Space-filling problems in ramose trepostome bryozoans as exemplified in a giant colony from the Permian of Greenland

In order to maintain branch strength and a confluent outer membrane, trepostome bryozoans had to maintain a continuous colony surface without any structural gaps. This put great constructional demands on colonies with relatively thick exozones to fill the exozonal space while preserving a suitable autozooecial spacing for colony-wide feeding currents. This situation was magnified in a giant colony of the trepostome Tabulipora from the Early Permian Kim Fjelde Fm. in eastern North Greenland. This single branch colony fragment had a diameter of 37.5 mm. A block was cut out of the 8- mm thick exozone, and 20 serial tangential peels were made at varying distances from the endozone. Exilazooecial and autozooecial chamber cross-sectional area, packing, spacing, and wall thickness were measured in the maculae and intermacular areas. Results indicate that, in this colony, volumetric space in the exozone was occupied by budding new exilazooecia in the maculae and by exozonal budding: budding of new exilazooecia in the intermacular areas that transform into autozooecia. Exilazooecia played a dominantly space-filling role in the maculae as well as helped to maintain regular spacing of autozooecia in the intermacular areas.