Hu et al.—A possible early Cambrian pterobranch hemichordate
31
as secondary tierers (Zhang et al., 2010), but they were only able to occupy lower levels (less than 5cm above the seafloor; Wang et al., 2012), whereas those on M. kuangshanensis could reach slightly higher elevations. In addition to the miniature brachio- pods, five juvenile individuals of the sponge Choia sp. are attached to the tubarium of the holotype of M. kuangshanensis (Fig. 2.1, 2.5). The sponges are 4–5mm tall (Fig. 2.5), implying probable secondary tiering. Benthic pterobranchs may have occurred in some early Cambrian shallowmarine communities in dense accumulations and provided firm substrates and shelter for other benthicmetazoans as secondary tierers. Pterobranchs played an important role in early Cambrian marine benthic communities. The interpretation of M. kuangshanensis as a pterobranch also indicates that a brachiopod–pterobranch association had likely developed by theCambrian Series 2, representing an early form of metazoan commensalism.
Acknowledgments
Figure 3. Dalyia racemata from the Cambrian Stage 5 Burgess Shale, No. USNM 194121, Smithsonian Institution. (1) Repeated dichotomous branching of tubarium with distinct threads within the partly decayed tubarium, interpreted as a sclerotized stolon system; thinner filaments besides the branched tubarium represent separated stolons from decayed colonies (arrows). (2) Branched tubarium with internal stolons; separated, decayed branching shows that tube material was decayed and only thickened round basal part of tube and fragments of stolons are left (arrows). (3) Partly decayed tubarium with retained left tube with inner stolon and decayed right tube with exposed stolon (arrow). (4) Branched tubarium with stolon in the left tube; remains of stolons of decayed colonies (arrow). Scale bars=1.0mm.
to microbial decay before final burial; consequently, the zooids disappeared due to protracted decay.
Paleoecology
Tiny brachiopods are observed on some of the specimens (Fig. 2.1, 2.2, 2.4, 2.5, 2.7). The brachiopods range from 0.8 to 2.2mm in diameter. The shells of the brachiopods are complete and articulated, with the two valves fitted together. These tiny brachiopods are called Kuangshanotreta malungensis Zhang, Holmer, and Hu in Wang et al., 2012 and are assigned to the acrotretoids (Wang et al., 2012). All individuals of K. malungensis are directly in contact with the tubes and are oriented with the posterior margin toward the tubes (Fig. 2.5, 2.7). Judging from the overall colony arrangement and the attachment of brachiopods, it seems likely that M. kuangshanensis was a benthic, erect pterobranch that was attached to a firm substrate while alive. The attached small brachiopods are interpreted as secondary tierers, which are benthic suspension feeders that use primary tierers to reach higher water levels above the seafloor to obtain a feeding advantage. This interpretation is also supported by the fact that these brachiopods are very rare in the surrounding matrix. Some other brachiopod species have also been interpreted
Thisworkwas supported by theNationalBasicResearch Program (2013CB835006); the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18030304); and the National Natural Science Foundation of China (Grant Nos. 41425008, 41472012). We are grateful to M. Zhu, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, and Z. Zhang, Northwest University, for constructive discussion and suggestions. The help of J. Thompson, University of Southern California, USA, and L. Muir, Amgueddfa Cymru – National Museum of Wales, UK, in improving the English of the manuscript is gratefully acknowledged. We also thank J. Jin and two reviewers for their most valuable comments and criticisms.
References
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Figure 2. Malongitubus kuangshanensis and attached brachiopods and sponges as secondary tierers. (1) Holotype, NIGP-165028, showing the associated brachiopods and sponges. (2) NIGP-165032, showing the attachment of numerous brachiopods onto the tubes. (3) NIGP-165033. Densely arranged tubes. Close-up of theareaframedin(6). (4) ELI-B CLP K010B, counterpart: a colony with both adult and juvenile brachiopods. (5)Close-upofthe area framed in (1), showing two sponges and one brachiopod on the tubes. (6) Close-up of the area framed in (3), showing the dark, continuous stolon. (7) Close-up of the two juvenile brachiopods indicated by the framed area in (2). (8) BSE image of the area marked by the frame in (4). White arrows point to the organic remains. (9–15) Energy-dispersive spectroscopic analysis of the tubarium in (8). (1–3)Scale bars=10mm; (4)scale bar=5mm; (5, 6)scale bars=1mm; (7)scale bar=0.5mm.
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