The early Cambrian Saccorhytus is a non-feeding larva of a scalidophoran worm.

Abstract

With its bag-like appearance, spiny ornament and single opening, Saccorhytus coronarius is one of the most enigmatic animals of the early Cambrian Kuanchuanpu Formation (ca 535 Ma) and has been at the heart of debates concerning the origin of two major animal lineages: the deuterostomes and the ecdysozoans. Although Ecdysozoa is now the most plausible option, key aspects of its palaeobiology have remained elusive. I suggest here that Saccorhytus was the possible larval stage of one of the numerous scalidophoran worms that co-existed with it in the Kuanchuanpu biota, especially those with bilateral pairs of broad-based sclerites such as Eokinorhynchus. This new interpretation is mainly based on the development of extant priapulid worms (Scalidophora). It would imply that Saccorhytus is a non-feeding larva and that its enigmatic single aperture with a pleated structure is not a mouth but more likely the opening through which the juvenile exited. Thus, the strange anatomy of Saccorhytus could be explained in a simple way without involving complex evolutionary processes (e.g. simplification, miniaturization, loss of anus, etc.). Instead of being a relatively basal member of the total group Ecdysozoa, Saccorhytus would find a more likely position among the total group Scalidophora.