjubella djagoran (Kruse, 1990) and Schizopholis napuru (Kruse, 1990). Abbreviations: M = Miaolingian; W = Wuliuan; A. abacus = Archaeocyathus abacus; S. favus s.s. = Syringocnema favus sensu stricto; J. tardus = Jugalicyathus tardus; S. tenuis = Spirillicyathus tenuis; W. wilkawillinensis = Warriootacyathus wilkaillinensis; C. opimolumum = Corollasphaeridium opimolumum; V. pseudofaveolata = Vulcanisphaera pseudofaveolata; C. aliquolumum = Corollasphaeridium aliquolumum; C. spinuconum = Ceratophyton spinuconum; V. trisentium = Veryhachium trisentium; K. rostrata = Kulparina rostrata; S. imbricata = Sunnaginia imbricata.

Öpik (1956, pp. 21–23) made note of Whitehouse’s (1939) upper Cambrian Eugonocare, Glyptagnostus, Rhodonaspis and Elrathiella stages, but did note that the Elrathiella stage may be below the Glyptagnostus stage and that the correlation of some localities with any of these stages may be doubtful.

Öpik began his numerous monographic contributions on Cambrian agnostides and trilobites with his ‘Geology and palaeontology of the headwaters of the Burke River, Queensland’ (Öpik 1961), but did not subdivide the Cambrian beyond establishing a few zones in the late middle and earliest upper Cambrian. In fact, to denote the early part of the upper Cambrian he used the North American Stage name Dresbachian, albeit in inverted commas (Öpik 1961, fig. 14).

It was in a later paper on trilobites in which he (Öpik 1968, p. 135) erected the Ordian and Templetonian stages, maintaining both to be middle Cambrian. The Ordian was proposed as a name for the ‘Redlichia-bearing beds, or strata’, whereas the Templetonian was stated to have not been ‘stabilized’, but referred to the Xystridura templetonensis fauna in Queensland and the Xystridura browni fauna in the Northern Territory, with the ‘Zone of Ptychagnostus gibbus’ being taken as the topmost zone of the Templetonian. The base of the Templetonian was eventually defined at the first appearance datum (FAD) of Pentagnostus anabarensis Lermontova in Vologdin, 1940 by Laurie (2006a), based on faunas from the southern Georgina Basin in the Northern Territory.

Öpik (1963, p. 7–8) defined two more stages, which he proposed were within the upper Cambrian: the Mindyallan and overlying Idamean (Fig. 2). The Mindyallan was vaguely defined as comprising the Glyptagnostus stolidotus Zone and an unnamed pre-stolidotus Zone (later defined as the Erediaspis eretes and Cyclagnostus quasivespa zones; Öpik 1967, pp. 9, 10), whereas the base of the Idamean was defined at the base of the Glyptagnostus reticulatus Zone. Given that the basal zone of the Idamean contains Glyptagnostus reticulatus, its base is therefore now known to be coeval with the base of the global Furongian Series. The uppermost zone in the Mindyallan is that of Glyptagnostus stolidotus, so it is now part of the Miaolingian Series.

Further, Jones et al. (1971) defined three overlying conodont-based stages: the Payntonian, Datsonian and Warendian (now Warendan; Kruse et al. 2009) (Fig. 2). Only the Payntonian Stage was considered Cambrian, with the base of the Datsonian being drawn at the first appearance of the Cordylodus proavus assemblage. However, with the definition of the global Cambrian–Ordovician boundary now defined as the FAD of Iapetognathus fluctivagus Nicoll, Miller, Nowlan, Repetski & Ethington, 1999, the Datsonian is now considered to be Furongian, and the Cambrian–Ordovician boundary lies within the early Warendan (Peng et al. 2020). The base of the Payntonian was redefined by Shergold et al. (1991) and Shergold & Nicoll (1992) as the first appearance of the conodont Hispidodontus resimus Nicoll & Shergold, 1991 (Shergold 1993).

Öpik (1979) completed the subdivision of the Australian Miaolingian with the erection of the Floran, Undillan and Boomerangian stages (Fig. 2). The base of the Floran was effectively defined at the FAD of the agnostide Acidusus atavus (Tullberg, 1880), which coincides with the base of the global Drumian Stage (Peng et al. 2020). The base of the Undillan was effectively defined at the FAD of the agnostide Ptychagnostus punctuosus (Angelin, 1851), and the base of the Boomerangian at the FAD of the agnostide Lejopyge laevigata (Dalman, 1828), the latter of which coincides with the base of the global Guzhangian Stage (Peng et al. 2020).

Shergold (1993, see also Laurie et al. 2023) completed the subdivision of the Australian Furongian with the erection of the Iverian Stage, the base of which was defined at ‘the first appearance of elements of the Irvingella tropica Trilobite Zone’ (Fig. 2). Hence, all the constituent stages of the Australian Miaolingian and Furongian were defined in central-northern Australia. This stadial scheme from central-northern Australia has been applied in Victoria (Paterson & Laurie 2004, Jell 2013, and references therein), Tasmania (Jago et al. 2016a, Bentley et al. 2020), New South Wales (see overview in Percival et al. 2011), the Warburton Basin of northeastern South Australia (Sun et al. 2021 and references therein), New Zealand (Smith et al. in press) and northern Victoria Land, Antarctica (Jago et al. 2019).

South Australian biostratigraphy

At the time of Shergold’s (1995) overview of the Cambrian in Australia, the stadial nomenclature used for the lower Cambrian was that of Siberia, with Tommotian, Atdabanian, Botoman and Toyonian being used in his Fig. 1. Alongside this was a zonation based on archaeocyaths established by Zhuravlev et al. (1994), based primarily on the work of Gravestock (1984). These latter authors established a succession of three archaeocyath zones in the lower Hawker Group of the Arrowie Basin—in ascending order, the Warriootacyathus wilkawillinensis, Spirillicyathus tenuis and Jugalicyathus tardus zones—beneath a widespread horizon known as the Flinders Unconformity (James & Gravestock 1990).

Daily (1956, 1972, 1976a, b) had earlier introduced twelve faunal assemblages based on various taxa including trilobites, archaeocyaths and small shelly fossils, but most of these faunas remained undescribed and lacking accurate biostratigraphic information; as such, they could only be used as informal ‘yardsticks’ for intercontinental correlations. Bengtson et al. (1990) described some of the shelly fossils from the Stansbury and Arrowie basins, but did not attempt to construct a biostratigraphic scheme from their data. In that same publication, Jell (in Bengtson et al. 1990) described trilobites from the Stansbury and Arrowie basins and erected four zones based on their ranges, though these were largely based on collections from spot localities (Fig. 3).

Daily (1972, 1990) and Gravestock et al. (1990, 2001) outlined the basic biostratigraphy of the Stansbury Basin, with the last of these publications describing key phosphatic fossils from two short surface sections and several drill cores. As a result, three small shelly fossil zones (Demidenko in Gravestock et al. 2001), four molluscan zones (Parkhaev in Gravestock et al. 2001), and seven acritarch zones (Zang in Gravestock et al. 2001, Zang et al. 2007) were erected (Fig. 2). However, Jacquet et al. (2016) later contended that the molluscan biozones of Gravestock et al. (2001) had very poorly defined boundaries and were based on poorly preserved, long-ranging steinkerns that Jacquet et al. (2019) considered, with a few exceptions, to have weak taxonomic validity. In addition, Jacquet et al. (2016) also noted that biostratigraphic data presented by Gravestock et al. (2001) revealed clear temporal discrepancies between the sections on Yorke and Fleurieu peninsulas (see also Betts et al. 2016b), indicating major problems with these basin-wide correlations.

Detailed taxonomic treatment of key lower Cambrian taxa collected systematically from more than 15 stratigraphic sections and subsurface cores in the Arrowie Basin was initiated in the 1990s and expanded during the 2000s, with comprehensive descriptions of small shelly fossils (Brock & Cooper 1993, Holmer et al. 2008, 2011b, Skovsted et al. 2008, 2009a, 2009b, 2011a, 2011b, 2015a, 2015b, 2016, Topper et al. 2009, Larsson et al. 2014), trilobites (Jell et al. 1992, Paterson & Edgecombe 2006, Paterson & Brock 2007, Paterson et al. 2007a, 2007b), bradoriide arthropods (Skovsted et al. 2006, Topper et al. 2011b, Betts et al. 2014, 2016a, 2016b), lobopodians (Topper et al. 2010, 2011a), brachiopods (Holmer et al. 2006, 2011a, Topper et al. 2013, Betts et al. 2019) and molluscs (Brock & Paterson 2004, Paterson et al. 2007a, 2009, Skovsted et al. 2007, 2012, 2014, Jacquet & Brock 2016, Jacquet et al. 2014, 2016). This comprehensive taxonomic dataset, together with new collections and illustrations of key taxa, resulted in a new high-resolution biostratigraphic scheme (Betts et al. 2016b, 2017) and the erection of three new shelly fossil biozones: in ascending order, these are the Kulparina rostrata, Micrina etheridgei and Dailyatia odyssei zones (Fig. 3). The two lower zones approximately correspond to the three archaeocyath zones of Zhuravlev et al. (1994), whereas the D. odyssei Zone correlates broadly with the Syringocnema favus beds of those authors. The integration of δ¹³C and δ¹⁸O isotope data (chemostratigraphy) from the sections that provided the new biozonation, together with several ²³⁸U/²⁰⁶Pb dates derived from ID-TIMS analyses of zircon grains from ash beds in the Mernmerna Formation and overlying Billy Creek Formation, facilitated the development of a new multiproxy chronostratigraphic scheme for the lower Cambrian succession in the Arrowie Basin (Betts et al. 2018). Further dates from these ash beds should improve definition of the scheme (Langsford & Jago 2023).

Modern taxonomic treatment of key shelly fossil groups from the Stansbury Basin has been provided by Paterson et al. (2007a), Jacquet et al. (2017), Betts et al. (2019) and Holmes et al. (2021b). Importantly, Jacquet et al. (2017) formally described Watsonella crosbyi Grabau, 1900 and the sinistral conchs of Aldanella sp. cf. golubevi Parkhaev, 2007 from the lower units of the Normanville Group, and reported new occurrences of the widespread tommotiide Sunnaginia imbricata Missarzhevskiy in Rozanov et al., 1969 from the Sellick Hill Formation, suggesting that the lower Cambrian package in the Stansbury Basin is older than the shelly fossil biozones established in the Arrowie Basin. This is supported by initial chemostratigraphic data presented by Betts et al. (2018) for the Wangkonda and Sellick Hill formations, interpreted by those authors to correlate with major global Zhujiaqing Carbon Isotope Excursion (ZHUCE) and Shiyantou Carbon Isotope Excursion (SHICE) events—reinforcing an older Terreneuvian, Stage 2 age for this package.

The Kangaroo Island Group on the north coast of Kangaroo Island includes the Emu Bay Shale Konservat-Lagerstätte, which contains over 50 species of panarthropods, molluscs, brachiopods, sponges, vetulicolians and other taxa (Paterson et al. 2016, 2023, Jago et al. 2016b, Edgecombe et al. 2017, Schroeder et al. 2018, Yun et al. 2019, Holmes et al. 2020, and references therein). The biostratigraphically important trilobites Estaingia bilobata Pocock, 1964, Balcoracania dailyi Pocock, 1970 and Redlichia takooensis Lu, 1950 are known from the Emu Bay Shale (Pocock 1964, 1970, Paterson & Jago 2006, Holmes et al. 2020, 2021a), with B. dailyi Pocock, 1970 also recognized from the underlying Marsden Sandstone (Gehling et al. 2011), and from the Smith Bay Shale further to the west, where it co-occurs with R. takooensis Lu, 1950 (Jago et al. 2021). These trilobites indicate correlation with the Series 2, Stage 4 Pararaia janeae Zone of the Arrowie and Stansbury basins.

Correlation between central-northern and South Australia

A longstanding impediment to the establishment of lower Cambrian stages in Australia arises from the historical fact that the entire Australian stadial scheme for the Miaolingian and Furongian series was established in the cratonic basins of central and northern Australia, whereas the lower Cambrian is best developed in South Australia (Laurie et al. 2023). Uppermost Series 2 (Ordian) rocks are widespread in central-northern Australia, but correlation with the succession in South Australia has proven difficult. Biostratigraphic studies targeting rocks of this age in central and northern Australia and South Australia will be essential for correlating these areas (Castle-Jones et al. 2023, Birksmith et al. 2023, Betts et al. 2023, Smith et al. 2023).

Among the existing Australian stages, the Ordian (Öpik 1968)—the stratigraphically lowest Australian stage recognized at present—is the only one that remains undefined. A base for the Ordian needs to be designated before any lower stages can be identified and formally defined. However, a suitable base for the Ordian Stage cannot be defined in central-northern Australia because the classical Ordian of Öpik (1968) in many places rests on the widespread Kalkarindji Volcanic Group (Glass 2002, Glass & Phillips 2002, Kruse in Rawlings et al. 2008), which extends from northern Western Australia across the Northern Territory into western Queensland, thereby underlying the Ord, Bonaparte, Daly and Wiso basins and much of the northern Georgina Basin. This extrusive flood-basalt province has been dated by Jourdan et al. (2014) at 510.7 ± 0.6 Ma, placing it within the unnamed and undefined global Stage 4.

In the southern Georgina Basin where this volcanic unit is absent, Ordian rocks rest upon the Red Heart Dolostone (assigned to the Micrina etheridgei Zone of Betts et al. 2016b) or older units. Hence, a lengthy hiatus lies beneath the classical Ordian succession. In the northeastern Amadeus Basin, the M. etheridgei Zone interval of the Todd River Dolostone is directly overlain by the Giles Creek Dolostone. The Giles Creek Dolostone likely straddles the Ordian–Templetonian boundary (e.g., Öpik 1968, 1982, Shergold 1986, Smith et al. 2023). Previously, Smith et al. (2014, 2015b, 2016) had assigned the unit’s type section entirely to the Templetonian. However, an update to the range for Pagetia silicunda Jell, 1975, as well as the likely synonymy of Xystridura gayladia Öpik, 1975 and Xystridura fracta Öpik, 1975, supports a latest Ordian age for the lower portion of the formation (Smith et al. 2023). In the central Amadeus Basin, the Ordian Tempe Formation is disconformably underlain by the unfossiliferous Chandler Formation or, where this is absent, the Arumbera Sandstone of latest Ediacaran to earliest Cambrian age (Smith et al. 2015a). A similar (albeit shorter) hiatus is also seen in the Koonenberry Belt of northwestern New South Wales. Here, the Ordian Coonigan Formation rests disconformably on the ‘late Botomian’ Cymbric Vale Formation (Roberts & Jell 1990, fig. 2 ‘second trench’), which is placed low in the Pararaia janeae Zone (Jago et al. 1997, Paterson 2005, Jell & Smith unpublished data). The basal part of the Pincally Formation (of likely Ordian to early Templetonian age) near Mount Arrowsmith, further northwest, is entirely covered by unconsolidated colluvium and impossible to access (Brock & Percival 2006, Percival in Greenfield et al. 2010, p. 68, Percival et al. 2011, p. 9).

For the above reasons, most, if not all, central-northern Australian sections are unsuitable for defining the Ordian base as traditionally conceived. Recourse to South Australian sections is unavoidable.

The youngest known trilobite occurrence in the entire South Australian succession is represented by four partial pygidia assigned to Pagetia cf. edura Jell, 1975, recorded from the Coobowie Limestone in Port Julia 1A well in the Stansbury Basin (Jago & Kruse 2020). Three relatively poorly preserved cephala of a taxon comparable to P. edura have also recently been reported in the Stansbury Limestone in the CUR-D 9 core (Birksmith 2022, Birksmith et al. 2023). Pagetia edura Jell, 1975 has previously been recorded from the Templetonian Xystridura templetonensis Zone by Jell (1975), and P. aff. edura has been recorded from the early Templetonian Jigaimara Formation in the Arafura Basin, Northern Territory (Laurie 2006b). Accordingly, Jago & Kruse (2020) assigned the Coobowie Limestone occurrence to the global Wuliuan Stage, the basal stage of the Miaolingian.

Öpik (1968) recorded species of Onaraspis Öpik, 1968 from the Ordian of central-northern Australia, specifically from localities Rd10 and AS33 (see Smith et al. 2015b) in a unit that he attributed to the Giles Creek Dolostone of the Amadeus Basin; locality CG51/1 in the Tarrara (= Blatchford) Formation in the Bonaparte Basin; the First Discovery Limestone Member of the Coonigan Formation (Roberts & Jell 1990, Smith et al. 2023) in the Koonenberry Belt; and in an unknown formation at locality M262 in the northeastern Georgina Basin. In South Australia, Onaraspis rubra Jell, 1990 is known from several localities in the Moodlatana Formation in the Arrowie Basin (Jell in Bengtson et al. 1990, p. 13) and is assumed to be of similar age.

In addition, Redlichia cf. versabunda Öpik, 1970 (= R. guizhouensis of Jell in Bengtson et al., 1990) is known from the Ramsay Limestone in the Stansbury Basin, and the Wirrealpa Limestone (and probably Aroona Creek Limestone) in the Arrowie Basin (Paterson & Brock 2007, Holmes et al. 2021b). Redlichia versabunda Öpik, 1970 has been described from several localities in the northeastern Georgina Basin, including the ‘chertified dolostone’ of the Yelvertoft Bed (locality M426, now considered within the Thorntonia Limestone; Southgate & Shergold 1991), as well as from what Öpik (1970) stated was an equivalent chert bed at Cornford Bore ∼40 km east of Mount Isa (locality M262, mentioned above), and from the Thorntonia Limestone ∼110 km south of Mount Isa (locality D41) (Öpik 1970, fig. 1).

Below these isolated occurrences, the South Australian early Cambrian trilobite zones erected by Jell (in Bengtson et al. 1990) comprise, in ascending order, the Parabadiella huoi, Pararaia tatei, Pararaia bunyerooensis and Pararaia janeae zones (Fig. 3). Regrettably, none of the representative taxa have yet been found in central-northern Australia (although P. janeae Jell, in Bengtson et al. 1990 is known from the Cymbric Vale Formation in New South Wales; Jell & Smith unpublished data). Hence, correlation between these two regions is difficult using available trilobite faunas. This apparent rarity of trilobites in the upper parts of the South Australian Cambrian means that other fossil groups are required to correlate between the central-northern Australian basins and those in South Australia.

Small shelly fossils are abundant and diverse in South Australian successions (e.g., Skovsted et al. 2007, 2008, 2011a, 2011b, 2015a, 2015b), with the Kulparina rostrata, Micrina etheridgei and Dailyatia odyssei zones erected (Betts et al. 2016b, 2017, 2018). In central-northern Australia, these fossils are known only from one unit each in the Amadeus (Todd River Dolostone; Laurie 1986) and southern Georgina (Red Heart Dolostone [= Errarra Formation]; Laurie & Shergold 1985) basins, both of which are bounded by disconformities that likely represent considerable intervals of time. The small shelly fossils indicate that both formations lie within the Micrina etheridgei Zone, and these are the only two units in the Amadeus and Georgina basins from which archaeocyaths have been recorded (Walter et al. 1979, Kruse & West 1980).

Brachiopods also appear to be a viable option for correlation, as several species are widespread. For example, Kostjubella djagoran (Kruse, 1990), widely reported from central-northern Australian cratonic sedimentary basins (Kruse 1990, 1991a, 1998, Percival & Kruse 2014), is also found in the Ramsay, Stansbury and Coobowie limestones in the western Stansbury Basin (Holmer & Ushatinskaya in Gravestock et al. 2001, Jago & Kruse 2020, Birksmith 2022, Birksmith et al. 2023), and the Wirrealpa Limestone in the Arrowie Basin (Brock & Cooper 1993, fide Holmer et al. 1996, but see Claybourn et al. 2020). Indeed, Brock (in Jago et al. 2006, 2012) recognized four informal brachiopod assemblages in the Arrowie and Stansbury basins, the uppermost of which was the K. djagoran assemblage. In central-northern Australia, Kostjubella djagoran is known from the Thorntonia Limestone in the southern Georgina Basin (Percival & Kruse 2014), Top Springs Limestone in the northern Georgina Basin (Kruse 1991a), Tindall Limestone in the Daly Basin (Kruse 1990), Montejinni Limestone in the eastern Wiso Basin (Kruse 1998), and Tempe Formation in the Amadeus Basin (Smith et al. 2015a). Further afield, it is also reported from erratics on King George Island in the South Shetland Islands, Antarctica (Holmer et al. 1996).

Other brachiopods widely distributed in South Australia include Askepasma toddense Laurie, 1986, Askepasma saproconcha Topper, Holmer, Skovsted, Brock, Balthasar, Larsson, Petterson Stolk & Harper, 2013, Schizopholis yorkensis (Holmer & Ushatinskaya in Gravestock et al., 2001) and Kyrshabaktella davidi Holmer & Ushatinskaya in Gravestock et al., 2001; whereas the Ordian Kyrshabaktella mudedirri Kruse, 1990, Westonia nyapungensis Kruse, 1990 and Schizopholis napuru (Kruse, 1990, fide Popov et al., 2015) among others, are widespread in central and northern Australia. Schizopholis yorkensis (Holmer & Ushatinskaya in Gravestock et al., 2001) is also known from the Transantarctic Mountains, Antarctica (Claybourn et al. 2020), and the Xinji Formation of South China (Pan et al. 2020). Uniquely among these species, S. napuru (Kruse, 1990) is represented in both central-northern and South Australia (where it often partially overlaps the range of Kostjubella djagoran [Kruse, 1990]). Schizopholis napuru (Kruse, 1990) has also been reported from King George Island, Antarctica (Holmer et al. 1996) and the Himalayas (Popov et al. 2015).

Although acritarch studies have been carried out in the South Australian Cambrian, resulting in the identification of seven assemblages (Zang in Gravestock et al. 2001, Zang et al. 2007), work on acritarchs in the central-northern Australian Cambrian has been limited, with a single assemblage identified from two units—the Adam Shale, which was considered to be ‘Tommotian’ (i.e., latest Terreneuvian) in age (Walter et al. 1979), and the Ordian Tempe Formation (Zang & Walter 1992).

Carbon isotope excursions, when integrated with biostratigraphy, are another useful tool for correlation. Smith et al. (2015c) identified a strong negative δ¹³C excursion in the lower Arthur Creek Formation of the southern Georgina Basin, first reported by Lindsay et al. (2005), as the Redlichiid–Olenellid Extinction Carbon Excursion (ROECE), although the weak correlation of C and O isotopes in the crossplot data (Smith et al. 2015c, p. 19) suggests a potential diagenetic signal for this event. Lindsay et al. (2005) interpreted this excursion as due to microbial alteration. In the Stansbury Basin, the ROECE has been identified by Birksmith (2022, see also Birksmith et al. 2023) in two cores in the upper Stansbury Limestone. Lower in the succession, a negative event in the Ramsay Limestone is also captured in the correlative Wirrealpa Limestone in the Arrowie Basin (GAB, unpublished data) and may represent the Archaeocyath Extinction Carbon Excursion (AECE) event. The youngest archaeocyaths in the entire Australian succession, which are correlated with the equivalent youngest archaeocyaths in the Toyonian Stage of Siberia (e.g., Debrenne et al. 1989, 1990), are found in the medial Wirrealpa Limestone (Kruse 1991b).

The quest for regional lower Cambrian stages in Australia

Despite possessing a series of world-class and well documented lower Cambrian (Terreneuvian–Series 2) successions, no regional stages have been defined for this interval in Australia. As outlined above, such stages will need to be defined in South Australia, likely within the Arrowie and Stansbury basins. The Arrowie Basin in particular has one of the best-exposed successions of lower Cambrian sedimentary packages anywhere in the world, as summarized by Jago et al. (2020). Calcareous and organophosphatic shelly fossils ‘… are ubiquitous across carbonate-dominated parts of the basin and have been described in a series of papers’ (Betts et al. 2016b, p. 177). In addition, the Stansbury Basin has well-documented successions on the Fleurieu Peninsula, Kangaroo Island, and Yorke Peninsula (the last largely from drill core).

In erecting an internally consistent succession of six named regional stages for North America, Palmer (1998, see also Babcock et al. 2011) stated that the value of regional stages was in their utility for communication. Similarly, Siberia (Spizharskiy et al. 1983, Rozanov & Varlamov 2008), South China (Peng et al. 2020) and Iberia/Morocco (Gozalo et al. 2003, Geyer & Landing 2004) also have well-developed stadial schemes. The Siberian scheme was long used as a proxy for the lower Cambrian in Australia (Shergold 1995, Kruse et al. 2009).

Given that we now have a well-developed understanding of the distribution of numerous taxa (archaeocyaths, trilobites, bradoriides, brachiopods, tommotiides, molluscs, acritarchs, etc.; Zhuravlev et al. 1994, Betts et al. 2016b, 2018, Kruse & Debrenne 2020), it is time to consider the introduction of a regional stadial scheme for the lower Cambrian in Australia.

When the remaining global stages for the Terreneuvian and Cambrian Series 2 are ratified, there is no guarantee that the definitions will be easily applicable in Australia. As an example, the base of the Miaolingian Series and conterminous Wuliuan Stage is defined (Zhao et al. 2019) by the FAD of Oryctocephalus indicus (Reed, 1910), which has not been found in Australia despite the record of numerous oryctocephalid species from the Amadeus, Arafura and Georgina basins, and the Koonenberry Belt (Shergold 1969, Laurie 2004, 2006a, 2006b, Smith et al. 2015b, 2023). As a consequence, a local stadial scheme should be based on multiple independent chronological datasets to allow successful correlation of Cambrian packages across regional and intercontinental scales.

Ideally, a local stadial scheme should be based on features that can be shown to be most useful in correlating as widely as possible within a broad sequence stratigraphic context. With regard to the Cambrian Terreneuvian and Series 2 in South Australia, usable taxa include trilobites, brachiopods, small shelly fossils (tommotiides, molluscs, etc.), archaeocyaths, acritarchs and ichnofossils, could be integrated with robust chemostratigraphy based on stable isotope data (δ¹³C and δ¹⁸O) and, wherever possible, radiometric dates based on ID-TIMS analysis of zircon grains from volcanic-ash beds.

The obvious levels to define stadial boundaries are where the potential for correlation is the greatest, and therefore where taxa are the most diverse and numerous. Previously established biozones, correlatable across multiple successions and facies, are likely to provide the best options in this case. At stratigraphically higher levels in the South Australian Cambrian successions (i.e., above the Hawker Group of the Arrowie Basin and equivalents), trilobite abundance and diversity are limited, with only four species known (Bengtson et al. 1990, Jago & Kruse 2020, Jago et al. 2020). Thus, in this interval, other shelly fossils are likely to be more useful in defining biozones and stage boundaries; however, trilobites can still be used as auxiliary index taxa, as genera such as Redlichia Cossmann, 1902 sensu lato are fairly widespread in Gondwana (Álvaro et al. 2013).

Below this, trilobites of the Parabadiella huoi, Pararaia tatei, Pararaia bunyerooensis and Pararaia janeae zones are found in the Wilkawillina and Ajax limestones, Mernmerna Formation and Oraparinna Shale of the Arrowie Basin and equivalents in the Stansbury Basin (e.g., Parara Limestone on Yorke Peninsula), and the Marsden Sandstone, Emu Bay Shale and Smith Bay Shale on Kangaroo Island (Bengtson et al. 1990, fig. 7, Jell et al. 1992, Paterson & Jago 2006, Paterson & Brock 2007, Betts et al. 2018, fig. 17, Holmes et al. 2020, 2021a, Jago et al. 2021). At least the P. janeae Zone fauna (including the eponym) are also found in the Cymbric Vale Formation of the Koonenberry Belt in New South Wales (Jago et al. 1997, Paterson 2005, Jell & Smith unpublished data). However, their numbers and occurrence are limited relative to those of small shelly fossils recovered from acid digestion.

Brachiopods are limited in diversity, but are relatively numerous and can be found in several horizons within units such as the Wilkawillina Limestone. Small shelly fossils are very diverse and are found in numerous horizons in many formations, such as the Wilkawillina, Ajax, Wirrapowie, Wirrealpa and Aroona Creek limestones and Mernmerna Formation (Arrowie Basin), as well as the Sellick Hill Formation and Fork Tree, Parara, Kulpara, Ramsay, Stansbury and Coobowie limestones (Stansbury Basin), among others (e.g., Gravestock et al. 2001, Betts et al. 2016b, 2017, 2018). This makes them probably the most useful taxa for correlation throughout the Arrowie Basin, Stansbury Basin, Stuart Shelf and other areas. However, their zonation only extends a short distance above the base of the Pararaia janeae Zone. Above this, zones will need to be established based on different taxa. Work currently in progress within the ‘Ordian’ interval in the Stansbury, Arrowie and Georgina basins will help to refine the biostratigraphy of these successions and correlations between them.

The base of the Templetonian was defined by Laurie (2006a), which implicitly placed an upper boundary on the Ordian, and is at the tail end of what has been suggested to be the ROECE event. This event is recorded in the Stansbury Limestone (Birksmith 2022, Birksmith et al. 2023). Lower in the Wirrealpa (Arrowie Basin) and Ramsay (Stansbury Basin) limestones are the brachiopods Hadrotreta primaeva (Walcott, 1902) (= Kostjubella djagoran [Kruse, 1990] according to Holmer et al. 1996, fide Percival & Kruse 2014), Schizopholis napuru (Kruse, 1990) and a possible Kyrshabaktella Koneva, 1986 species (Brock & Cooper 1993). The FAD of K. djagoran (Kruse, 1990) as reported by Brock & Cooper (1993) is only some 11–14 m above the base of the Wirrealpa Limestone. In the Ramsay Limestone—and presumably above the medial Wirrealpa Limestone, as there are archaeocyaths in the medial interval of that unit—is a δ¹³C excursion that may represent the AECE event (Birksmith et al. 2023). The first appearance of such brachiopod taxa near the AECE event could be a potential datum to define the base of the Ordian Stage. Such a horizon is most likely to be identified in the Wirrealpa Limestone or Ramsay Limestone. It is noteworthy that Öpik (1968) listed the Wirrealpa Limestone among the units included in his Ordian Stage.

If the base of the Ordian is defined at the FAD of K. djagoran (Kruse, 1990) and/or S. napuru (Kruse, 1990) (or a related taxon), then horizons lower in the Cambrian that potentially lend themselves to stage definitions include the following (in descending order):

• FAD of Pararaia janeae Jell in Bengtson et al., 1990, which appears very late in the Dailyatia odyssei Zone.

• FAD of Pararaia bunyerooensis Jell in Bengtson et al., 1990, which occurs late in the D. odyssei Zone.

• FAD of Pararaia tatei (Woodward, 1884), which occurs just above the base of the D. odyssei Zone.

Note that data on the geographic distribution of the above three species are currently limited (Jell in Bengtson et al. 1990, Paterson & Brock 2007, Betts et al. 2018).

• FAD of Dailyatia odyssei Evans & Rowell, 1990 and/or Stoibostrombus crenulatus Conway Morris & Bengtson in Bengtson et al., 1990, which define the base of the D. odyssei Zone (Betts et al. 2016b). Betts et al. (2018) interpreted a positive δ¹³C peak in the Mernmerna Formation and Ajax Limestone as the Cambrian Arthropod Radiation Event (CARE, or excursion IV), which straddles the boundary between the Micrina etheridgei and Dailyatia odyssei zones. However, new data from the Parara Limestone in the western Stansbury Basin suggest that this positive event could represent the younger MICE event (Castle-Jones et al. 2023).

• FAD of archaeocyathan fauna of the Syringocnema favus assemblage sensu lato (Zhuravlev et al. 1994, Kruse & Debrenne 2020). This assemblage coincides with the base of the D. odyssei Zone and with the widespread Flinders Unconformity of James & Gravestock (1990). The S. favus assemblage is known from Yorke Peninsula and Kangaroo Island, South Australia (Zhuravlev et al. 1994, Kruse & Moreno-Eiris 2013), the Mount Wright Volcanics and Cymbric Vale Formation in the Koonenberry Belt of western New South Wales (Kruse 1978, 1982), at numerous localities along the Transantarctic Mountains and adjacent seas and islands (Gordon 1921, Debrenne & Kruse 1986, 1989, Stone et al. 2012), as well as in southern Africa (Debrenne 1975, Perejón et al. 2019). In addition to S. favus Taylor, 1910, Kruse & Debrenne (2020) proposed the FADs of Stapicyathus stapipora (Taylor, 1910), Tumuliolynthus irregularis (Bedford & Bedford, 1934) and Sigmofungia flindersi Bedford & Bedford, 1936 as potential guide species for the base of this assemblage.

• FAD of Parabadiella huoi Zhang, 1966, which occurs late in the M. etheridgei Zone, and which also represents the eponymous zone in South China (Betts et al. 2018).

• FAD of Micrina etheridgei (Tate, 1892) and/or Askepasma toddense Laurie, 1986, which defines the base of the M. etheridgei Zone (Betts et al. 2016b). This is interpreted by Betts et al. (2018) to be generally coincident with δ¹³C excursions II and III, which, according to those authors, straddle the boundary between the Kulparina rostrata and Micrina etheridgei zones, but peak in the lower part of the M. etheridgei Zone. New data suggest that this boundary may be coincident with the CARE δ¹³C excursion (Castle-Jones et al. 2023).

• FAD of Kulparina rostrata Conway Morris & Bengtson in Bengtson et al., 1990 and/or Askepasma saproconcha Topper, Holmer, Skovsted, Brock, Balthasar, Larsson, Petterson Stolk & Harper, 2013, which define the base of the K. rostrata Zone (Betts et al. 2016b). This assemblage is reported from the upper part of the Sellick Hill Formation and co-occurs with an archaeocyathan fauna (Debrenne & Gravestock 1990) that was correlated with the Atdabanian Stage of Siberia (lower global Stage 3) by Gravestock et al. (1995, p. 4); it is stratigraphically above negative δ¹³C data that Betts et al. (2018) interpreted as the SHICE, although is more likely to be negative values below the CARE. SHICE is a negative δ¹³C excursion in the Shiyantou Member of the Chiungchussu (Qiongzhusi) Formation in eastern Yunnan, China, and is associated with the extinction of many shelly taxa around the middle of Stage 2.

• FAD of Watsonella crosbyi Grabau, 1900 in the Mount Terrible Formation (Jacquet et al. 2016). This species is a candidate indicator for the base of global Stage 2. The ZHUCE (I’ in Siberia, 6p of Maloof et al. [2010], L4 of Landing et al. [2013]) is a positive δ¹³C excursion and follows a major radiation of the first biomineralized organisms in the Cambrian. Elsewhere, positive ZHUCE δ¹³C values occur just after the probable transition between the as yet undefined Fortunian − Stage 2 boundary. A peak within the Sellick Hill Formation is interpreted by Betts et al. (2018) as ZHUCE; it is within the stratigraphic range of Watsonella crosbyi.

• FAD of vertical burrows of Skolithos Haldeman, 1840 and Diplocraterion Torell, 1870 in the Parachilna Formation, which is also a potential proxy for the onset of Stage 2 (Mángano & Buatois 2014) in the global scale. These traces are also known from the Wangkonda Formation in the eastern Stansbury Basin (Betts et al. 2018). Skolithos Haldeman, 1840 is also present in Arumbera Sandstone 4 in the Amadeus Basin, and the Neutral Junction Formation [= Donkey Creek beds of Walter (1980)] in the Georgina Basin; Diplocraterion Torell, 1870 is reported from the Octy Formation and/or Neutral Junction Formation, Red Heart Dolostone and Mount Birnie beds in the Georgina Basin (Walter et al. 1989, Haines et al. 1991).

• FAD of Treptichnus pedum (Seilacher, 1955) in the Uratanna Formation (Droser et al. 1999, Jago et al. 2020). This ichnospecies is the primary indicator taxon for the global base of the Cambrian. It is also present in Arumbera Sandstone 4 in the Amadeus Basin, and the Octy and Neutral Junction formations of the Georgina Basin (Walter et al. 1989, Haines et al. 1991). The first occurrence of Sabellidites Yanishevskiy, 1926 ‘worms’ immediately above Treptichnus Miller, 1889 is also within the Uratanna Formation, and the genus is additionally present in the Mount Terrible Formation. Sabellidites Yanishevskiy, 1926 is globally widespread among basal Cambrian terranes (e.g., South China, Norway, Iran and elsewhere; see Ebbestad et al. 2022).

Concluding remarks

After nearly 100 years of development of the Cambrian stage scale in Australia, and with the Miaolingian and Furongian regional stages completed in 2006 and much new information on Terreneuvian and Series 2 faunas now available, a workable lower Cambrian stage scale is at last in view. Brachiopods appear to be best suited to establishing the base of the Ordian Stage, after which several faunal levels offer scope for proposing new lower Cambrian stages, thereby facilitating the completion of a comprehensive Australian Cambrian stage scale.

Acknowledgements

The authors thank the Adnyamathanha People, generous homestead owners and leaseholders, and the National Parks and Wildlife Service South Australia for permission to sample lower Cambrian successions across the Flinders Ranges over the past 30 years. JRL thanks his colleagues at Geoscience Australia; Natalie Schroeder and Steven Petkovski, for their assistance and support. GAB would also like to personally thank the late Tom Bradley, David Mathieson and a bevy of Honours and postgraduate students and volunteers from Macquarie University for assistance in the field and for collecting, processing and providing large portions of the biostratigraphic, chemostratigraphic and lithostratigraphic data from the numerous lower Cambrian sections in the Arrowie and Stansbury basins that are discussed here as potential regional stage boundaries. Similarly, MJB and JRP would like to thank all students and volunteers at the University of New England for their contributions to these datasets. PMS thanks Peter Jell, Ian Percival and Yong Yi Zhen for providing access to material from New South Wales for study. The authors thank the staff of the South Australian Drill Core Reference Library, Northern Territory Geological Survey Core Library and Geoscience Australia for access to relevant Cambrian drill holes. JBJ thanks the University of South Australia for logistic support. Funding has been provided by various Australian Research Council (ARC) grants (especially Discovery Project #120104251 to GAB and JRP). The authors also thank Dr S. Jacquet (Geological Sciences, University of Missouri) and an anonymous reviewer for improving the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).