Feeding currents: a limiting factor for disparity of Palaeozoic fenestrate bryozoans
Abstract The Palaeozoic bryozoan Order Fenestrata is represented almost exclusively by erect unilaminate forms, most of which consist of a fan-shaped, conical or spiral reticulate meshwork. Fewer taxa developed pinnate or branching growth habits, and encrusting or bifoliate colonies occurred only exceptionally. Fenestrate disparity apparently peaked and then declined within the Devonian with the appearance of singular encrusting, bifoliate and large branching-conical morphologies in the Emsian and their decline in diversity in the Eifelian, together with the proliferation and morphological diversification of superstructures lying above the meshwork. In contrast, conventional reticulate and pinnate growth habits show a wider stratigraphical range and geographical distribution. Colony shapes are interpreted in terms of zooid-generated feeding currents; the most common morphologies are inferred to have developed more effective unidirectional feeding currents. The ephemeral Devonian encrusting, bifoliate and branching-conical fenestrates, all of them with superstructures, have features that hindered or prevented unidirectional water flow. Except for the subcolonies of Ernstipora , there is no evidence of chimneys or maculae in fenestrates such as those commonly present in other bryozoan groups with these habits, so it can be concluded that these fenestrate forms did not develop bidirectional currents that would have enhanced the efficiency of feeding activity. Unidirectional feeding currents were universal in Palaeozoic fenestrates; optimization of colony shape for colony-wide currents was a major factor limiting the morphological disparity within this group of bryozoans.