Han et al.—Olivooides-like tube aperture in carinachitids
large specimen ELISN148-52, is superficially dome-shaped (Fig. 1.6–1.9). The maximum diameter of the tube is approximately 1.3mm, but near its apertural end the tube tapers rapidly, with the faces curving smoothly toward the longitudinal axis of the tube and becoming more or less perpendicular to it. Close to the longitudinal axis, the faces and intervening corner sulci are inclined toward the aboral end of the tube (Fig. 1.6, 1.7, 1.9). Present on each face, at the summit of the aperture, is a triangular tongue-shaped structure, and the distal ends of the faces almost meet near the longitudinal axis of the tube, leaving just a narrow central opening. One of the tongue-shaped structures projects much farther than the others beyond the aperture (Fig. 1.6). The tongue-shaped structures are not flat features but rather fold-like structures having two main, arched sides separated by two flanks. For this reason, we term the tongue-shaped structures ‘plicate apertural lobes’ (pal). The vertical abapertural side of the apertural lobes extends far into the apertural opening and exhibits a medial subtriangular groove bordered by two elevated flanks (Fig. 1.8, 1.9). The abapertural side is either flat or outwardly convex with a central ridge, and there are many longitudinal striations on the two sides. These striations converge on the tip of the lobes, and there are some oblique, irregular cracks on the striated surface (Fig. 1.11, 1.12). Situated peripheral to the apertural lobes are four longitudinal rows of nearly evenly spaced, nose-shaped, thorn-like spines aligned along the lateral sides of the faces. In addition, the distance between the apertural lobes and the marginal thorn-like spines is approximately equal to the distance between adjacent thorn-like spines. The adapertural side of the lobes, which is more or less perpendicular to the lateral faces, is concave aborally. The abapertural side, like the bridge of a nose, is inclined at approximately 30º to the lateral faces. The four corner sulci, which are substantially lower than the apertural lobes, follow the inward foldings of the adjacent faces and extend far into the tube cavity (Fig. 1.9). The external surface of the sulci is highly and irregularly folded and exhibits fine parallel striations. The summit of the four corner sulci is evidently lower than that of the faces (Fig. 1.9). Clearly, then, the tube aperture, including the inwardly folded portion, is a smooth continuation of the faces and corner sulci.
The apertural region, preserved only in the relatively
Tube wall and internal anatomy.—Micro-CT observations confirm the presence of a narrow apertural opening and the continuation of the tube walls into the inwardly and down- wardly folded apertural lobes (Fig. 2.1a–e). The tube wall of carinachitids generally exhibits a prismatic inner layer of uniform thickness and a granular outer layer that is much thinner in the corner sulci than in the faces (e.g., Conway Morris and Chen, 1992, fig. 8.19; Qian et al., 1997, plate 2, 1c, 3c; Liu et al., 2005, plate 2, 1e, j; Liu et al., 2011, fig. 2f-g). The prismatic layer originally was thought to consist of overgrowths of diagenetic apatite, while the granular layer was thought to have been originally organic but later replaced by diagenetic apatite (Qian et al., 1999). Although we are mindful of possible preservational artefacts, Micro-CT imaging revealed that the thickness of the tube wall in specimen ELISN148-52 appears to vary, and that the apertural walls are much thinner than the lateral tube walls (Fig. 2.1a, 2.1c, 2.1e). High magnification
7
imaging revealed a single-layered wall in the apertural region (Fig. 2.1c) and bilayered lateral tube walls (white arrows in Fig. 2.1a, 2.1g). In addition, the facial walls are thicker than those of the corner sulci (Fig. 2.1a, 2.1g). Finally, both the thorn-like spines and the apertural lobes are hollow (Fig. 2.1b). Present within the tube is a short, subcylindrical mass
measuring ~200 µm in diameter and 400 µm in length. The upper part of this feature is in direct contact with the inward folds of the faces and corner sulci (Fig. 2.1b–e), and it is
connected to the lateral tube wall by numerous fine, straight filaments (mmf) (Fig. 2.1a–g). No additional details of the subcylindrical mass can be discerned.
Discussion
Relic soft-tissue of Carinachites spinatus.—Similar fine filaments commonly occur within associated fossils, including poorly preserved Carinachites (Conway Morris and Chen, 1992, fig. 7.9–7.11), other tubular microfossils (e.g., Steiner et al., 2014, figs. 7.21, 11.9, 11.12, 11.15), and egg envelopes with partly decayed embryos (e.g., Steiner et al., 2014, fig. 4.6– 4.9). Because these internal filaments are generally interpreted as microbial in origin and derived from partly decomposed soft tissue (Yue and Bengtson, 1999), we interpret the subcylindrical mass surrounded by these fine filaments in specimens of C. spinatus as remains of soft tissue that underwent partial decay. Relic soft tissue similar to that near the tube aperture of
Carinachites spinatus also is present in Hexaconularia (Steiner et al., 2014, fig. 7.18), Olivooides (e.g., P. Li et al., 2007, fig. 4d; Steiner et al., 2014, figs. 10.3, 11.6, 11.12, 11.15), and Quadrapygites (e.g., Steiner et al., 2014, figs. 14.19, 15.13, 15.15). In addition, an intact, trumpet-shaped mass of relic soft tissue consisting of an upper calyx and a slender basal stalk extending to the aboral end has been documented in Olivooides (see Steiner et al., 2014, fig. 12.7–12.10). The presence of relic soft tissues in Olivooides embryos has been demonstrated convincingly by the discovery of exceptionally well-preserved, primary internal anatomy (Han et al., 2016b). All of these relic soft-tissue structures are smaller in diameter than the surrounding tube wall. Whether the soft tissues of Carinachites spinatus reached the aboral end of the tube cannot be determined at present.
Growth of the tubes of Carinachites spinatus.—The thorn-like spines on the transverse ribs of this taxon resemble the plicate apertural lobes in many respects including shape, size, surface ornament, spacing, and tip directions. Thus, it seems clear that the spines and lobes are substantially the same kind of structure and are simply located in different positions on the faces. The tube aperture was a kind of extracellular matrix that was most likely secreted by epithelial tissue at the oral end, as only in this area was the tube in direct contact with the soft body. If this hypothesis is correct, then one can make the following addi- tional inferences: (1) the thorn-like spines most probably were derived from the plicate lobes and not vice versa. Delimited by the corner sulci, the transverse extension of the two flanks of the plicate lobes may have undergone ontogenetic transformation into the lateral face ribs. The abapertural groove of the plicate lobes was transformed into the middle groove or the central ridges between adjacent ribs. The converging striations on the
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