Bryozoan astogeny and evolutionary novelties: their role in the origin and systematics of the Ordovician monticuliporid trepostome genus Peronopora

Abstract

Modifications of primitive astogenetic patterns were central to the origin of bifoliate Peronopora. Outgroup comparison with Prasopora indicates that fundamental events in the former's origin included vertical growth of basal lamina skeleton to form the median lamina and heterochronic modifications of early and later-stage astogeny. Heterochronic modifications of early astogeny included acceleration of budding rates in the ancestrular disc, disc enlargement, and reduction of the basal expansion. Later-stage heterochronic modifications included reduction of zooecial length and width of “endozonal” growth and parallel orientation of cystiphragms about maculae. Also important were “2-D” budding in longitudinal ranges and a unique mode of secondary frond formation. Paedomorphosis resulted in constraints on zoarial form, autozooecial morphology, cystiphragm patterning, and increased colonial integration. Coordination of early and later-stage astogenetic events suggests developmental integration linked to median lamina formation. Heterochronic modifications are inferred to have been products of spatial competition in early astogeny and competitive avoidance and resource exploitation in later astogeny. Restricted biogeographic distribution and characteristics of the ancestrula suggest that the larvae of bifoliate Peronopora were nonplanktotrophic.A shared derived suite of characters including the median lamina unite the bifoliate Peronopora clade. Results of cladistic analyses indicate that bifoliate Peronopora comprise a cohesive, statistically significant clade. Character analysis, phylogenetic results, and the restricted biogeographic distribution of bifoliate species support the hypothesis of a monophyletic bifoliate Peronopora clade of generic rank. The generic concept of Peronopora is revised and limited to bifoliate species.