PALYNOLOGICAL ZONATION OF MID-PALAEOZOIC, NW SPAIN 



127 



Ludlow - Pridoli 



Early Devonian 



Fig. 9 Oceanic circulation through the Ludlow-Lower Devonian. Based on Le Herisse et al. (1997). 



contrast to some of the Upper Silurian trilete associations showing 

 regional variations in species distribution. 



Well illustrated accounts of spore data from Upper Silurian- 

 Lower Devonian sequences are limited to comparatively few areas, 

 viz. Britain (Richardson & Lister, 1969), the Cantabrian Mountains 

 (Rodriguez, 1983),andNorthAfrica(Richardson&Ioannides 1973, 

 and work in progress). More limited stratigraphical data are avail- 

 able from Belgium, France. Poland, and China. Upper Silurian and 

 Lower Devonian spore assemblages have been described from 

 graptolite-bearing sequences only in England and Wales (Richardson 

 & Lister, 1969, Burgess & Richardson. 1995) and Podolia 

 (Arkhangel'skaya, 1980). However, many of the published illustra- 

 tions of the Podolian material are insufficient for detailed comparisons 

 and key taxa cannot be identified with certainty at the species level. 

 Nevertheless the assemblages enable general correlation of these 

 important graptolite-bearing sequences with Britain. The following 

 discussions are therefore based mainly upon the authors" previous 

 and current work, and consequently the present discussion is depend- 

 ent on data from a limited geographic area representing three 



separate blocks, namely Laurussia, Iberia-Aquitaine and northern 

 Gondwana. 



Several spore taxa (eg. Streelispora neivportensis), although rare 

 in parts of Gondwana, appear to be geographically widespread. 

 Some cryptospore taxa. eg. Tetraletes variabilis Cramer 1 966a. may 

 have a restricted distribution and so far have been recorded only from 

 Gondwana, Iberia and Brittany, whereas other cryptospores (PI. 1, 

 figs 1, 7, 9-1 1) occur in Laurussia as well. The planar tetrads of 

 Tetraletes occur in Silurian-Lower Devonian sections from north- 

 west Spain and North Africa, with a single specimen recorded from 

 Brittany (d'Erceville, 1979). It is interesting to note that similar 

 planar tetrads are found in the modern thalloid liverwort Riccia 

 perssonii from South Africa (Perold. 1 989), Both the fossil Tetraletes 

 and the modern Riccia spores are permanently adherent and are 

 joined by more or less smooth bands of attachment separating areas 

 of spinose sculpture. Many cryptospore taxa are found in the Silurian 

 of North America. Britain, Western Europe, the Baltic States. Podolia, 

 Spain, and North Africa, whereas others are apparently regionally 

 confined. As more records are forthcoming it should be possible to 



