First report of a Palaeozoic fenestrate bryozoan with an articulated growth habit

Abstract

The main goal of this paper is the description of the erect articulated growth habit for the first time within the Palaeozoic Order Fenestrata, and the interpretation of its occurrence within the evolutionary history of colonial morphology in fenestrates. The present paper is based on a single specimen composed of fifteen disarticulated internodes. The specimen was extracted from the outcrop as two minute slabs, one of which was embedded in a two-component epoxy resin for preparation of acetate peels and the other one preserved without subsequent preparation for observation of exterior features. A new Devonian fenestrate bryozoan genus, Arnaopora, and its type species, A. sotoi, are described from the Emsian-Eifelian Moniello Formation (Asturias, NW Spain). Zooecial characters as well as the development of articulated colonies support the definition of this new genus, which is the first reported representative with an articulated growth habit in the Order Fenestrata, almost exclusively composed of rigidly erect unilaminar forms. The occurrence of Arnaopora in the early Eifelian coincides with a peak of disparity in fenestrate bryozoan growth forms during the transition between the Early and Middle Devonian. While all previously known morphological variations of growth forms in Palaeozoic fenestrates derive from the basic reticulate habit, articulation evolved from a rigidly branching phylloporinid ancestor. Arnaopora is extremely rare within the abundant and diverse bryozoan fauna of shallow platform facies where it occurs, which is in contrast with the relatively common presence of cellariform colonies in assemblages from modern shallow environments.