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, 

 1 985 ). 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 cryptospores from the Llandovery and Wenlock, 

 and to a lesser extent in the Upper Ordovician are apparently closely 

 similar and widespread. Such assemblages occur in North America, 

 Europe, North Africa, the Middle East and China. Was this wide 

 dispersal due to efficient spore dispersal mechanisms, closer prox- 

 imity of the main continents when their parent plants evolved, 

 climate, or some other factors? The Ordovician data may not be 

 representative, as few assemblages have been described in detail, but 

 the uniformity of Lower Silurian cryptospore assemblages form a 



Proto Arctic 

 Ocean 



Ural 

 Ocean 



Pacific 

 Ocean 



I 



Fig. 8 Emsian palaeotectonic-palaeogeographic map. Simplified from Ziegler ( 1988). 



