The biological affinity of Amsassia: new evidence from the Ordovician of North China



Amsassia shaanxiensis sp. nov. occurs in the Middle Ordovician part of the Jinghe Formation in Yongshou and the lower part of the Upper Ordovician Beiguoshan Formation in Longxian, Shaanxi Province, north-central China. In addition to module increase by bipartite longitudinal fission, which is also known in other species of Amsassia, tripartite and rare quadripartite fission are recognized in A. shaanxiensis. All species previously assigned to Lichenaria from the Middle to Upper Ordovician of Shaanxi probably belong to Amsassia. Therefore, Amsassia, rather than the tabulate coral Lichenaria, should be credited as an important contributor to reef-building in this area. Reports of Lichenaria from elsewhere in the North China Platform require confirmation in the light of the present study. Some morphological characteristics of Amsassia are comparable to those of tabulate corals, tetradiids and chaetetid sponges. Consequently, various authors have assigned Amsassia to the Lichenariida, Tetradiida (now Prismostylales; florideophycean rhodophyte algae) and Chaetetida. Other important characters, however, seem to exclude Amsassia 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 Amsassia remains unresolved, but it is unlikely to have been a coral, tetradiid or sponge. Perhaps, like the tetradiids, Amsassia was an alga.