Ordovician geography and faunal provinces deduced from trilobite distribution

Abstract

Lists of families and genera of trilobites from the stratigraphical series of the Ordovician have been compiled, using information considered adequate from various areas in the world. Dissimilarities between these faunas have been assessed at both generic and family level, using Simpson's index. These indices have been analysed by a non-metric multidimensional scaling technique, and the groupings revealed are interpreted as indicating faunal provinces. Four provinces are recognized in the Lower Ordovician, the faunal characteristics and geographical extent of three of them (the Bathyurid, Asaphid and Selenopeltis provinces) being as previously described. A fourth province, here named the Asaphopsis province, is proposed for South American and Australian faunas in the Arenig and Llanvirn; the affinities of faunas from southeast Asia are indeterminate. In the Caradoc, faunas from areas occupied earlier by Bathyurid, Asaphid and Asaphopsis faunas, together with trilobites from south-east Asia, constitute a single province, here named the Remopleuridid. The Selenopeltis province persists in southern Europe and North Africa. These two provinces are recognizable only by a generic analysis in the Ashgill, and the latest faunas of that series are world-wide. It is assumed that a faunal province originally extended over parts of a single continental mass (cf. Wilson 1966), and that migration between provinces is inhibited by width of seas or temperature differences. An assemblage of continental masses consistent with these assumptions, and with palaeomagnetic data, is proposed to give four palaeogeographical maps for the Ordovician period. The position of the proto-Atlantic ocean proposed by Wilson (1966), and Gondwanaland of McElhinney & Luck (1970), are accepted; the remainder of Eurasia is divided into four blocks. The maps suggest relative movements between continental blocks that may have removed barriers to migration and resulted in progressive merging of the faunal provinces. They are models to be tested against distributions of other animal groups, new palaeomagnetic and palaeoclimatic evidence, and theories on lithosphere plates. The geographical reconstructions imply that the faunas of South America, southern Europe and North Africa inhabited cool waters, those of North America, northern Europe, north-east Asia, south-east Asia and Australia, warm waters. Cooler water faunas appear to have been less diverse. Decreasing provinciality of faunas during the Ordovician appears to have been accompanied by a reduction in total diversity; supposed climatic deterioration does not seem to have resulted in increased diversity. The differences between contemporaneous faunas within the Bathyurid and Remopleuridid provinces are investigated, and are shown to reflect differences in environment and evolution and diversification of groups of trilobites which took place in these environments.