An early Cambrian ecdysozoan with a terminal mouth but no anus

The ecdysozoans are the most diverse animal group on Earth1, 2. Molecular clock studies indicate that the ecdysozoans may have diverged and diversified in the Ediacaran Period3, 4, but unambiguous ecdysozoan fossils first appear in the earliest Cambrian and are limited to cycloneuralians5-7. Here we report new material of the early Cambrian microscopic animal Saccorhytus coronarius, which was previously interpreted as a deuterostome8. Saccorhytus coronarius is reconstructed as a millimetric and ellipsoidal meiobenthic animal with a spinose armor and an anterior mouth but no anus. Purported pharyngeal gills in support of the deuterostome hypothesis8 are shown to be taphonomic artifacts. Phylogenetic analyses indicate that Saccorhytus coronarius belongs to the total-group Ecdysozoa, highlighting the morphological and ecological diversity of early Cambrian ecdysozoans.