Revision of Triplicatella (Orthothecida, Hyolitha) with preserved digestive tracts from the early Cambrian Chengjiang Lagerstätte, South China

The cap-shaped shells of Triplicatella are known almost exclusively from small shelly fossil assemblages with articulated specimens showing unequivocally that they represent the operculum of a hyolith. Abundant specimens of Triplicatella opimus from the fine-grained shales of the Chengjiang Lagerstätte of South China with soft-part preservation are documented herein. The soft tissues, including the feeding apparatus and complex digestive system, in T. opimus strongly suggest that Triplicatella was a deposit feeder. The digestive tract of T. opimus consists of two limbs, a spiral loop folded into a chevron-like structure and a slightly recurved to straight anal tube, which are preserved as reddish-black traces enriched in iron. The new anatomical information obtained from T. opimus in the Chengjiang Biota suggests an intermediate stage in the development of the characteristic folded gut of orthothecid hyoliths. The new anatomical information reported here shows that Triplicatella is one of the best-preserved early members of the Orthothecida and promotes our understanding of the general anatomy and evolution of the Hyolitha.