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Abstract The Cambrian Explosion saw the widespread development of mineralized skeletons. At this time, nearly every major animal phylum independently evolved strategies to build skeletons through either agglutination or biomineralization. Although most organisms settled on a single strategy,SalterellaBillings, 1865 employed both strategies by secreting a biocalcitic exterior shell that is lined with layers of agglutinated sediments surrounding a central hollow tube. The slightly older fossil,VolborthellaSchmidt, 1888, shares a similar construction with agglutinated grains encompassing a central tube but lacks a biomineralized exterior shell. Together these fossils have been grouped in the phylum Agmata Yochelson, 1977, although no phylogenetic relationship has been suggested to link them with the broader metazoan tree, which limits their contribution to our understanding of the evolution of shells in early animals. To understand their ecology and place them in a phylogenetic context, we investigatedSalterellaandVolborthellafossils from the Wood Canyon and Harkless formations of Nevada, USA, the Illtyd Formation of Yukon, Canada, and the Shady Formation of Virginia, USA. Thin-section petrography, acid maceration, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray tomographic microscopy were used to provide new insights into these enigmatic faunas. First, morphological similarities in the aperture divergence angle and ratio of central tube diameter to agglutinated layer thickness suggestSalterellaandVolborthellaare related. Second, both fossils exhibit agglutinated grain compositions that are distinctive from their surrounding environments and demonstrate selectivity on the part of their producers. Finally, the calcitic shell composition and simple layers of blocky prismatic shell microstructure inSalterellasuggest a possible cnidarian affinity. Together these data point to these organisms being sessile, semi-infaunal filter or deposit feeders and an early experimentation in cnidarian biomineralization chronicling a hypothesized transition from an organic sheath inVolborthellato a biomineralized shell inSalterella. 

Archaeocyaths are biocalcified sponges largely restricted to the early Cambrian Period. Their perforated cup-shaped body facilitated filter feeding. Many of them were clonal modular animals that formed the earliest metazoan skeletal reefs. In Siberia, archaeocyaths extend from late Age 2 to Age 4 of the early Cambrian, representing an ∼15 m.y. range (ca. 525−510 Ma). Elsewhere, archaeocyaths emerged later than in Siberia and, in places, survived to the middle-late Cambrian. The existing fossil record thus indicates an out-of-Siberia scenario and delayed biomineralization in archaeocyaths relative to many other animals, which acquired biomineralization in the Fortunian Age of the early Cambrian. Here we report two microscopic archaeocyath species—Primocyathus uniseriatus Wang and Xiao, gen. et sp. nov. and Sinocyathus biseriatus Wang and Xiao, gen. et sp. nov.—from the Fortunian Kuanchuanpu Formation (ca. 533 Ma) in South China. Preserved as phosphatized internal molds, they are interpreted to have had a biomineralized, two-walled, perforated, cup-shaped skeleton. They were likely filter feeders, but their solitary habit and millimetric body size indicate that they were unlikely reef framework builders. They substantially extend the stratigraphic range of archaeocyaths, challenge the out-of-Siberia hypothesis, support archaeocyath biomineralization in the beginning of the Cambrian explosion, and imply a Precambrian divergence of sponge classes.