Specific, generic, and familial diversity of Devonian bryozoans

Abstract

Lack of an adequately calibrated Devonian time scale precludes satisfactory comparisons of Recent and Devonian bryozoan diversities. Nevertheless, Devonian bryozoans apparently are 3–100 times less diverse than Recent bryozoans. This variation is a function of how Devonian time is divided (whole period, stages, per million years) as well as the diverse skeletal architecture of cheilostome bryozoans, the most abundant Recent bryozoan order.The Givetian has the largest specific, generic, and familial diversity of any Devonian stage and the drop in bryozoan diversity from the Givetian to the Frasnian is greater than the rise or fall of diversity between any other adjacent stages. Diversity hardly changes among bryozoans across the Frasnian–Famennian boundary. Among bryozoans the Givetian–Frasnian extinction was a major event.Devonian taxonomic diversity is less than nomenclatorial diversity, although both exhibit the same trends from stage to stage during the Devonian. The amount of difference between the two measures of diversity is probably a reflection of the intensity of research on Devonian Bryozoa, especially revisionary studies of previously described faunas. Monographic bursts in diversity are present and are a measure of the episodic character of taxonomic studies on less popular phyla. Examination of the literature commonly provides a finer stratigraphic resolution than indicated in compilations such as the Zoological Record.Although some significant areas of the exposed continents have not received adequate study, Devonian bryozoans show marked geographic changes in diversity that can not be ascribed entirely to lack of study.