Journal of Paleontology, 92(1), 2018, p. 3–13 Copyright © 2017, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.10


Olivooides-like tube aperture in early Cambrian carinachitids (Medusozoa, Cnidaria)


Jian Han,1 Guoxiang Li,2 Xing Wang,1 Xiaoguang Yang,1 Junfeng Guo,3 Osamu Sasaki,4 and Tsuyoshi Komiya5


1Department of Geology and State Key Laboratory of Continental Dynamics, Northwest University, 229 Taibai Road, Xi’an 710069, P.R. China


⟨elihanj@nwu.edu.cn⟩, ⟨799179701@qq.com⟩, ⟨lqzy0301@gmail.com⟩ 2State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences,


Nanjing 210008, China ⟨gxli@nigpas.ac.cn⟩ 3School of Earth Science and Land Resources, Key Laboratory of Western China’s Mineral Resources and Geological Engineering, Ministry of


Education, Chang’an University, Xi’an 710054, China ⟨junfengg@chd.edu.cn⟩ 4Tohoku University Museum, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, Japan ⟨Sasaki@museum.tohoku.ac.jp⟩ 5Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo 153-8902, Japan ⟨komiya@ea.c.u-tokyo.ac.jp⟩


Abstract.—The early Cambrian Carinachitidae, a family in the subclass Conulata, are intriguing and important small shelly fossils. Their gently tapering, tube-shaped skeletons consist of convex faces separated from each other by broad, deep corner sulci, and they exhibit triradial, pentaradial, or predominantly tetraradial symmetry. However, the morphology of the aperture and the modes of growth of carinachitid skeletons as well as the anatomy of their soft parts are unknown. Examination of a single new, exceptionally well-preserved specimen of tetramerous Carinachites spinatus Qian, 1977, collected from the lower Cambrian Kuanchuanpu Formation in South China, reveals: (1) that its aperture is connected to a small mass of relic soft tissue and (2) that the apertural end of each of the four faces is developed into a subtriangular lappet or oral lobe that is smoothly folded toward the long axis of the tube, partially closing the tube aperture. Similarities between thorn-like spines on the faces and the oral lobes indicate that the transverse ribs were periodically displaced from the perradial portion of the aperture during formation of new ribs. In addition, the tube walls may have undergone secondary thickening during growth. The growth pattern of the tube and the spatial relationships between the tube aperture and soft parts are analogous to those of co-occurring olivooids. These findings further strengthen the previously proposed hypothesis that coeval carinachitids, olivooids, hexangulaconulariids, and Paleozoic conulariids are closely related taxa within the subphylum Medusozoa. Finally, carinachitids most likely represent an evolutionary intermediate between olivooids and hexangulaconulariids.


Introduction


The abrupt appearance of diverse small shelly fossils (SSFs) during the earliest Cambrian signals the initial stages of the Cambrian explosion (G. Li et al., 2007; Maloof et al., 2010). It therefore seems axiomatic that SSFs are of great importance for understanding the early rise of metazoan phyla and the origins of animal skeletogeny. Paleoecological reconstruction of SSF communities is a challenging task as the majority of SSFs are fragmentary or consist of isolated sclerites. A critical exception to this rule is the set of phosphatic SSFs in Orsten-type Lagerstätten, for example the Kuanchuanpu Biota in South China (ca. 535 Ma), which together have the potential to provide unique insights into the nature and significance of these fossils thanks to their high potential for exceptional pre- servation of both hard parts and soft tissues. Carinachitids are an important component of early


Cambrian SSFs in South China (Conway Morris and Chen, 1992). Their gently conical skeletal tubes exhibit several (three


3


to five) transversely ribbed faces separated from each other by wide and deep corner sulci that usually bear fine transverse wrinkles (tw) (Fig. 1). To date, three genera and six species of carinachitids—Emeiconularia trigemme Qian in Qian et al., 1997; E. amplicanalis Liu et al., 2005 (Fig. 1.1); Pentaconularia ningqiangensis Liu et al., 2011 (Fig. 2); Carinachites spinatus Qian, 1977 (Figs. 3–5); C. tetrasulcatus Jiang in Luo et al., 1982; and C. curvatornatus Chen, 1982—have been reported from the Kuanchuanpu Formation and equivalent horizons in South China (Qian, 1977; He, 1987; Conway Morris and Chen, 1992; Qian et al., 1997; Liu et al., 2005, 2011). These fossils collectively exhibit tri-, tetra-, or pentaradial symmetry in transverse sections (Liu et al., 2011), and these symmetries may have arisen inde- pendently in different lineages (Han et al., 2016a, b). In addition, the tube wall appears to have been flexible and composed of organic material and/or calcium phosphate (Conway Morris and Chen, 1992; Qian et al., 1999). Carinachitids, together with co-occurring hexangulaconular- iids (Yue and Bengtson, 1999; Van Iten et al., 2010) and olivooids


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