Institute of Geology, Adam Mickiewicz University, Poznań
Abstract: The formation of Pangea resulted in an unique global geography, which in turn influenced the distribution, development and extinction pattern of the Rugosa. The mid-Carboniferous free connection between the main oceans allowed common roots to develop for several Permian rugose coral lineages. Permian development within those lineages was independent, but their presence obscured the distinction between the Rugosa living near eastern and western sides of Pangea.
Although four families (Kepingophyllidae-Waagenophyllidae in the East and Durhaminidae-Heritschioididae in the West) appeared in the Middle Carboniferous, they were permanently restricted geographically to become characteristic for two distinct faunal realms which appeared near the end of the Carboniferous as a result of the formation of Pangea. These Realms were: 1. The Cordilleran-Arctic-Uralian Realm, extending from the Ural Mountains along the northern and western shelves of Pangea and including carbonate platforms (future American accreted terranes), located not far off shore in the Palaeopacific Ocean. 2. The Tethys Realm, extending westward from China and Indochina to Tunesia. It includes several accreted terranes as well, with the Japanese terranes best known. The history of development and extinction of rugose coral faunas in the two realms was almost totally different in timing and faunal content.
The Asselian-Artinskiam was the most significant period in the Cordilleran-Arctic-Uralian Realm. The Durhaminidae-Heritschioididae faunas were restricted to that interval. They disappeared first from the North, possibly as a result of the northward movement of Pangea. The non-dissepimental solitary Rugosa that continued to develop in the realm until the Capitanian or early Wuchiapingian belong to old cosmopolitan families and genera. In the Wordian and early Wuchiapingian, there were rare, exotic Tethyan genera in some accreted North American terranes. It is postulated that the appearance of carbonate platforms far off shore was necessary to allow larvae of some Tethyan taxa to cross the Palaeopacific Ocean. That dispersal opportunity was too late for the Durhaminidae-Heritschioididae, which were already extinct. Thus, true exotic faunas are absent from the Asiatic accreted terranes. No corals are known to occur in the Cordilleran-Arctic-Uralian Realm above the Early Wuchiapingian.
Two periods during which the rugose coral faunas fluorished - i. e., the Asselian and the Chihsian-Maokouan - occurred in the Tethys Realm, with the second constituting a peak in the development. A rapid but sequential decrease, which began in the Late Maokouan, led to the total extinction of the Rugosa before the end of the Changhsingian.
Except for the Asselian, which is characterised by mixed Carboniferous and Permian faunas, the periods of increase and decrease in rugose corals development in both realms do not coincide. Also, they do not exactly follow global eustatic sea level changes. The latter deviation is most pronounced in the Cordilleran-Arctic-Uralian Realm.
The disappearance of the Rugosa from individual areas and their final extinction from both realms were highly diachronous. Unfavourable environmental conditions can in many instances be identified as a direct factor causing their disappearance. This applies most clearly to earlier periods but was less significant during the last steps of the extinction. Although continuously important, the environmental factor was not the only important one at that time. The following factors were also in effect during the final developmental steps of the Rugosa and eventually caused their extinction: patchy distribution of distinctly isolated faunas, restriction of shelf areas available for the Rugosa to inhabit, changes in coastlines and current directions, resulting from a drastic drop in sea level, a continuously decreasing number of populations and specimens within individual species, drasticly restricting their gene pools (i. e., the possibility to properly respond to changing environmental conditions), insufficient production of larvae to colonize and/or re-colonize areas offering favourable conditions. The combination of all these factors eventually resulted in the extinction of the Rugosa independently and diachronously from the two Realms.
Key words: Permian - Rugosa - distribution - development - extinction
