Journal of Paleontology, 92(1), 2018, p. 26–32 Copyright © 2017, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.134


Malongitubus: a possible pterobranch hemichordate from the early Cambrian of South China


Shixue Hu,1 Bernd-D. Erdtmann,2 Michael Steiner,3 Yuandong Zhang,4 Fangchen Zhao,4 Zhiliang Zhang,5 and Jian Han5


1Chengdu Center, China Geological Survey, Chengdu, 610081, P.R. China ⟨hushixue@126.com⟩ 2Institut für Angewandte Geowissenschaften, Technische Universität Berlin, D-13355 Berlin, Germany ⟨berni1739@gmail.com⟩ 3Institut für Geowissenschaften, Freie Universität Berlin, D 12249, Berlin, Germany ⟨michael.steiner@FU-Berlin.de⟩ 4CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road,


Nanjing 210008, China ⟨ydzhang@nigpas.ac.cn⟩, ⟨fczhao@nigpas.ac.cn⟩ 5Early Life Institute, State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China ⟨zhangtensor@hotmail.com⟩, ⟨elihanj@nwu.edu.cn⟩


Abstract.—Malongitubus kuangshanensis Hu, 2005 from the early Cambrian Chengjiang Lagerstätte of China is redescribed as a pterobranch and provides the best evidence to demonstrate that hemichordates were present as early as Cambrian Stage 3. Interpretation of this taxon as a hemichordate is based on the morphology of the branched colony and the presence of resistant inner threads consistent with the remains of an internal stolon system. The presence of fusellar rings in the colonial tubes cannot be unambiguously proven for Malongitubus, probably due to early decay and later diagenetic replacement of the thin organic material of the tubarium, although weak annulations are still discernible in parts of the tubes. The description of M. kuangshanensis is revised according to new observations of previously reported specimens and recently collected additional new material. Malongitubus appears similar in most features to Dalyia racemata Walcott, 1919 from the Cambrian Stage 5 Burgess Shale, but can be distinguished by the existence of disc-like thickenings at the bases of tubarium branching points in the latter species. Both species occur in rare mass-occurrence layers with preserved fragmentary individuals of different decay stages, with stolon remains preserved as the most durable structures. Benthic pterobranchs may have occurred in some early Cambrian shallow marine communities in dense accumulations and provided firm substrates and shelter for other benthic metazoans as secondary tierers.


Introduction


The Pterobranchia are a group of colonial or pseudocolonial hemichordates with a long evolutionary history ranging from the Cambrian to the present day. Extant pterobranchs are benthic and classified into two major orders: the Cephalodiscida and the Rhabdopleurida. A close relationship between rhabdo- pleurid pterobranchs and graptolites was suggested by some previous studies (Urbanek, 1986, 1994; Urbanek and Dilly, 2000; Mierzejewski and Kulicki, 2003) and confirmed by a cladistic investigation (Mitchell et al., 2013), which indicated that the extinct Graptolithina should be included in the Pterobranchia. On the basis of scanning electron microscope back-scatter electron (SEM-BSE) analyses (Maletz et al., 2005), some fossils previously presumed to represent Cambrian algae have been reinterpreted as pterobranchs (Maletz et al., 2005; Maletz, 2014; LoDuca et al., 2015a, 2015b; Maletz and Steiner, 2015), greatly improving our knowledge of the early fossil record of the Pterobranchia. However, the age of the oldest pterobranchs and even of the Hemichordata is still not settled. The earliest potential candidate for hemichordate fossils so far known is acid-isolated organic fragments of Sokoloviina costata


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Kirjanov, 1968 representing tubaria with zigzag-shaped fusellar collars from the basal Cambrian Rovno Formation of Ukraine (Sokolov, 1997, pl. 8.4). Other remains have been reported from Cambrian Stage 5 (Series 3), including the potential colonial pterobranch Yuknessia simplex Walcott, 1919 (LoDuca et al., 2015a) and the tubicolous enteropneust hemichordates Spartobranchus tenuis Caron, Conway Morris, and Cameron, 2013 and Oesia disjuncta Walcott, 1911 (Caron et al., 2013; Nanglu et al., 2016) from the Burgess Shale, and undescribed fragments from the Kaili Formation in Southwest China (Harvey et al., 2012). Frond-like fossil remains with a pair of tentacles from the Chengjiang Lagerstätte were described as Galeaplumosus abilus Hou et al., 2011 and interpreted as a possible hemichordate zooid (Hou et al., 2011). The assignment of the sole known specimen to the hemichordates, however, was questioned by recent studies (LoDuca et al., 2015a; Maletz and Steiner, 2015; Ou et al., 2017). Another potential rhabdopleurid pterobranch from the Niutitang Formation of China (Cambrian Stage 2; Zhao et al., 1999) requires further study, and its pterobranch affinity remains uncertain (Maletz, 2014; LoDuca et al., 2015a; Maletz and Steiner, 2015). The earliest widely accepted occurrences of pterobranchs are from the basal part of


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