An extinct clade sister to Eumetazoa: On the phylogeny of the Cambrian chancelloriids

The notable disparity of animal body plans can be traced back to the morphological innovations during the Cambrian explosion and represented by a number of exceptionally preserved soft-bodied and skeletal fossils that provide a compelling narrative for animal evolution. Chancelloriids, one of the extinct groups of Cambrian animals, have a distinctive body that characterized by a sclerite-bearing, flexible integument and a single apical opening leading into a central cavity devoid of unequivocal internal organs. Their phylogenetic position within the Metazoa, however, is controversial. Here, we describe new soft-bodied fossils of chancelloriids from the 518-million-year-old Chengjiang biota of China, which corroborate the unique bauplan pattern and reveal exquisite integument microstructures. The tiny protuberances and wrinkling structures of the integument are interpreted to be related to primitive epithelial contraction, suggesting that chancelloriids were a group of basal epitheliozoans and constitute an evolutionary clade that branched below all extant eumetazoans while above or close to the placozoans. Thus, the chancelloriid body plan likely filled one of the anatomical gaps between the Placozoa and the Eumetazoa.