Annals of Botany

Wagnerian approach to invasion of Europe

For the third time this month the topic is angiosperm evolution. Here, however, instead of using the present to dissect the past, a direct analysis of the past is discussed. Coiffard et al. (Villeurbanne and Rennes, France, pp. 545–553) note that by the start of the late Cretaceous era angiosperms were already widespread and in many habitats had taken over from the previously dominant gymnosperms. The authors are interested in how this situation arose, especially in relation to the invasion of Western Europe by the angiosperms. Previously, they have demonstrated that a clustering method normally used in molecular phylogenetics (Wagner’s Parsimony Method, WPM) may be used to construct relationships between fossil assemblages and palaeoenvironments. For the present paper a new database of European Cretaceous plant fossils was constructed and WPM was used to group the localities according to their species content. This provided a fascinating picture of the changing character of the flora in relation to changing environments through the early and mid-Cretaceous. In the early Cretaceous (Barremanian) phase, characterized by warm, dry conditions, matoniaceous ferns (of the order Gleicheniales) dominated the land; some gymnosperms were also present. Angiosperms were mainly confined to freshwater habitats, although fossil pollen from the next phase (the lower Aptian) suggests that some colonization of non-aquatic habitats was under way. Climatic changes in the Aptian led to a dominance of gymnosperms and a decline in the matoniaceous ferns, while a return to warmer conditions in the Albian phase led to some recovery of the ferns. Throughout these phases, angiosperms were gradually becoming established away from aquatic habitats, firstly on the floodplains and then in a wide range of habitats, leading to their presence in all environments by the Cenomanian (mid-Creaceous) phase. Further, it is possible that the pole-wards migration of the angiosperms, initially from Africa and then through Europe, was driven by global warming.