A new bryozoan genus from the Jurassic of Switzerland, with a review of the cribrate colony-form in bryozoans

Abstract

Very few Jurassic bryozoans have been recorded from Switzerland. The discovery in the collections of the Universitat Zurich of a very distinctive cyclostome bryozoan from the Aalenian of Gelterkinden in the Basel-Country Canton, warrants the creation of a new monospecific genus, Rorypora gen nov. The type species of Rorypora, Diastopora retiformis Haime, 1854, was originally described from the French Bajocian. This genus has a ‘cribrate’ colony-form comprising flattened bifoliate fronds that bifurcate and coalesce regularly to enclose ovoidal lacunae. Unlike the much commoner fenestrate colony-form, apertures of feeding zooids open on both sides of the fronds. The taxonomic distribution of cribrate colonies in bryozoans is reviewed. They are most common in Palaeozoic ptilodictyine cryptostomes but are also found among Palaeozoic cystoporates and Cenozoic cheilostomes, as well as the single known cyclostome example described here. Cribrate colonies therefore provide a striking example of multiple evolutionary convergence. In contrast to fenestrate colony-forms, feeding currents cannot be channelled through the lacunae in cribrate colonies to generate a one-way flow of water for efficient suspension feeding. Instead, the holes are inferred to be the locations of chimneys over which exhalent currents are vented in opposite directions on either side of the planar colony.