Gall wasps and gall midges i i ! Leaf miners
Nectar gatherers ill Leaf gall wasps 1 i 1 Ichneumon wasps
Ml ! Insect pollination ! t ] Mycophageous gali midges
Plant sap suckers
Spore and pollen eaters
Equisetaceae Eurosp.ferns Leptosp. ferns
Cycads fig. 4.1.3. Age and development of important groups of vascular plants, and of phyto- and entomophageous insects, as well as evidence of pollination and herbivory. (After Zwölfer 1978)
which remained until the Palaeozoic (about 220 million years ago; see Fig. 4.1.1) with changing positions towards poles and equator.
Traces of life and the first single-cell prokar-yotes (bacteria, cyanobacteria) existed for about 3 billion years in an oxygen-poor atmosphere. Eukaryotes and multicellular forms could only develop with increasing oxygen content. This period, extending into the Silurian (about 400 million years ago), is called the eophytic (proterophytic) or the algal period.
The most important developmental step occurred with the settling of the land at the change from the eophytic to the palaeophytic during the Silurian period. The first land plants possessed cells with large vacuoles, already had stomata and had developed supporting tissues. These autotrophic producers lived together with fungi and bacteria as decomposers and formed the first bioscenoses.
Evolution of animal and plant species and communities progressed in parallel (see Table 4.1,1 and Fig. 4.1.3). Psilophytes (archetypal ferns) are regarded as precursors of the Pterido-phyta (ferns and their allies), to which Filicinae (ferns proper), Equisitinae (horsetails) and Lyco-podiniae (club mosses) belong. At that time Bryophyta (mosses) split off and remained at this developmental stage, but the other groups in the warm and humid climate of the Carboniferous formed luxurious forests with tree-like horsetails, club mosses and Cordiates, which later became extinct, on the swampy and boggy sites. Analyses of deposits of bituminous coal allow exact reconstruction of this very structured vegetation, including indications of symbiotic interactions (mycorrhiza) and the animal kingdom linked to it.
At the transition from the Carboniferous to the Permian climatic conditions became dryer. Many species did not survive and many land plants were unable to adapt their water relations. Thus the transition from the palaeophytic to the mesophytic is characterised by a decrease in species diversity. At the beginning of the Triassic, the Tethys Ocean separated off the eastern part of Pangaea (Fig. 4.1.2). During the Jurassic, the North Atlantic developed, and during the Cretaceous period the South Atlantic formed; in the early Tertiary the Antarctic and Australia seperated. The continents moved into their present position only in the Pliocene.
Up to the Cretaceous period the flora was very similar worldwide. The oceans forming between the drifting continents were no obstacles to the exchange of flora. The history of vegetation up to the mesophytic is also called the period of the gymnosperms. After the extinction of the larger club mosses and horsetails, gymnosperms gained space, particularly conifers and the Ginkgoaceae. The only present-day representative of the Ginkgoaceae is Ginkgo biloba, regarded as a "living fossil". In the northern regions the first representatives of the Pinaceae and the genus Juniperus have been found. Cu-
pressaceae, as well as the genus Araucaria, which today is limited to the tropics of the southern hemisphere, occurred worldwide.
The boundary between the Jurassic and Cretaceous periods also divides the mesophytic from the neophytic and the period of gymno-sperms from that of the angiosperms. The first angiosperms occurred at the end of the Jurassic period. In a relatively short period, 25 million years of the Cretaceous period, flowering plants developed very rapidly and suppressed many of the gymnosperms which had dominated until then. Almost at a stroke all main angiosperm groups developed; all available sites which could be occupied were filled with adapted species and the intercontinental flora exchange was still not too difficult. A much stronger floristic separation occurred during the upper Cretaceous period. The so-called plant kingdoms developed and - corresponding to the slightly earlier division in the south - distinction is made between the three floristic realms in the southern hemisphere (Antarctic, Australis and Capensis), two tropical equatorial floristic realms (Neotropics and Pa-laeotropics) and only one northern hemisphere floristic realm (Holoarctic).
Zwölfer (1978) has given an overview of the geological occurrence of vascular plants as well as phytophageous and entomophageous insects, and thus of the development of communities. There is proof that pollen and dead plant material were eaten as early as the Devonian period and in the Permian leaves were consumed. In the Triassic period insect pollination of flowers occurred and there were entomophageous parasites (ichneumon wasps) and plant galls. From the Tertiary period onwards all present-day phytophages are represented: herbivores, nectar collectors, leaf miners, gall flies and gall gnats. Figure 4.1.3 shows that development of phytophageous and entomophageous forms is geologically fairly recent and occurred at the same time as the development of angiosperms.
Mutual adaptations of floristic and faunistic partners is a consequence of the selection pressure linked to this development. Plants developed thorns, spines or chemical defence substances, animals responded with adaptive changes to their mouth parts or resistance against plant toxins. Some flowering plants were, however, particularly successful as they were able to protect themselves against herbivory; e.g. gentians (defence by indoalkaloids) or deadly nightshades (defence by tropanalkaloids). Dur ing this evolution, some insects used secondary plant metabolites for their own defence, e.g. against entomophagous enemies.
The most important groups of flowering plants were present at the beginning of the Tertiary period and, therefore, the period of the history of vegetation from then to the present day is called the neophytic or angiosperm period. Amongst the flowering plants, specialists adjusted to stress environments such as deserts and saline soils developed. Along with the formation of mountain ranges, in the northern hemisphere, including the central Asiatic and European Mediterranean mountains, young mountain floras developed (oreophytes, closely related to flora in plains). Often individual alti-tudinal steps are characterised by vicarious species and genera.
Holoartic flora has been found in Palaeozoic to Eocene lignite deposits in Spitzbergen, where not only remains of plants (e.g. Acer, Betula, Fa-gus, Quercus, Salix, Tilia, Pinus and Picea) growing today in central Europe under conditions of a temperate climate were found, but also those which now grow only in humid, subtropical conditions (e.g. Taxodium, Magnolia, Liriodendron), as well as species which survived in Ice Age refuges in North America and east Asia. Comparable plants (e.g. Aesculus, Castanea, Plantanus, Vitis) found in Greenland represent the so-called arcto-tertiary flora, the basis of the Holoarctic flora.
From the Eocene flora from southern England and the "flora from the Geiseltal" near Merseburg, Germany, plant lists were compiled containing families of plants (e.g. species of Anno-naceae, Pandanaceae, Sterculiaceae) which now occur in seasonally moist tropical regions. At that time the vegetation in Europe was similar to that in mountain regions of Southeast Asia. Europe only took its current position during the Tertiary, evidenced by fossiles from the early Tertiary (Miocene and Pliocene).
In plant remains of Pliocene in central Europe, tropical species could no longer be found, but representatives of present-day genera (Fagus, Quercus, Salix, Fraxinus, Populus, Pinus), and also species of genera which are at present extinct in this region (Liriodendron, Sequoia). Comparable fossiles from north-east Asia and North America indicate worldwide progressive cooling. In Europe, climate and vegetation zones shifted to the south. The west-east extension of the Alps and the Mediterranean made it difficult for species adapted to a warm climate to survive; some got extinct, others were pushed into limited regions. The relative uniformity of the Pliocene flora in the Holoarctic was lost. Large distribution areas were separated into several spatially limited regions, the so-called large Tertiary disjunctions (e.g. Sequoia and Metase-quoia, Magnoliaceae). For some species, e.g. of the genera Styrax, Plantanus, Melia and Casta-nea, the moist warm sites in the eastern Black Sea region (Colchis) became important refuges.
Development of Mediterranean sclerophyllic vegetation and the recent steppe, semi-desert and savannah flora is also often connected to the general cooling of global climate in the Tertiary period. Cooling in the Pliocene may be regarded as a precursor of the multiple quaternary climate changes in the most recent neophytic, with lasting influence on the central European and American flora.
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