Schroeder et al.—Eldonioids from the Cambrian Emu Bay Shale of South Australia
83
sinuous burrow running between the margin of the disc and its center (Fig. 1.2). It does not extend beyond the disc margin into the surrounding matrix. Near the disc margin, the trace on SAM P45196 is a clear, shallow groove about 1mm across, flanked with low levees, but it becomes less distinct nearer the disc center. The fine concentric ornament on the disc surface also crosses the contours of the groove and levees of the trace without disruption, which indicates it was caused by activity just below, rather than at or above, the disc surface. In situ orientation of the EBS specimens was not recorded,
Figure 2. Eldonioid from the early Cambrian Emu Bay Shale, Kangaroo Island, South Australia, SAM P46361. Lighting from top left. White arrows indicate tubercular protuberances. Scale bar=4mm.
SAM P45196 is almost complete and provides the most
morphological information for this description. About one- seventh of the disc margin is missing (Fig. 1.1, 1.2, top of specimen). SAM P46361 comprises about two-thirds of a disc (Fig. 2). Where preserved, the margins of both discs are sharply delineated, with no evidence of any structures protruding beyond the margins. Eldonioids are characterized by a series of internal radial
(sensu Zhu et al., 2002, fig. 1) or stomach (sensu Caron et al., 2010b), generally considered to be the digestive organ overlying the internal lobes. This structure may be represented on SAM P45196 by a slightly convex ridge (shown by black arrows in Fig. 1.2) that is in a similar position to the ‘coiled sac’ in some eldonioid specimens from China (e.g., Chen et al., 1995, fig. 4; Zhu et al., 2002, fig. 4a), which also exhibit convex relief. The external surface in both specimens consists of very
lobes, which extend in all directions from near the center of the disc toward its margin. In SAM P45196, the internal lobes appear as raised ridges on approximately one-quarter of the disc, imprinting through the integument (Fig. 1). They are indistinct near the center of the disc and terminate about 3mm from the margin. Each lobe bifurcates twice, with the primary bifurcation situated near the marginal side of the coiled sac, and the sec- ondary bifurcations occurring close to the distal end (Fig. 1.4). It is estimated that the whole disc bore approximately 30–35 lobes. Another distinctive feature of eldonioids is the ‘coiled sac’
but most eldoniids and rotadiscids from China (Chen et al., 1995; Zhu et al., 2002) are found with the dorsal surface facing upward relative to the bedding plane. Such an orientation in the EBS specimens is supported by the lack of ventral structures (e.g., circumoral tentacles) and the presence of more robust structures visible through the disc, such as the internal lobes and probable coiled sac. Assuming this is the correct orientation for SAM P45196, burrowing activity beneath the disc is preserved in negative epirelief. SAM P45196 also bears two circular indentations and a
small, protruding, tubercle-like feature (Fig. 1.2), while SAM P46361 has two tubercles (Fig. 2). Similar protuberances (e.g., Wang et al., 2009, figs. 2E, 3D, 6A) and circular indentations (e.g.,Wang et al., 2009, figs. 2A, 3C, 5B) can be seen on discs of Pararotadiscus guizhouensis.
Discussion
fine, evenly and closely spaced concentric corrugations. This ornament is well preserved around the outer portion of the disc (Figs. 1.3, 2) but becomes gradually more indistinct toward the center. In SAM P45196, the concentric ornament clearly drapes over the ridges and valleys of the internal lobes without interruption (Fig. 1.3, 1.4). The internal lobes seem to have been resistant to compression, so the integument must have had some degree of flexibility.
Associated trace fossils.—The EBS eldonioids also host distinctive trace fossils. SAM P45196 has a narrow,
Taxonomic considerations.—Despite there being only two EBS specimens, with imperfect preservation, the size of the discs, the number and structure of radial internal lobes, and the fine con- centric dorsal ornament strongly support an eldonioid affinity. While the EBS discs have some superficial similarities to features of the brachiopod Heliomedusa orienta Sun and Hou, 1987 from the Chengjiang Biota (see Jin and Wang, 1992; Chen et al., 2007; Zhang et al., 2009), an affinity can be easily dismissed. Apart from the EBS discs and valves of H. orienta being similar in size and possessing a fine concentric ornament, the EBS discs do not exhibit radial striae like H. orienta (e.g., Chen et al., 2007, fig. 4.5). The radial lobes of the EBS discs bear a faint resemblance to the lophophore of H. orienta, but the lophophore tentacles in H. orienta do not bifurcate (e.g., Zhang et al., 2009, figs. 2, 3, 6). Other important features of H. orienta (that are absent in the EBS discs) include mantle setae that extend beyond the margins of both valves, a pseudointer- area, and a well-developed visceral cavity (Jin and Wang, 1992; Chen et al., 2007; Zhang et al., 2009). At about 21mmin diameter, the EBS specimens are among
the smallest eldonioids known (Table 1). In their redescription of Pararotadiscus, Zhu et al. (2002) measured the diameter of 75 specimens, which ranged from ~12 to ~105mm. They concluded that the disc becomes larger and increasingly sclerotized throughout ontogeny. In their smallest specimens, the disc margin is unclear and the disc easily deformed. Specimens greater than 20mmin diameter, with a relatively stiff disc and clear disc margins, are considered to be adults. Using these criteria, the EBS specimens would appear to be adult animals, but whether they represent two young (similar-sized)
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