Schroeder et al.—Eldonioids from the Cambrian Emu Bay Shale of South Australia


on SAM P45196 suggests a more complex situation than the form of the burrow implies (discussed in the following). Tubercule-like features preserved on both EBS specimens


have also been observed on eldonioid discs and arthropod exoskeletons from the Kaili Biota and from Cambrian sites in the Czech Republic. These have been interpreted as attempts at upward movement by the trace-maker, which was unable to pierce the sclerotized disc or exoskeleton (Wang et al., 2009; Mikuláš et al., 2012). Priapulids are proposed as possible contenders for the trace-makers at Kaili and other Lagerstätten (Wang et al., 2009). Interestingly, small palaeoscolecid worms with a body width (or diameter) similar to the observed burrow have been described from the EBS (García-Bellido et al., 2013), implicating them as suspected trace-makers. The circular indentations on SAM P45196 (Fig. 1.2) are


similar to those appearing with Gordia-like traces on some of the Kaili specimens illustrated, but not discussed, by Wang et al. (2009, figs. 2A, D, 3C, 5). These may represent sites of epibiont attachment, or perhaps, contrary to Wang et al. (2009), are the result of successful perforations of the disc surface by the trace-maker. The presence of different trace forms on a single specimen


(e.g., Fig. 1.2) suggests that either multiple trace-makers were present in the immediate vicinity of the individual and converged on it, or one trace-maker was able to produce several trace forms.


Mode of life and taphonomic considerations.—Previous studies have suggested that eldonioids were either pelagic (Walcott, 1911a; Durham, 1974; Sun and Hou, 1987; Zhu et al., 2002; Hu et al., 2007; Chen, 2009, 2012), nektobenthic (Caron et al., 2010b), or benthic (Dzik et al., 1997; MacGabhann and Murray, 2010). Taxa envisaged as living at or near the seafloor have been portrayed as vagile, oriented with the disc facing convex side upward and the tentacles combing the substrate (Caron et al., 2010b), or sessile, with tentacles facing upward and the convex side of the disc lying passively on or within the sediment (Dzik et al., 1997). Eldonioids were also a common, and occa- sionally abundant, element of the early to middle Paleozoic fauna, with some taxa known from hundreds or thousands of specimens (see Table 1). The two EBS eldonioid specimens represent isolated


individuals, found in different stratigraphic levels within Buck Quarry, separated by about 1.5 m. Both specimens are undistorted, with the discs preserved parallel to laminations within mudstone beds. This suggests that these individuals were buried under relatively calm conditions, as opposed to being swept up by the intermittent sediment gravity flows that captured other faunal elements (including benthic and nekto- benthic taxa) of the EBS biota prior to rapid burial (Gehling et al., 2011; García-Bellido et al., 2014; Paterson et al., 2016). Unfortunately, there is limited taphonomic (and certainly no


morphologic) evidence to elucidate the lifestyle of the EBS eldonioid. The paucity of specimens in the EBS—coupled with the otherwise depauperate diversity of the autochthonous benthic fauna in the exaerobic zone (see Paterson et al., 2016)—would indicate that a sessile life mode is unlikely, unless individuals were infrequently relocated fromtheir original habitat.Moreover, the style of preservation of the EBS specimens neither supports


85


nor contradicts amobile benthic, nektic, or pelagic habit, but their rarity suggests that these individuals either were transported or settled into the ‘preservational trap,’ as represented by the Konservat-Lagerstätte interval. Irrespective of lifestyle, the associated trace fossil evidence suggests that the EBS eldonioids were exposed on the substrate for a short period before burial, allowing organisms to exploit these particular carcasses before the entombing sediment became truly anoxic (Hall et al., 2011; McKirdy et al., 2011; Mángano et al., 2012; Gaines, 2014; Paterson et al., 2016).


Acknowledgments


This research was supported by grants from the Australian Research Council (FT120100770 to JRP) and National Geo- graphic (#8991-11), with additional financial assistance from Beach Energy Ltd. and the South Australian Museum, and logistical support from SeaLink. We are grateful to the Buck family for access to the field area, and to our regular EBS collaborators G. Edgecombe, D. García-Bellido, J. Gehling, J. Jago, and M. Lee. We thank R. Atkinson, M.-A. Binnie, A. Camens, A. Daley, R. Gaines, M. Gemmell, J. Holmes, K. Kenny, P. Kruse, J. Laurie, B. McHenry, M. Mills, L. Reid, D. Rice, E. Thomson, and members of the South Australian Museum Waterhouse Club for assistance in the field and lab, and for discussions. Thanks also to the two anonymous referees for their helpful reviews of the manuscript.


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