faunal provinces

faunal provinces Faunal provinces are large geographical areas within which the organisms, even perhaps up to the family level, have a distinct identity. Provinces may be separated by sharp or gradational boundries, and in some cases the fauna may be endemic, that is, not found outside a particular province. As marine invertebrates are the commonest fossils, most interest in past faunal provinces concentrates on the marine realm and particularly on the continental shelves. The main modern zoogeographical regions are controlled particularly by temperature and, therefore, by latitude so that we can delineate tropical shelf, warm temperate, cold temperate, and polar regions. Within these areas there are further subdivisions termed realms whose boundaries are controlled by the presence of warm or cold currents or the position of land masses. Thus the Isthmus of Panama separates two different tropical shelf faunas, so that of the 517 species on the Caribbean side and the 805 species on the Pacific side only 24 are held in common. The cold Humboldt current that sweeps up the western side of South America likewise restricts the tropical fauna to a few degrees south of the equator.

Similar faunal provinces can be detected in the fossil record and can help to corroborate ideas about continental movements through time, as well as providing information about palaeotemperatures and even the pattern of ancient oceanic current systems. The distribution of rugose corals in the Devonian of North America has been described in terms of faunal realms. During the Early Devonian two realms were separated by a ridge extending through the centre of the continent. Through the Middle and into the Late Devonian, however, a steady mixing of the faunas showed that the influence of the barrier was waning until it became inundated towards the end of the Devonian. Separation and amalgamation of faunal realms has been instrumental in tracing the movements of Gondwana (southern hemisphere) continents during the Late Palaeozoic and early Mesozoic. The mammal-like reptile Lystrosaurus is found in Antarctica, India, and South Africa but clearly could not have travelled across oceans to establish such a range. In the nineteenth century hypothetical land bridges were postulated to explain distributions such as this that we now know were caused by plate motions. The opening and closing of ocean basins can be timed by the increasing similarity or dissimilarity of faunas on the opposing coastlines. In the early Palaeozoic, North America and Europe were separated by the Iapetus ocean. During the Ordovician, only planktonic animals such as graptolites were able to establish themselves on both sides; during the Silurian benthonic organisms with swimming larvae such as brachiopods were able to bridge the gap; and by the Devonian the northern part of the ocean had closed and even freshwater fish occurred in both faunas. Accreted terranes, that is, portions of present continents that have come from other former continents or oceans, can also be recognized because they contain fossils from different faunal provinces. For instance, the Avalon terrane, which occurs on the east coast of North America, contains fossils otherwise known only from the Palaeozoic of western Europe. The explanation for their distribution is that after the Iapetus ocean closed in the late Palaeozoic it opened again a little further to the east, leaving part of the western shore of Europe attached to North America.

David K. Elliott