Geological Magazine Ontogeny of the trilobite Redlichia from the www.cambridge.org/geo lower Cambrian (Series 2, Stage 4) Ramsay Limestone of South Australia 1 2 3,4 Original Article James D. Holmes , John R. Paterson , James B. Jago and Diego C. García-Bellido1,4 Cite this article: Holmes JD, Paterson JR, Jago JB, and García-Bellido DC (2021) 1School of Biological Sciences, University of Adelaide, North Terrace, Adelaide, South Australia 5005, Australia; Ontogeny of the trilobite Redlichia from the 2 lower Cambrian (Series 2, Stage 4) Ramsay Palaeoscience Research Centre, School of Environmental and Rural Science, University of New England, 3 Limestone of South Australia. Geological Armidale, New South Wales 2351, Australia; University of South Australia, STEM, Mawson Lakes, South 4 Magazine 158: 1209–1223. https://doi.org/ Australia 5095, Australia and South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia 10.1017/S0016756820001259 Abstract Received: 19 March 2020 Revised: 23 September 2020 Studies that reveal detailed information about trilobite growth, particularly early developmental Accepted: 13 October 2020 stages, are crucial for improving our understanding of the phylogenetic relationships within this First published online: 2 December 2020 iconic group of fossil arthropods. Here we document an essentially complete ontogeny of the Keywords: trilobite Redlichia cf. versabunda from the Cambrian Series 2 (late Stage 4) Ramsay Limestone Redlichiida; Redlichiidae; protaspid; meraspid; of Yorke Peninsula in South Australia, including some of the best-preserved protaspides (the arthropod; early Cambrian earliest biomineralized trilobite larval stage) known for any Cambrian trilobite. These protaspid stages exhibit similar morphological characteristics to many other taxa within the Suborder Author for correspondence: James D. Holmes, Email: [email protected] Redlichiina, especially to closely related species such as Metaredlichia cylindrica from the early Cambrian period of China. Morphological patterns observed across early developmental stages of different groups within the Order Redlichiida are discussed. Although redlichiine protaspides exhibit similar overall morphologies, certain ontogenetic characters within this suborder have potential phylogenetic signal, with different superfamilies characterized by unique trait combinations in these early growth stages. 1. Introduction Trilobites are one of the most recognized and well-studied Palaeozoic groups. However, despite over 150 years of research and >22 000 described species, the relationships between higher-level groups within this iconic class of fossil arthropods remain unclear (Adrain, 2011; Paterson, 2020). Advances in our understanding of the ontogeny of different trilobite groups will help to resolve these problems by providing crucial information for phylogenetic analyses (e.g. Edgecombe et al. 1988; Fortey & Chatterton, 1988; Chatterton et al. 1990; Chatterton & Speyer, 1997; Hughes et al. 2017). Determining the relationships between Cambrian trilobites in particular will help to illuminate not only the origins of the class (e.g. by providing informa- tion on character polarities; Paterson et al. 2019), but also assist in resolving how some post- Cambrian clades are related to Cambrian taxa (the so-called ‘cryptogenesis problem’; see Paterson, 2020). Developmental information from early Cambrian trilobites considered to be ‘primitive’, such as those belonging to the Order Redlichiida, is therefore of high importance. Within the Redlichiida (specifically the suborder Redlichiina sensu Adrain, 2011), instances where multiple, well-preserved protaspides (the earliest biomineralized larval stage) are known is limited to less than 10 species. The most informative of these are phosphatized specimens of the estaingiid Ichangia ichangensis Zhang, 1957 and several indeterminate species described by Zhang & Pratt (1999) and Zhang & Clarkson (2012). Protaspides from several other ellipsoce- phaloid species preserved in shale or mudstone are also well known, including the ellipsoce- phalid Ellipsostrenua granulosa (Ahlberg, 1983) (Laibl et al. 2018) and the estaingiid Estaingia sinensis (Zhang, 1953) (Dai & Zhang, 2012a). Early ontogenetic information for the other major redlichiine superfamilies Redlichioidea and Paradoxidoidea is more limited, although Laibl et al.(2017) described ‘giant’ protaspides of two paradoxidid species from the Miaolingian of the Czech Republic, and Dai & Zhang (2012b) presented well-preserved examples of the redlichiid Metaredlichia cylindrica (Zhang, 1953) from Cambrian Stage 3 of Hubei Province, China. Protaspides are either unknown from other examples of redlichiine © The Author(s), 2020. Published by Cambridge ontogenies (e.g. Dai & Zhang, 2013), or represent relatively poorly preserved or isolated University Press. examples that do not provide detailed morphological information of protaspid stages (e.g. Westergård, 1936; Lu, 1940; Kautsky, 1945; Whittington, 1957; Šnajdr, 1958; Pocock, 1970; Öpik, 1975; Zhang et al. 1980; Pillola, 1991; Palmer & Rowell, 1995; Hou et al. 2017). Here we describe a relatively complete post-embryonic ontogenetic series of the trilobite Redlichia cf. versabunda Öpik, 1970 from the Cambrian Series 2 (late Stage 4) Ramsay Downloaded from https://www.cambridge.org/core. University of Athens, on 29 Sep 2021 at 07:47:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756820001259 1210 JD Holmes et al. Fig. 1. (Colour online) (a) Map of southeastern South Australia showing the location of the Stansbury and Arrowie basins and the study area. (b) Map of central Yorke Peninsula showing the locality within the Ramsay Limestone where material considered in this study was collected (pentagon), as well as the locations of several drill holes that intersect the formation in the vicinity (stars). Limestone of Yorke Peninsula (South Australia), including some of intersected by the Minlaton-1, Minlaton-2 and Cur-D1B drill the best-preserved redlichioid protaspides known to date. holes, with the formation represented in these by dark- to light- grey, mottled, nodular limestones interbedded variously with shales, siltstones, planar stromatolites and evaporites (Ludbrook, 2. Geological setting 1965; Daily, 1990; Gravestock et al. 2001). Redlichia was reported by B Daily (unpub. report, 1957) from the basal c. 25 m of the for- The Ramsay Limestone on Yorke Peninsula, and its temporal equiv- mation in Minlaton-1. The Ramsay Limestone conformably over- alents the Wirrealpa and Aroona Creek limestones (Flinders lies conglomerates, arkoses and evaporites of the Minlaton Ranges), form part of the second major depositional sequence of Formation and conformably underlies the shales, siltstones and the Cambrian succession in the Stansbury and Arrowie basins (as calcareous sandstones of the Corrodgery Formation (Fig. 2) defined by Gravestock, 1995; Gravestock & Shergold, 2001), repre- (Daily, 1990; Gravestock et al. 2001). senting relatively warm, shallow-marine environments during the last major Cambrian transgression in South Australia. These units are considered contemporaneous based on the co-occurrence of 3. Occurrence of Redlichia in the Ramsay and Wirrealpa Daily’s(1956) ‘Faunal Assemblage 10’ (e.g. Horwitz & Daily, limestones 1958; Daily, 1990;Brock&Cooper,1993). The Ramsay Limestone is known from central Yorke Peninsula Trilobites within the upper part of the Cambrian succession in in the Stansbury Basin (Fig. 1a), where it crops out in a small area South Australia (i.e. above the first major depositional sequence c. 7 km south of Curramulka as ‘dark, blue-grey, mottled, argilla- of Gravestock, 1995 and Gravestock & Shergold, 2001) are rare. ceous limestone’ (Daily, 1990, p. 223), as well as in the subsurface Only three instances in three successive formations are currently from a number of drill holes in the general vicinity (Fig. 1b). It has a recognized from the Arrowie Basin in the Flinders Ranges (Jago maximum thickness of 85 m in the Stansbury West-1 well, within et al. 2006, 2020): the emuellid Balcoracania dailyi Pocock, 1970 which Daily (1990) reported shelly fossils (including Redlichia) from the Billy Creek Formation (Paterson & Edgecombe, 2006; from oolitic, sandy and dolomitic limestones c. 15 m below the Paterson et al. 2007); ‘Redlichia guizhouensis’ (re-assigned here top of the formation. The Ramsay Limestone is 68.6 m thick in to R. cf. versabunda) from the Wirrealpa Limestone and equivalent the Stansbury Town-1 well, although only a 3-m-section in the Aroona Creek Limestone (Daily, 1976; Jell in Bengtson et al. 1990; upper part of the formation was cored, consisting of interbedded Jago & Zang, 2006; Paterson & Brock, 2007); and Onaraspis rubra grey, nodular, argillaceous limestone and black, pyritic, micaceous Jell in Bengtson et al. 1990 from the Moodlatana Formation siltstone and/or mudstone containing Redlichia and other shelly (Fig. 2). In the Stansbury Basin on Yorke Peninsula, Redlichia fossils (B Daily, unpub. report, 1968; Daily, 1990; Gravestock has been reported from the Ramsay and Stansbury limestones et al. 2001). Lower portions of the Ramsay Limestone were also (e.g. Öpik, 1970; Daily, 1990), and Pagetia cf. edura has been Downloaded from https://www.cambridge.org/core. University of Athens, on 29 Sep 2021 at 07:47:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.