Paper No. 60-2
Presentation Time: 1:15 PM
EARTH’S OLDEST KNOWN BRYOZOAN (PYWACKIA, LATE CAMBRIAN) AND THE CAMBRIAN DIVERSIFICATION OF BIOMINERALIZED METAZOA
Restudy of Pywackia Landing, 2010, confirms it to be a secondarily phosphatized, late Cambrian stenolaemate bryozoan with colonial habit; originally calcareous zooarium; granular/rarely granular-prismatic histology of its trilamellar walls; and polymorphism shown by deep autozooecia with diaphrams and hemiphragms, axial zooecia with diaphragms, and probable nanozooecia. Thus, bryozoans and all biomineralized metazoan phyla extend back into the Cambrian. The irregular form of Pywackia reflects growth as a14-hedron that could not branch and the absence of structures, as thickened walls or styles, that maintain regular autozooecial spacing in later stenolaemates. Pywackia, a stem group stenolaemate with a stolon modified into a budding axial zooid and autozooid budding, is morphologically simpler than the highly evolved late Tremadocian bryozoans of South China which have styles, cystiphragms, thickened zooecial walls, and massive or branching colonies. Pywackia lacks holdfasts and has lineated living chambers and variably sized autozooecia as some bryozoans. The late Cambrian origin of bryozoans, euconodonts, polyplacophorans, and cephalopods set the stage for more complex communities of the Ordovician radiation. Pywackia is not a pennatulacean octocoral. It lacks the pennatulacean axial rod’s “wood fiber” histology. In addition, pennatulacean budding zooids remains confluent with daughter autozooids; by contrast, Pywackia walled off its axial zooid. Similarity of the 6- and 12-sided Pywackia zooarium with circular to 4-sided pennatulacean axes only includes calcareous composition and the general shapes of Pywackia zooaria and some Lituaria axial rods. Pennatulaceans do not extend from the Mesozoic back into the Cambrian, and early cnidarians were not phosphatic.