Distinguishing Earth’s oldest known bryozoan (Pywackia, late Cambrian) from pennatulacean octocorals (Mesozoic–Recent)

Abstract

AbstractBryozoans and all biomineralized metazoan phyla extend back into the Cambrian.PywackiaLanding, 2010 is confirmed as a secondarily phosphatized, late Cambrian stenolaemate bryozoan with colonial habit; mineralized zooarium (originally calcareous); granular/rarely granular-prismatic histology of its trilamellar walls; and polymorphism shown by deep autozooecia with diaphragms and hemiphragms, axial zooecia with diaphragms, and probable nanozooecia. The irregular form ofPywackiareflects growth as a 14-hedron that could not branch and a lack of structures such as thickened walls or styles that maintain regular autozooecial spacing in later stenolaemates.Pywackiais a stem group stenolaemate with a stolon modified into a budding axial zooid and autozooid budding. It is morphologically simpler than the highly evolved late Tremadocian bryozoans of South China with features such as styles, cystiphragms, thickened zooecial walls, and massive or branching colonies. As with some bryozoans,Pywackialacks holdfasts but has lineated living chambers and variably sized autozooecia. The late Cambrian origin of bryozoans, euconodonts, polyplacophorans, and cephalopods set the stage for the Ordovician Radiation’s complex communities.Pywackiais not a pennatulacean octocoral. It lacks both a pennatulacean axial rod histology and a budding zooid that remains confluent with daughter autozooids. Indeed,Pywackiawalled off its axial zooid. Similarity of the 6- and 12-sidedPywackiazooarium with circular to 4-sided pennatulacean axes only includes calcareous composition and the general shapes ofPywackiazooaria and someLituariaaxial rods. The pennatulacean record does not extend from the Mesozoic into the Cambrian, and early cnidarians were not phosphatic. The diagnosis ofPywackiais modified.