The absence of echinoderms from the Lower Cambrian Chengjiang fauna of China: Palaeoecological and palaeogeographical implications

Although deuterostome phylogeny has now achieved a good degree of consensus, echinoderm emergence and early evolution remain highly enigmatic. Different phylogenetic scenarios have emerged based partially on early Cambrian enigmatic animals of the Chengjiang Biota (Yuanshan Formation, Cambrian Series 2, Yunnan, China) interpreted as primitive echinoderms. They include Cambrofengia yunnanensis Hou et al., 1999, Cotyledion tylodes Luo and Hu, 1999, and two vetulocystids (Dianchicystis jianshanensis and Vetulocystis catenata Shu et al., 2004). Their interpretation as echinoderms is here critically revised and C. tylodes, based on newly sampled specimens, is tentatively re-interpreted as a cnidarian polyp. As a result, it appears that the Chengjiang biota, one of the most diverse and earliest recorded communities, is most probably devoid of echinoderm representatives. This is not interpreted as a taphonomic artefact but as a palaeoecological signal of the radiation of echinoderm during the early Cambrian on carbonate substrates. Up to ten exceptional biotas are described from the Yangtze Platform (South China) from Cambrian Stage 2 to (early) Stage 5 in siliciclastic environments varying from shallow platform to basin-slope. The absence of echinoderms in those Burgess Shale-type deposits from Cambrian Stages 2 and 3 along with their report from laterally coeval carbonate sediments argue for a proximal radiation of echinoderms during early Cambrian times in carbonate environments. They would have later colonized siliciclastic proximal and distal environments. The same colonization sequence is generally observed at a global scale, with the first appearance of echinoderms varying in time according to local palaeogeographical controls.