1 New insight into the soft anatomy and shell microstructures of early Cambrian 2 orthothecids (Hyolitha) 3 4 Luoyang Li1,2, Christian B. Skovsted1,2, Hao Yun2, Marissa J. Betts2,3, Xingliang 5 Zhang2, 6 7 1Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, SE- 8 104 05 Stockholm, Sweden. 9 2State Key Laboratory of Continental Dynamics, Shaanxi Key laboratory of Early 10 Life and Environments, Department of Geology, Northwest University, Xi’an 710069, 11 PR China. 12 3Palaeoscience Research Centre, School of Environmental and Rural Science, 13 University of New England, Armidale, NSW, Australia, 2351 14 E-mail for correspondence:
[email protected] 15 16 Abstract 17 Hyoliths (hyolithids and orthothecids) were one of the most successful early 18 biomineralizing lophotrochozoans, and were a key component of the Cambrian 19 evolutionary fauna. However, the morphology, skeletogenesis and anatomy of earliest 20 members of this enigmatic clade, as well as its relationship with other 21 lophotrochozoan phyla remain highly contentious. Here we present a new orthothecid, 22 Longxiantheca mira gen. et sp. nov. preserved as part of the secondarily phosphatized 23 Small Shelly Fossil assemblage from the lower Cambrian Xinji Formation of North 24 China. Longxiantheca mira retains some ancestral traits of the clade with an 25 undifferentiated disc-shaped operculum and a simple conical conch with a two- 26 layered microstructure of aragonitic fibrous bundles. The operculum interior exhibits 27 impressions of soft tissues, including muscle attachment scars, mantle epithelial cells 28 and a central kidney-shaped platform in association with its feeding organ. Our study 29 reveals that the muscular system and tentaculate feeding apparatus in orthothecids 30 appear to be similar to that in hyolithids, suggesting a consistent anatomical 31 configuration among the total group of hyoliths.