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Journal of Paleontology 92(1):80–86
individuals that could potentially have grown larger, or members of a miniature species, cannot yet be ascertained. The number of internal lobes is fairly consistent for each of
the well-known Cambrian eldonioid species (Table 1). Eldonia ludwigi Walcott, 1911a has a maximum of 30, E. eumorpha (Sun and Hou, 1987) has ~44, Rotadiscus grandis Sun and Hou, 1987 has up to 88, and Pararotadiscus guizhouensis has ~40. The EBS specimens are estimated to have ~30–35 lobes, thus falling within the range observed in both eldoniids and rotadiscids. However, internal lobes that have primary and secondary bifurcations, as observed in SAM P45196, have not been documented in other Cambrian eldonioids. The three families of eldonioids can be distinguished by the
ornament on their dorsal surface. Simple concentric corrugations, as found on the EBS specimens, are characteristic of the Rotadiscidae. The post-Cambrian paropsonemids are also ornamented, but they have a far more complex pattern, and the ornament of the Eldoniidae is radial, not concentric (MacGabhann and Murray, 2010). The concentric corrugations observed in rotadiscids have been interpreted as lines of accretionary growth at the disc margin (Dzik, 1991; Chen et al., 1995; Dzik et al., 1997; Chen, 2012). An alternative interpretation is that, at least in some instances, the concentric corrugations are taphonomic artefacts, due to compression of a dome- or bell-shaped (and variably sclerotized) body (Zhu et al., 2002; Caron et al., 2010b). Given the small size of the EBS specimens, and the regularity and spacing of the fine concentric corrugations, it seems unlikely that the taphonomic interpretation applies in this instance. Zhu et al. (2002) compared the degree of sclerotization of
the dorsal disc in the three Chinese eldonioid taxa, using the frequency of specimens represented by folded or deformed discs as a proxy for disc stiffness or rigidity. Specimens of Eldonia eumorpha were most often folded or distorted, so were interpreted as being the most lightly sclerotized. Specimens referred to Pararotadiscus and Rotadiscus showed progres- sively lesser degrees of folding. Deformation of the dorsal disc to reveal the form of the underlying internal lobes is typical of Pararotadiscus, but does not occur in Rotadiscus (Zhu et al., 2002). The EBS specimens show no sign of folding, but the internal lobes inSAM P45196 imprint through part of the dorsal surface, suggesting a stiff, but not completely rigid, disc, most similar to that of Pararotadiscus. At present, lack of important morphological detail in the
two EBS specimens precludes assignment to a new or existing genus and species. While their concentric ornament and stiffened dorsal disc would suggest placement of the EBS eldonioids within the family Rotadiscidae, the unknown condition of their circumoral tentacles prevents an assignment to either of the included genera. On comparison with other characteristics, Pararotadiscus seems the most similar, as both it and the EBS specimens have a stiff but not completely rigid disc and a similar number of internal lobes. However, internal lobes that show secondary bifurcations (as in the EBS specimens) would warrant designation to a separate species. Until further information regarding the precise number of internal lobes, the condition of the circumoral tentacles, and perhaps their size range becomes available, we consider the EBS specimens as being representative of the Rotadiscidae, but leave the genus and species under open nomenclature.
The eldonioid–trace fossil association.—Examples of body and trace fossil associations can be found in the major Cambrian Konservat-Lagerstätten. For example, in the Burgess Shale, Chengjiang, Kaili, Sirius Passet, and Stanley Glacier assem- blages, traces can be found on, or immediately below, body fossils, in the sediment surrounding them, or crossing between sediment and body fossil. These burrows and trails meander, intersect, or branch and occur on a variety of hosts (e.g., Zhang et al., 2007, Wang et al., 2009; Caron et al., 2010a; Lin et al., 2010; Mángano, 2011; Mángano et al., 2012). In the Burgess Shale and Stanley Glacier assemblages, traces are found on and around various arthropod exoskeletons (such as Hurdia Walcott, 1912 and Tuzoia Walcott, 1912), and the vetulicolian Banffia Walcott, 1911b (Caron, 2006; Caron et al., 2010a; Mángano, 2011). In the Sirius Passet Lagerstätte, exoskeletons of the large, trilobite-like arthropod Arthroaspis Stein et al., 2013 (see Mángano et al., 2012), and rarely the trilobite Buenellus Blaker, 1988 (see Babcock and Peel, 2007), host trace fossils. The Chengjiang Biota hosts include vetulicolians, the bivalved arthropods Isoxys Walcott, 1890 and Branchiocaris Resser, 1929, and other nonbiomineralized arthropods (Zhang et al., 2007). In the Kaili Biota, typical hosts include arthropods such as Canadaspis Novozhilov, 1960, Naraoia Walcott, 1912, Skania Walcott, 1931, Tuzoia, and Waptia Walcott, 1912. One of the most common Kaili fossils, the rotadiscid Pararotadiscus, is also the most common trace fossil host (Wang et al., 2009; Lin et al., 2010). Aside from large coprolites containing trilobite fragments
(Nedin, 1999; Daley et al., 2013), trilobite exoskeletons showing injuries (Conway Morris and Jenkins, 1985), and perhaps the ‘epibionts’ seen in some specimens of the EBS
vetulicolian Nesonektris (cf. García-Bellido et al., 2014, figs. 1B, C, 4A), trace fossils are notably scarce from the body-fossil-bearing layers of the EBS Lagerstätte interval (Paterson et al., 2016). The near-complete absence of traces associated with EBS body fossils is clearly not due to a lack of suitable hosts—similar taxa to those listed, including Nesonektris García-Bellido et al., 2014 and the bivalved arthropods Tuzoia and Isoxys (García-Bellido et al., 2009), are common to very abundant in the EBS. Therefore, the scarcity of EBS body–trace fossil associationsmost likely relates to specific (probably restrictive) paleoenvironmental conditions, particu- larly at the sediment-water interface (Paterson et al., 2016). The discovery of eldonioids with associated traces in the
EBS is important, not only for expanding the ichnological diversity of this fossil deposit, but also for providing new paleoecological information on the biota. For example, the simple, sinuous burrow on one of the discs (SAM P45196; Fig. 1.2) may represent a feeding trace. The form of the trace, a groove with flanking levees, is typical of a burrow produced by sediment displacement (Carbone and Narbonne, 2014). Smooth, curved, unbranched burrows such as this suggest a single-use, nonselective feeding strategy (Mikuláš et al., 2012). Burrowing seems to have occurred just below the disc, perhaps to feed on bacteria or other nutrients situated beneath it (Mángano et al., 2012), with the disc subsequently draping over the burrow during (or possibly before) burial of the eldonioid. The simplicity and short length of the burrow may be due, in part, to the small size of the disc, but the presence of additional traces
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