Platyhelminths as paleogeographical indicators

Abstract

Turbellarians do not feature as examples in the present discussions on the theory and method of analytical biogeography. It is argued, however, that turbellarian distributional records form good examples of large-scale biogeographic patterns resulting from continental breakup. Some turbellarian taxa also indicate biogeographic links across the Pacific Ocean, which can be visualized readily by means of track construction. Amphi-pacific organismal distributions form the ingredients of trans-Pacific biogeographic tracks. Such tracks may be explained historically either as the result of dispersal or of vicariance. In the case of the flatworm examples, as well as many other organisms, dispersal explanations are the least satisfactory. However, under a vicariance paradigm the classical pre-drift reconstruction of Pangea cannot adequately explain trans-Pacific tracks. Therefore, alternative paleogeographic models may be invoked as explanatory hypotheses: the lost continent Pacifica, island integration, a new reconstruction of eastern Gondwanaland, an expanding earth. None of these alternative models is fully compatible with all geological and biogeographic data available at present. It is stressed that biogeographic data and theories should not be made subservient to geological theories. Biogeographical data on flatworms may indicate paleogeographical relations which are worthy of examination by geologists.