Journal of Paleontology, 92(1), 2018, p. 80–86 Copyright © 2018, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2018.6
Eldonioids with associated trace fossils from the lower Cambrian Emu Bay Shale Konservat-Lagerstätte of South Australia
Natalie I. Schroeder,1 John R. Paterson,1 and Glenn A. Brock2
1Palaeoscience Research Centre, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia ⟨
nschroed@myune.edu.au⟩, ⟨
jpater20@une.edu.au⟩ 2Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia ⟨
glenn.brock@
mq.edu.au⟩
Abstract.—Rare specimens of eldonioids recovered from the lower Cambrian (Series 2, Stage 4) Emu Bay Shale (EBS) Konservat-Lagerstätte represent the first record of the group for the Cambrian of East Gondwana. The disc-shaped body of the EBS taxon bears fine concentric corrugations on the dorsal surface and, ventrally, a series of internal lobes that have primary and secondary bifurcations, as well as a coiled sac. It appears to be most similar to Rotadiscus and Pararotadiscus of the Cambrian Chengjiang and Kaili biotas of South China, respectively. While the structure of the internal lobes would indicate that this occurrence in the EBS represents a new taxon within the Rotadiscidae, lack of detail regarding the precise number of internal lobes and the condition of the circumoral tentacles warrants a more conservative approach in leaving the genus and species under open nomenclature. The EBS specimens also host trace fossils, including the remains of a burrow, which are generally lacking in the body-fossil- bearing layers of the Konservat-Lagerstätte interval. These traces appear to have been made by small organisms and are similar to traces associated with the discs of Pararotadiscus guizhouensis (Zhao and Zhu, 1994) from the Kaili Biota. The available taphonomic, paleoenvironmental, and ichnological evidence indicates that the EBS eldonioids are most likely vagrants that were transported or settled into the ‘preservational trap’ and subsequently exposed on the substrate for a brief period before burial, thereby allowing organisms to exploit their carcasses for nutrients or other purposes.
Introduction
Recent intensive excavation of the lower Cambrian (Series 2, Stage 4) Emu Bay Shale (EBS) at Buck Quarry on the northeast coast of Kangaroo Island has significantly increased the known diversity of this Konservat-Lagerstätte to over 50 species (Paterson et al., 2016). Like most Cambrian Konservat- Lagerstätten, the fauna is dominated by arthropods, both in diversity and, especially, abundance of individual animals (Ivantsov et al., 2005; Zhao et al., 2014; Paterson et al., 2016). Ongoing work at Buck Quarry continues to reveal new taxa; the diversity of nonarthropod fauna is also becoming better known (García-Bellido et al., 2013, 2014; Paterson et al., 2016). Many of these newly discovered taxa are enigmatic, and are often only represented by one or two specimens. One such group is the Eldonioidea Dzik, 1991, which includes disc- shaped organisms whose complex internal structure belies their superficially medusoid appearance. Anatomical features observed across the group include dorsal and ventral discs enclosing a distinctive coiled digestive system, with a ventrally located mouth and anus, two circumoral tentacles, and a suite of variously observed and interpreted radial structures and concentric rings. Only two eldonioid specimens are known from more than 5,500 specimens collected from Buck Quarry and registered in the South Australian Museum paleontology collection.
80 The affinity of eldonioids remains problematic, with sug-
gestions ranging across the major metazoan groups, including cnidarians and lophophorate protostomes (Dzik, 1991; Zhao and Zhu, 1994; Chen et al., 1995; Dzik et al., 1997; Zhu et al., 2002), as well as deuterostomes (Conway Morris, 1993a, 1993b, figs. 2, 4; Chen et al., 1995; Chen, 2012; Zhang et al., 2013), such as holothurian echinoderms (Durham, 1974; Chen, 2009) and other ambulacrarians (as part of the informal, unranked stem-group called the ‘cambroernids’ sensu Caron et al., 2010b; Conway Morris et al., 2015). A taxonomic review of the group is long overdue, but two families that include Cambrian taxa are recognized, the Eldoniidae Walcott, 1911a and the Rotadiscidae Dzik, 1991, with a third post-Cambrian group informally referred to as the paropsonemids. A summary of the geographic and stratigraphic distribu-
tions of eldonioids, along with some of their key morphological characteristics, is provided in Table 1. Here we document a rare eldonioid from the EBS that
displays similarities to Pararotadiscus guizhouensis from the middle Cambrian (Series 3, Stage 5) Kaili Biota of South China, including the presence of associated trace fossils on the discs of both forms. This adds support for close faunal affinities between the Konservat-Lagerstätten of South Australia and those of South China (Paterson et al., 2016) and extends the known geographical range of Cambrian eldonioids to East Gondwana.
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