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Journal of Paleontology 92(1):3–13
plicate lobes are equivalent to the striated folds on the lateral thorn-like spines. (2) The distance between neighboring ribs on the same face is approximately equal to or less than the radius of the tube aperture and the depth of the inward portion of the plicate lobes. The displacement of the ribs reflects the dis- placement of the segmented faces and the corner sulcus and thus the migration of the entire tube aperture. (3) Following the previous inward portion of tube aperture, new inward portion skeleton was secreted by the epithelium of soft tissue at the oral region. The new skeleton may have been primarily attached with the epithelium, and afterward the new apertural parts may have detached with the epithelium of the oral region and been pushed onward and outward with centrifugal expansion, finally being displaced to the lateral side of the tube and becoming the lateral components of the lateralwalls. (4) Periodic renewal of the tube aperture necessarily led to orally addition of iterated ribs and ‘segmentation’ of the faces. Together, these processes reflect the growth of the tube by apertural extension (Fig. 4.4–4.6). The multiple thorn-like spines on specimens ELISN93-45
(Fig. 1.5) and ELISN148-52 (Fig. 1.6) indicate that the morphology of the ribs on the faces of a single individual is essentially uniform and constant, without gradual transforma- tion from welts to arcuate ribs or other, more complex folds. Probably, this replacement began at the basal end of the tube and continued to the upper part without metamorphosis. If this hypothesis is correct, then the specimens of C. spinatus described in Conway Morris and Chen (1992) are likely a mixture of several species. Specimens with sharp, thorn-like spines or arcuate ribs as well as welts should be reinterpreted as different species rather than different developmental stages, unless these variants can be shown to co-occur in the same individual of C. spinatus. In addition to sequential adoral addition of ribs on the faces,
growth of the Carinachites tube also involved increase in the diameter of the tube and the width of the corner sulci. Along the longitudinal axis, the corner sulcus expands gradually toward the aperture (Fig. 1.5). In the most complete specimen (ELISN93-45), which however lacks the apex and apertural margin, at least 35 ribs are present on each face (Fig. 1.5). If the tube could grow up to 1.3mm in width, as indicated by specimen ELISN148-52 (Fig. 1.6), then we infer that a single face may have contained at least 130 ribs over a total length of approximately 25mm. With regard to the apertural extension model mentioned in the preceding, neighboring ribs at the same level indicate that the plicate apertural lobes were replaced synchronously by new ones (Fig. 4.4–4.6). In contrast, long- itudinal offset of ribs along the corner sulci may reflect diachronous replacement of previously formed plicate lobes, indicating that the tube opening was always more or less partly closed. The corner sulci are generally thinner and more flexible than the faces (Qian et al., 1997), and in most fragmentary specimens, the sulci exhibit secondary breakage. Probably in life the flexible corner sulci served as buffer zones that prevented tearing of the tube during diachronous replacement of the ribs on neighboring faces. The reason for longitudinal offset of the ribs along the facialmidline (ConwayMorris andChen, 1992, fig. 6.1) remains unclear, though probably each apertural lobe was formed asynchronously by two adjacent subunits of soft tissue. It is important to note that longitudinal offset or asynchronous
displacement of the ribs also can be seen on hexangulaconulariids, but it has never been observed in olivooids.
Tube morphology and growth of other carinachitids.—Since the ribs most likely constitute displaced plicate lobes, the four uniform plicate lobes in tetraradiate Carinachites spinatus correspond to the four rows of lateral facial ribs. Similarly, triradiate Emeiconularia and pentamerous Pentaconularia ningqiangensis Liu et al., 2011 most likely possessed three (Fig. 4.1, 4.2) and five centripetal plicate lobes (Fig. 4.7–4.9), respectively. Similarly, specimens with arcuate ribs reflect the presence of a set of centripetal arcuate lobes in the apertural region. Nevertheless, the apertural region of Carinachites tetrasulcatus (Chen, 1982) is difficult to reconstruct as its ribs
are low and inconspicuous (Conway Morris and Chen, 1992, fig. 8.22). Probably the aperture of this species resembled a four-sided pyramid with deep, concave corner sulci similar to those of Hexaconularia sichuanensis He and Yang, 1986 (e.g., Steiner et al., 2014, fig. 7.13–7.16, 7.19–7.21). We hypothesize that this species exhibited periodical growth similar to that of Carinachites spinatus. However, such eversion probably was possible only in organic or lightly sclerotized exoskeletons and not in those with thick and rigid hard parts such as tubes of Emeiconularia trigemme (Qian et al., 1997). Thus, secondary, subsequent thickening of the lateral walls, suggested by the double-layered wall structure (Fig. 2), is proposed here to resolve conflicts between the flexibility of the primary apertural wall and the rigidity of the thick secondary tube wall. The thin primary apertural wall in Carinachites spinatus, represented by the outer layer with fine transverse wrinkles, may have been rich in organic material. The outer layer may have undergone sub- sequent thickening on its inner surface by a mixture of inorganic materials (represented by the smooth granular layer), thus resulting in a double-layered structure similar to that of con- ulariids (Brood, 1995; Ford et al., 2016). This model could account for: (1) the flexibility of the external tube surface (as indicated by the striations and welts shown in Conway Morris and Chen, 1992) and the relative rigidity of the entire tube wall, (2) the high abundance of fragmentary specimens of carinachitids and the extremely rare preservation of their aperture, and (3) probable variation in mechanical properties between the different layers as reflected in the secondary cracks on the tube surface (Fig. 1.11, 1.12). However, because the relic soft tissue is much smaller in diameter than the host tube, the sides of the soft body may not have been in direct contact with the lateral tube wall. How the organic or inorganic material was deposited on the inner surface of the outer layer remains unknown.
Feeding habits of carinachitids.—Despite the absence of in situ preservation of carinachitid tubes, it was originally assumed that carinachitids were solitary sessile forms having their aboral end attached to hard or firm substrates (He, 1987) as in extant medusozoan polyps.Apelagic habit for carinachitids is unlikely as their skeletonized tube appears to have been too dense to float in seawater. However, because the soft body was almost entirely enclosed within the tube, filter-feeding on microorganisms seems highly likely. Besides the function of supporting the growing soft tissues, periodic tube growth and thickening of
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