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

  • Ed Landing (a1), Jonathan B. Antcliffe (a2), Martin D. Brasier (a3) and Adam B. English (a4)

Abstract

Bryozoans and all biomineralized metazoan phyla extend back into the Cambrian. Pywackia Landing, 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 of Pywackia reflects 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. Pywackia is 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, Pywackia lacks 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. Pywackia is not a pennatulacean octocoral. It lacks both a pennatulacean axial rod histology and a budding zooid that remains confluent with daughter autozooids. Indeed, 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. The pennatulacean record does not extend from the Mesozoic into the Cambrian, and early cnidarians were not phosphatic. The diagnosis of Pywackia is modified.

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Deceased 16 December 2014

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