Saccorhytus is an early ecdysozoan and not the earliest deuterostome

The early history of deuterostomes, the group composed of the chordates, echinoderms and hemichordates(1), is still controversial, not least because of a paucity of stem representatives of these clades(2-5). The early Cambrian microscopic animal Saccorhytus coronarius was interpreted as an early deuterostome on the basis of purported pharyngeal openings, providing evidence for a meiofaunal ancestry(6) and an explanation for the temporal mismatch between palaeontological and molecular clocktimescales of animal evolution(6-8). Here we report new material of S. coronarius, which is reconstructed as a millimetric and ellipsoidal meiobenthic animal with spinose armour and a terminal mouth but no anus. Purported pharyngeal openings in support of the deuterostome hypothesis(6) are shown to be taphonomic artefacts. Phylogenetic analyses indicate that S. coronarius belongs to total-group Ecdysozoa, expanding the morphological disparity and ecological diversity of early Cambrian ecdysozoans.

Automated summary

The early history of deuterostomes, composed of chordates, echinoderms, and hemichordates, is still controversial due to a lack of representative fossils. The microscopic animal Saccorhytus coronarius was previously thought to be an early deuterostome, but new material and analyses have shown that it actually belongs to the total group Ecdysozoa. This expands the morphological and ecological diversity of early Cambrian ecdysozoans.