Early Cambrian epibolic gastrulation: A perspective from the Kuanchuanpu Member, Dengying Formation, Ningqiang, Shaanxi, South China Xiaoyong Yao, Jian Han ⁎, Guoxiang Jiao Early Life Institute and Department of Geology and State Key Laboratory of Continental Dynamics, Northwest University, 229 Taibai Road, Xi'an 710069, PR China abstract article info Article history: Received 16 July 2010 Received in revised form 14 April 2011 Accepted 15 April 2011 Available online 22 April 2011 Handling Editor: J.G. Meert Keywords: Gastrulation Olivooides Embryos Early Cambrian Penta-radial symmetry The Cambrian explosion is marked by the apparent sudden appearance of diverse metazoan skeletal fossils and an increase in the complexity of both body and trace fossils. Among them, fossil eggs and embryos from the Lower Cambrian Kuanchuanpu Member of the Dengying Formation at Ningqiang in southern China provide a unique window for investigating the ontogeny of metazoans. Gastrulation is the cell migration stage after cleavage, and can be viewed as the embryonic analog of the transition from protozoan to metazoan grades of complexity. As an example, the embryonic developmental sequences of Punctatus emeiensis is well- documented because of the stellate spines covering the body surface, that are shared with embryonic stages represented by Olivooides and their growth stages represented by P. emeiensis. Although Olivooides was considered to be a taxonomic mixture, consideration of its variation was neglected. A specific type of embryo with zigzag blastopore lips that differs from typical candidate embryos for P. emeiensis is identified; its gastrulation process is reconstructed and is comparable to the epibolic gastrulation of extant metazoans. Both the embryos and adults of this ‘type’ display radial symmetry, that suggests an affinity distinct from that of echinoderms. Moreover, the body plan of penta-radial symmetry seen widely in Early Cambrian ‘Small Shelly Fossils’ (SSF's) was probably deep rooted in the Ediacaran. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction During the Ediacaran to Mid-Cambrian time (635–505 Ma), the Earth underwent significant changes in palaeogeography that include the rifting of a possible supercontinent and the formation of a second, slightly smaller supercontinent (Meert and Lieberman, 2004; Jiang et al., 2011). The assembly of the Gondwana supercontinent during the waning stages of the Proterozoic seems to have provided a tectonic backdrop for the myriad biological, climatological, tectonic and geochemical changes leading to, and including, the Cambrian radiation (Meert and Lieberman, 2008). Evidence for this radiation event includes the enigmatic (but widespread) Ediacaran fauna, small Shelly fossils and fossils found in the Chengjiang, Burgess Shale and Sirius Passett lagerstatte (Walcott, 1909; Conway Morris et al., 1987; McCall, 2006; Shu, 2008). In particular, the SSF and the Ediacaran faunas are distinct from each other and the links between the two and their relation to Phanerozoic metazoans is controversial. It is possible that SSF's have the potential to record a mixture of some relicts of Ediacaran faunas and antecedents of the Chengjiang faunas. In particular, the occurrence of fossil eggs and embryos at the transition of the Neoproterozoic/Cambrian provides an exceptional develop- mental window for bridging the gap between the Ediacaran-SSF's and metazoan life in the Cambrian. Embryos from the late Neoproterozoic Weng'an biota in the Doushantuo Formation have been described as exhibiting cleavage, blastula and post-blastula stages (Xiao and Knoll, 2000; Xiao et al., 1998). Although some were later interpreted as giant sulfur bacteria (based on concerns of taphonomic bias; Bailey et al., 2007), the Doushantuo fossils contain convincing embryos (Xiao and Knoll, 2000; Xiao et al., 1998; Xiao et al., 2007). Nevertheless, the discovery of possible gastrula embryos from the Doushantuo Formation (Chen and Chi, 2005; Chen et al., 2000; Yin et al., 2001) is hotly debated (Dong, 2009; Xiao et al., 2000). In contrast, embryos, especially the Olivooides type, from the Lower Cambrian Kuanchuanpu Member of the Dengying Formation in Ningqiang County, southern Shaanxi Province are generally accepted as embryonic fossils (Bengtson and Yue, 1997; Hua et al., 2004; Liu et al., 2006; Steiner et al., 2004; Yue and Bengtson, 1999, Dong, 2009; Xiao et al., 2000). In spite of this general acceptance, the gastrulation processes in these embryos are poorly characterized. The extraordinarily well-preserved fossil em- bryos recovered from the Dengying Formation allow us to compare some embryos with the epibolic gastrulation process of extant forms. Gastrulation is the cell migration stage after cleavage, and can be viewed as the embryonic analog of the transition from protozoan to metazoan grades of complexity. It is the initial process in the formation of embryonic germ layers, especially ectoderm and endoderm, on which all subsequent developments eventually depend Gondwana Research 20 (2011) 844–851 ⁎ Corresponding author. Fax: +86 29 88302128. E-mail address: redsmall1982@sina.com (J. Han). 1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.04.003 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr