Three-dimensionally preserved digestive systems of two Cambrian hyolithides (Hyolitha)

Hyoliths are a group of Paleozoic marine bilaterian animals with conical aragonitic shells. Evolution of the hyoliths, and their relationships to other early bilaterians, are controversial. New exceptionally preserved hyolithide hyolith specimens from Cambrian Stage 4 strata of Nevada, USA, and Guizhou, China, provide information on the internal soft-part anatomy that is pertinent to understanding the early evolutionary history of the group. Two specimens show partial digestive tracts preserved in three dimensions. The guts comprise a modified U-shaped system consisting of a spiral intestine wound about a nearly straight tube. Similarity to the spiral U-shaped gut tract of sipunculans supports an inferred sister-group relationship between sipunculans and hyoliths.