Hyolithid-like hyoliths without helens from the early Cambrian of South China, and their implications for the evolution of hyoliths

Background: A small hyolith, with a triangular operculum and a conical-pyramidal conch with a sharp apex, originally documented as Ambrolinevitus ventricosus, is revised based on new material from the Chengjiang biota. The operculum of ‘Ambrolinevitus’ ventricosus displays strong morphological similarities with the operculum of Paramicrocornus from the Shuijingtuo Formation (Cambrian Series 2), indicating that the species should be reassigned to Paramicrocornus. Results: Based on the unusual morphology of Paramicrocornus, we herein propose a new family Paramicrocornidae fam. nov. A cladistic analysis of Cambrian and Ordovician hyoliths clearly delineates hyolithids as a monophyletic group which evolved from the paraphyletic orthothecids in the early Cambrian and with Paramicrocornidae as its closest relative. Conclusions: The phylogenetic analysis, together with the distribution of hyoliths from the Cambrian to the Ordovician, reveals the presumptive evolution model of both the skeleton and soft-part anatomy of hyoliths. The Family Paramicrocornidae plays an intermediate role in hyolith evolution, representing the transitional stage in the evolution from orthothecids to hyolithids.This work was supported from the National Natural Science Foundation of China [Grants 41720104002, 41890844, and 41621003 to ZZF, 41772002 to CBS], the Strategic Priority Research Program of the Chinese Academy of Sciences to the Early Life Institute [XDB26000000] and Overseas Expertise Introduction Centre for Discipline Innovation [111 Center: D17013] for the continuous fossil collections of the Xi’an group are sincerely acknowledged. This work was also funded by the Swedish Research Council to CBS [VR2016-04610] and TPT [VR2017-05183] and Young Thousand Talents Plan of China and National Science Foundation of China grant to TPT [42072003]. ZZF acknowledges the Changjiang Scholars [T2016155 to ZZF], Department of science and technology of Shaanxi Province [2022TD-11] and Wanrenjihua programmes [W03020685 to ZZF] for continuous supports for his laboratory and field works.