Cambrian origin of all skeletalized metazoan phyla—Discovery of Earth's oldest bryozoans (Upper Cambrian, southern Mexico)

Abstract

Exquisite Pywackia baileyi Landing n. gen. and sp. specimens from the lower Tinu Formation, southern Mexico, extend the bryozoan record into the Upper Cambrian. They are ∼8 m.y. older than the purported oldest bryozoans from South China, and show that all skeletalized metazoan phyla appeared in the Cambrian. The new form differs from similar, twig-like cryptostomes by its shallow autozooecia and an elongate axial zooid, which may be homologous to the stolon in nonmineralized ctenostomes. It may morphologically resemble mineralized stem group bryozoans that retained a stolon-like individual, although an ability to bud was acquired by the feeding individuals (autozooids). The latest Cambrian origin of bryozoans, several mollusk classes (polyplacophorans, cephalopods), and euconodonts was a major evolutionary development and can be considered the onset of the Ordovician radiation of more complex marine communities.