. Bulletin der natürlichen Histort Museum. Geologie Serie. 126 J.B. RICHARDSON, R.M. RODRIGUEZ UND S.J.E. SUTHERLAND LAURUSSIA - GONDWANA SPORE VERTEILUNG (Abb. 7-9) oberen Silur und Devon spore Floras sind vielfältig und Atavismus aber wenig von ihrer geographischen Verbreitung bekannt. Silurische paläogeographische Karten zeigen Europa und Nordamerika in niederen Breiten, zwischen dem südlichen Wendekreis und dem Äquator (Witzke & Amp; Meckel, 1988), getrennt durch einen Ozean von Gondwana (mit Iberia in der Nähe auf den nördlichen Flanken) in höheren südlichen Breitengraden (McKerrow&S

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. Bulletin of the Natural Histort Museum. Geology series. 126 J.B. RICHARDSON, R.M. RODRIGUEZ AND S.J.E. SUTHERLAND LAURUSSIA - GONDWANA SPORE DISTRIBUTION PATTERNS (FIGS 7-9) Upper Silurian and Lower Devonian spore floras are diverse and homosporous but little is known of their geographical dispersal. Silurian palaeogeographical maps show Europe and North America at low latitudes, between the Tropic of Capricorn and the equator (Witzke & Meckel, 1988), separated by an ocean from Gondwana (with Iberia in close proximity on its northern flanks) in higher southern latitudes (McKerrow & Scotese, 1990). The map (Fig. 7) based on Ziegler (1988) shows the Cantabrian Terrane separated from Gondwana by Proto-Tethys and on its northern flank the Rheic Ocean lies between the Aquitaine Cantabrian Terrane and northern France and Britain. McKerrow & Scotese show a wedge-shaped ocean narrowing westward to the point where the Americas were in contact. Using this reconstruction, and unless there was a major climatic barrier, there would have been a possible land plant migra- tion route between Laurussia and Gondwana. With favourable winds, airborne transport would have been feasible for many of the spores because of their small size (Mogensen, 1981) and it is worth noting that many moss species have achieved wide dispersal in the northern hemisphere since the last ice age, some 10, 000 years ago (Schofield, 1985). Such a time span is negligible in terms of geological time and is essentially instantaneous. Theoretically therefore, since there was no major physical barrier to dispersal apart from the availability of suitable habitats, and possibly climate, large parts of the tropical flora should have been uniform. As size affects dispersal (Mogensen, 1981), smaller spores, corresponding to homosporous plants with spores less than 25 |jm, and appearing in the Pfidoli and lowermost Lochkovian, should have had the widest distribution. Before the Pridloli, multiple unit crypt

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