Late Cambrian Pywackia is a cnidarian, not a bryozoan: Insights from skeletal microstructure

Abstract

The phylum Bryozoa had long been the only major phylum unknown from the Cambrian and by inference the Cambrian Explosion of biodiversity. When described in 2010 as a late Cambrian cryptostome bryozoan, Pywackia baileyi Landing in Landing et al., 2010 became the oldest known bryozoan (early Cambrian bryozoans have since been described). Controversy remains about the phylum-level identification of Pywackia Landing in Landing et al., 2010—one study proposed an interpretation of Pywackia as an octocoral. No previous studies of the skeletal microstructure of Pywackia have employed the analysis of petrographic thin sections and high-magnification scanning electron microscopy. These two methods, with the addition of data from previous studies, are employed in this analysis of skeletal microstructure, a feature often important for higher-level taxonomic identification. Although many candidate groups were considered, Pywackia's distinctive pillar and laminae, porous skeleton like many Cnidaria, topology of the body walls, and growth of modules are consistent with a cnidarian affinity. Pywackia skeletons with primary microstructure were 100% phosphate mineral and were collected from a setting of pervasive phosphatic replacement, which leaves uncertainty as to the original skeletal composition. Pywackia is not assigned here to a cnidarian class and likely represents an early, rare, short-lived cnidarian evolutionary group.