Han et al.—Olivooides-like tube aperture in carinachitids
exhibit two other types of apertural lobes (Sendino et al., 2011). Finally, the internal anatomy of the tube wall of conulariids is much more complex at the corners and midlines than in carinachitids, as summarized by Van Iten (1991, 1992b). For example, there are eight types of internal midline structures (Bischoff, 1978; Van Iten, 1991, 1992b; Jerre, 1994), including: (1) a single continuous (nonseriated) carina, and (2) a pair of continuous carinae (flanking the midline), (3) a pair of seriated carinae, (4) a single seriated carina (subsequently discovered by Hughes et al., 2000 in Baccaconularia), and (5) the Y-shaped continuous single carina documented by Jerre (1994) in Eoconularia loculata (Wiman). The corners may be: (1) nonthickened, (2) thickened without formation of a clear carina, (3) thickened and bearing a distinct nonseriated carina, or (4) thickened and bearing a seriated distinct carina. In summary, gross morphological comparisons of the
skeletons of olivooids, carinachitids, hexangulaconulariids, and Paleozoic conulariids support the previous hypothesis (He, 1987) that these fossil taxa represent closely related lineages within the Conulata. Since the olivooid soft body exhibits a manubrium within a subumbrellar cavity, tentacles, apertural lappets, and frenula (Han et al., 2016a, b), olivooids and hence all conulatans probably were medusozoans (Van Iten et al., 2006) that were related either to extant cubozoans (Han et al., 2013; Han et al., 2016a,b) or to scyphozoans (Dong et al., 2013; Liu et al., 2014b; Van Iten et al., 2014). The proposal that Conulata constitutes an independent phylum (Babcock et al., 1986; Brood, 1995) or the internal rachis of sea pens (Conway Morris and Chen, 1992) appears unlikely. Carinachitids, originally interpreted as the most primitive taxa within Conulata (He, 1987), are interpreted here as a stock of phylogenetically intermediate forms between olivooids and hexangulaconular- iids. The presence of corner sulci and faces with a median line (midline) probably represent synapomorphies of carinachitids, hexangulaconulariids, and conulariids. The general similarities shared by olivooids, hexangulaconulariids, carinachitids, and conulariids (i.e., radial symmetry), probably represent primitive conditions. Finally, the bimerous tetraradial symmetry of hexangulaconulariids may have been independently acquired in this lineage. However, these interpretations await future phylogenetical analysis.
Orientation of the radial symmetry planes in carinachitids.— Similarities in gross morphology between carinachitids, oli- vooids, and conulariids suggest that their peridermal apertural lobes are homologous structures. If this hypothesis is correct, then the orientation of the meridian planes of olivooids and Olivooides-like medusozoans (Han et al., 2013, 2016a, b) may shed new light on the orientation of these planes in carinachitids and conulariids. In the soft body of Olivooides, the perradial frenula and apertural lappets, which correspond in position to the perradial pockets (e.g., Han et al., 2013, fig. 3), probably were responsible for the formation and closure of the plicate lobes of the periderm (Han et al., 2016a, figs. 3–5). Apart from the adradial frenulae and apertural lappets, no frenulae or apertural lappets are present in the interradii, where the interradial septa connect the subumbrellar and exumbrellar walls, and there is no interradial apertural lobe on the peridermal tube (Fig. 5). Similarly in carinachitids, the bulging faces
11
and corner sulci may directly reflect the configuration of the tube aperture, and they may correspond in position, respec- tively, to the perradial pockets and interradial septa of the gastric cavity. This means: (1) that the midline of the facial ribs and the corner sulci were most likely located at the perradii and interradii, respectively; and (2) that the corner sulci may correspond to the former interradial septa/mesenteries (Fig. 3). This orientation may also apply to conulariids if indeed their apertural lobes are homologous with those of carinachitids and olivooids. It should be noted, however, that our suggested orientation of the interradial symmetry planes in conulariids differs from the traditional hypothesis, which is based on similarities between the conulariid and coronate periderms and between the midline carinae of Eoconularia loculata Wiman and the gastric septa of stauromedusans (Van Iten et al., 2006). According to this hypothesis, the apertural lobes and facial midlines in conulariids were situated at the interradii (Chapman, 1966; Werner, 1966; Van Iten, 1992a; Jerre, 1994). Confirming or disproving this hypothesis will require the discovery of additional and better-preserved relic soft tissues in conulariids.
Conclusions
A single, exceptionally well-preserved specimen of Carinachites spinatus, documented for the first time in the present paper, reveals that the apertural end of the skeletal tube of tetraradial carinachitids exhibits four plicate lobes that are similar to those of co-occurring olivooids and younger conulariids. Similarities between the lateral tube spines and the apertural lobes of carinachitids indicate that all of the transverse ribs on the faces were released adorally and were eventually displaced toward the edges of the tube, a pattern of growth similar to that of co-occurring olivooids. The internal anatomy and symmetry of Olivooides suggest a perradial and interradii disposition, respectively, for the four faces and corner sulci of carinachitids. These findings corroborate the previously proposed hypothesis that early Cambrian carinachitids, hexangulaconulariids, olivooids, and conulariids are closed related taxa within the subphylum Medusozoa, although olivooids may have retained certain primitive features.
Acknowledgments
We thank Drs. H. Van Iten (Hanover College, USA), C.B. Skovsted (Swedish Museum of Natural History), and G.A. Brock (Macquarie University) for their suggestions and lin- guistic improvement of the manuscript. We also thank H.J. Gong, J. Sun, J. Luo, and M.R. Cheng (Northwest Uni- versity, Xi’an, China) for their assistance in the field and with lab work. X. Han prepared the 3D drawings of the carinachitid specimens, and Y.H. Liu (Chang’an University) provided two carinachitid photos. This work was supported by the Natural Science Foundation of China (NSFC grant 41272019, 41621003, 41372021, 41472015), the ‘973 project’ of the Ministry of Science and Technology of China (grant 2013CB835002, 2013CB837100), the Chinese Academy of Sciences (XDB10010101), and the State Key Laboratory of Palaeobiology and Stratigraphy (No. 163107).
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