The first appearance of animal skeletons is one of the most significant events in the history of life. Knowledge of how early animals constructed their skeletons is generally poor, largely because taphonomic pathways obscure details of skeletal secretory mechanisms. Here, we report exceptionally preserved organic templates from the sclerites of chancelloriids, an enigmatic and widespread organism in the Cambrian. The organic templates form the mineralized layer of the sclerite and bear a striking resemblance, in both size and morphology, to the organic templates that control biomineralization in modern molluscs. Based on the similarities to modern molluscs, we postulate that the mineralized sclerites of chancelloriids were secreted entirely by epithelial cells within the organic template, reflecting a mechanism known to be used by a wide range of eumetazoans. This new awareness of chancelloriid microstructure provides further evidence that the close affinity between chancelloriids and poriferans should be rejected and that chancelloriids should be assigned to a eumetazoan clade.