AbstractA study of the Upper Ordovician trepostomate bryozoans belonging to the families Amplexoporidae and Monticuliporidae, from the eastern Anti-Atlas of Morocco, is presented here. They occur in the marly to fine-grained limestone, intermediate unit of the Khabt-el-Hajar Formation, late Katian in age, representing outer-ramp depositional environments. They inhabited the highest paleolatitude known for a bryozoan fauna during the Ordovician, estimated at more than 65–70ºS. A total of 11 species of the generaAnaphragma,Atactoporella,Homotrypa,Monotrypa,Monticulipora, andPrasoporaare described. Three species are already known from the equatorial-tropical paleocontinents of Baltica, Laurentia, and Siberia:Anaphragma mirabile,Monotrypa jewensis, andPrasopora falesi. Four new taxa are described:Anaphragma undulata,Atactoporella moroccoensis,Monticulipora globulata, andMonticulipora irregularis.The two species ofAnaphragmaand the one ofAtactoporelladisplay significantly larger zoarial sizes than congeneric species, representing gigantism among bryozoans. Polar gigantism is rejected for the two species ofAnaphragmaas is gigantism related to photosynthetic endosymbionts. An alternative proposal for their giant size is their long zoarial life span due to their well-balanced, robust branching form, with a relatively wide basal supporting surface, adapted to unconsolidated substrates in environments below wave base. Their great stability in outer-ramp environments, with infrequent storms, would allow the zoaria to grow for an extended time and reach large sizes before being overturned and buried.Atactoporella moroccoensis, has both zoaria and zooecia gigantic, suggesting a hypothesis of polar gigantism.
