Reply

Abstract

THE sentence which Teichert objects to is, when taken in context, a criticism of Nicholas's assumption¹ of the existence of an ocean between Southern Europe and Africa. In geological discussions an ocean may be defined as a deep basin floored by simatic crust generated by the accretion of basaltic material at linear ridges of active volcanism and seismicity (“mid-ocean ridges”). Oceanic growth is accomplished by the process unfortunately known as “seafloor spreading”. The Red Sea is an example of an incipient ocean and the Atlantic an example of a fully fledged ocean. This definition would exclude those expanses of salt water usually included in oceans which cover continental crust. These expanses are seas. Clear differentiation between oceans and seas is important in palaeogeographic and tectonic discussions. Nicholas¹ was clearly using his Tethys in the sense defined above with the plate tectonic embellishment of a subducting margin. My communication² arose out of a worldwide, quantitative analysis of Ordovician biogeography (unpublished). The conclusion that the faunal distributions found are best interpreted in terms of oceanic barriers rather than in terms of climate or other ecological factors is also justified elsewhere. Johnson's Provinciality Index³ was used to compare pairs of geographical areas. This index is calculated by dividing the number of common fossil genera by twice the number of endemic genera in the smaller faunal list. Values above one are taken to indicate some degree of co-provincialism. In this study values have ranged from 0–5.2, with most values falling within the range 0.1–0.7. Of all pairs of areas, the comparison between North Africa and Southern Europe gave, by far, the highest values (Fig. 1). On the other hand, indices between Southern Europe and Northern Europe were very low (Fig. 1). Somewhat more sophisticated computerized studies of Cambrian faunal provinces are in preparation by Burrett and Richardson. These studies further support free faunal interchange between Southern Europe and Africa but show a very clear dichotomy between Northern and Southern Europe. These connexions have been clearly shown by Repina⁴ in a series of maps. In addition the non-existence of archaeocyathids in Northern Europe but their abundance in Southern Europe is further evidence for a Mid-European Ocean but indicates nothing about a Tethyan Ocean. Faunal similarities between Southern Europe and Africa continued through the Palaeozoic⁵ and into the Triassic when the “Tethys itself could have been no more than a minor obstacle to the north-south migration of land animals”.⁶ Rifting probably started in the Triassic⁷ and by the Jurassic a reasonably large Tethyan Ocean was in existence⁸.