Tectonics from fossils? Analysis of reef-coral and sea-urchin distributions from late Cretaceous to Recent, using a new method

Biogeographical distribution is seen as a product of small- and large-scale factors, of which only the latter are important in palaeogeographical reconstruction. We review here previous methods for assessing the historical sequence of major events that have caused biogeographical distributions, and we argue that the most rigorous of these is cladistic biogeography. We also advocate our own new method, which uses parsimony analysis of taxa shared between sample localities. In not being dependent on prior phylogenetic analysis, this method is especially suitable for taxonomic groups which lack an adequate phylogeny. This new method takes contemporaneous distributional data from one or more narrow stratigraphic time planes (including the present), and generates a cladogram of sample localities for each of them. We interpret these cladograms as hypotheses of historical sequences of divergences between biotas, for which the sample localities provide geographical control. Congruence between cladograms from different organic groups and time planes provides an indication of those events which are most likely to have been the result of major (e.g. tectonic) historical factors. This method avoids prejudgement of areas of endemicity. We also argue that because younger major historical events can destroy the biogeographical patterns produced by older events, we can reach a more complete understanding of historical biogeography by using a combination of Recent and fossil data. The method however requires refinement and we summarize some of its present problems. We present area cladograms for Recent and Lower Miocene corals and echinoids, and for Eocene and late Cretaceous (Campanian-Maastrichtian) echinoids. Although these should be tested against area cladograms based on other taxa, we conclude, amongst our preliminary results, that: (1) the present Indo-Pacific fauna is a young biogeographical entity, not older than the Lower Miocene; (2) prior to this, Pacific faunas had been isolated since a time older than the Upper Eocene; and (3) the faunas of the Atlantic and western Tethys have been distinct from those of the Indian Ocean and eastern Tethys since a time long before the ‘terminal Tethyan event’ (Miocene), and since as far back, at least, as the late Mesozoic.