HOMOLOGOUS SHELL MICROSTRUCTURES INCAMBRIAN HYOLITHS AND MOLLUSCS

Hyoliths were among the earliest biomineralizingmetazoans in Palaeozoic marine environments. They havebeen known for two centuries and widely assigned tolophotrochozoans. However, their origin and relationshipswith modern lophotrochozoan clades have been a longstand-ing palaeontological controversy. Here, we provide broadmicrostructural data from hyolith conchs and opercula fromthe lower Cambrian Xinji Formation of North China, includ-ing two hyolithid genera and four orthothecid genera as wellas unidentified opercula. Results show that most hyolithconchs contain a distinct aragonitic lamellar layer that is com-posed of foliated aragonite, except in the orthothecid Newtaxon 1 that has a crossed foliated lamellar microstructure.Opercula are mostly composed of foliated aragonite andoccasionally foliated calcite. These blade or lath-likemicrostructural fabrics coincide well with biomineralizationof Cambrian molluscs rather than lophophorates, as exempli-fied by the Cambrian members of the tommotiid-brachiopodlinage. Accordingly, we propose that hyoliths and molluscsmight have inherited their biomineralized skeletons from anon-mineralized or weakly mineralized common ancestorrather than as a result of convergence. Consequently, fromthe view of biomineralization, the homologous shellmicrostructures in Cambrian hyoliths and molluscs stronglystrengthen the phylogenetic links between the two groups