The biological affinity of A msassia : new evidence from the O rdovician of N orth C hina

A msassia shaanxiensis sp. nov. occurs in the M iddle O rdovician part of the J inghe F ormation in Y ongshou and the lower part of the U pper O rdovician B eiguoshan F ormation in L ongxian, S haanxi P rovince, north‐central C hina. In addition to module increase by bipartite longitudinal fission, which is also known in other species of A msassia , tripartite and rare quadripartite fission are recognized in A . shaanxiensis . All species previously assigned to L ichenaria from the M iddle to U pper O rdovician of S haanxi probably belong to A msassia . Therefore, A msassia , rather than the tabulate coral L ichenaria , should be credited as an important contributor to reef‐building in this area. Reports of L ichenaria from elsewhere in the N orth C hina P latform require confirmation in the light of the present study. Some morphological characteristics of A msassia are comparable to those of tabulate corals, tetradiids and chaetetid sponges. Consequently, various authors have assigned A msassia to the L ichenariida, T etradiida (now P rismostylales; florideophycean rhodophyte algae) and C haetetida. Other important characters, however, seem to exclude A msassia from those taxonomic groups. The phacelocerioid organization of modules having separate walls would not be expected in sponges. The basic symmetry of individuals may have been radial, unlike the tetramerous symmetry of tetradiids. Module increase by longitudinal fission, involving infoldings of the wall, is fundamentally different from modes of increase in corals, tetradiids and chaetetids. The skeleton was probably aragonitic, whereas that of tabulates was calcitic. The affinity of A msassia remains unresolved, but it is unlikely to have been a coral, tetradiid or sponge. Perhaps, like the tetradiids, A msassia was an alga.