Carboniferous Rugosa in the Hina Limestone,Akiyoshi Terrane, Southwest Japan: fauna endemic to the Panthalassan Ocean

摘要

Rugose corals of the Akiyoshi Terrane in the Inner Zone of Southwest Japan are notable for two reasons, their role in reef construction and their highly endemic nature. Until now, little has been known about the faunal characteristics of the Rugosa-rich Hina Limestone (early Visean to late Bashkirian) in the Akiyoshi Terrane. The early Visean non-dissepimented solitary corals are replaced in the middle to late Visean by large, dissepimented solitary corals. These solitary faunas are followed by dominantly pseudopavonid corals of variable form. During the late Visean warmwater transgression, the Akiyoshi fauna contrasted markedly with Tethyan faunas (e.g. the Kueichouphyllum fauna), forming a characteristic Akiyoshian faunal type. Ozakiphyllum, Pseudopavona, and Omiphyllum exhibit their close phylogenetic continuity through intergradational morphology. Abundant new species of these genera are recognized in the Hina Limestone, and will help clarify genus-level phylogenetic relationships. Although Hiroshimaphyllum may be ancestral to this clade, its solitary representatives are not yet well known. Of special interest is the presence of the Australian "Orionastraea", which perhaps is separable from European Orionastraea. Phylogenetically close genera ("sister-groups") have been noted in Eastern Australia and the Akiyoshi Terrane. However, no pseudopavonid genera have yet been interpreted as exhibiting true "sister-group" relationships with any genera outside of the Japanese Islands. If true "sister-group" relationships are recognized within pseudopavonids and non-pseudopavonids, phylogenetic problems would arise as to the monophyly of the family Pseudopavonidae. More importantly, invaluable clues about the origins of the Akiyoshi fauna can be obtained by combining palaeogeographic and phylogenetic analyses.