The concepts of astogeny and ontogeny in stenolaemate bryozoans, and their illustration in colonies of Tabulipora carbonaria from the Lower Permian of Kansas

Abstract

Recognition of ontogeny within a stenolaemate bryozoan colony requires separating the individualistic aspects of a zooid's growth from those of its neighbors. The ancestrula is the only zooid within a colony that always displays a partially independent ontogeny that ceases when it starts to experience shared changes with its neighboring daughter zooids during subsequent accretionary skeletal growth.Stenolaemate astogeny (shared changes across multiple zooids during the growth of both the ancestrular zooid and its asexual descendants) includes all coordinated changes in the size, shape, number, and calcification of autozooids, polymorphs, and extrazooidal structures, as well as changes within autozooids or polymorphs, such as the formation of basal diaphragms and brown bodies. Despite the fact that many of these directional changes occur within individual zooids, they are not part of ontogeny because they are taking place simultaneously across zooids that share a common skeleton, extrazooidal tissues, and pseudocoelomic spaces.Shared directional multizooidal changes occurring during colony growth provide a confirmatory test for the existence of astogeny. Astogeny was statistically evaluated in 6–15 characters measured within the exozones of four colonies of Tabulipora carbonaria (Worthen in Worthen and Meek, 1875). Statistically significant (at P ≤ 0.05) directional changes took place across growth stages within the exozone in the following morphometric characters: zooecial density, zooecial wall surface area, acanthostyle density, zooecial wall thicknesses, maximum acanthostyle diameters, and intrazooecial diaphragm abundances. Overall, earlier exozonal growth stages differ statistically from those of the later exozone, with characteristics of intermediate growth stages intergrading between the two. Discriminant function analysis segregated intervals of exozonal growth into early-, intermediate-, and late-stage clusters, confirming patterns delineated by univariate statistical tests.Based on these exozonal growth patterns, heterochronic changes in exozone astogeny characterized evolution within and across species of Tabulipora. Onshore populations of T. carbonaria were astogenetically progenetic relative to offshore ones along an environmental gradient across Kansas, whereas local populations became temporally more hypermorphic in a short-term stratigraphic succession of similar environments. Tabulipora carbonaria originated by astogenetic recapitulation in populations of its probable ancestor, T. ramosa. Therefore, speciation, microevolution, and clinal variation in Tabulipora all involved heterochronic modifications of exozone astogeny.