Evolutionary significance of floristic changes in the Northern Hemisphere during the Late Cretaceous and Palaeogene, with particular reference to Central Europe

Abstract

Documented changes in the composition of the vegetation of Central Europe during the Late Cretaceous and Palaeogene are compared with those for other parts of the Northern Hemisphere in order to test the validity of the supposed global bioevents at the Cretaceous-Tertiary (K/T) and Eocene-Oligocene boundaries. Many heterosporous plant species, known mainly from megaspores, became extinct by the end of the Cretaceous. Most of the plants with which the enigmatic “mesofossils” Spermatites and Costatheca were associated also disappeared towards the end of this period. On the other hand, the angiosperms seem to have evolved without significant interruption at either the K/T or the Eocene-Oligocene transitions. Although there were many extinctions during the Late Cretaceous among certain groups, the plants concerned appear to have been continually replaced by other taxa. Recent studies of fruit and seed assemblages from Central and Western Europe have revealed 270 species of angiosperms and some conifers belonging to 28 families. Of the 13 genera recovered from Maastrichtian deposits, 11 occur in the Palaeocene and are partly represented by the same species. Although the vegetation may have been damaged by the kinds of catastrophic events that are suggested to have taken place at the K/T boundary, such as a bolide impact or an episode of major volcanism, the effects are likely to have been only temporary (a few years) and not necessarily on a global scale. Overall, the palaeobotanical evidence suggests that there were continuous rather than abrupt changes in the composition and physiognomy of the vegetation throughout the Late Cretaceous and Early Tertiary. The changes that took place during the Eocene and Oligocene correspond to stepwise climatic deteriotation. The uppermost Eocene-lowermost Oligocene transition was connected with subhumid to semi-arid summer seasons which resulted in the disintegration of forest cover and increased migration of terrestrial vertebrates. A lowering of mean January temperatures during the Oligocene led to the “withdrawal” or extinction of some palaeotropical elements and immigration of deciduous trees into Central Europe in several waves, the most pronounced of which was in the middle Oligocene.