Ursprung und Evolution der Charophyten

Zusammenfassung

This chapter presents a review of the most significant steps in charophyte evolution and its fossil record, which is amongst the most complete in fossil algae. It provides enough data to document the history of the group from the Silurian until the present. Fossil charophyte remains include mainly their calcified fructifications, i.e. utricles and gyrogonites, but also their fossil thalli. Palaeozoic represents the time when charophytes reached a maximum of diversity in the Bauplan of their fructifications, with the coexistence of three charophyte orders, Sycidiales, Moellerinales and Charales. The extinction of two of these orders at the end of the Palaeozoic led to a single type of fructification, the porocharacean gyrogonite that represents the charalean ancestor of all post-Palaeozoic charophytes. However, this ancestor diverged soon in two evolutionary lines, Polyplacata and Monoplacata, based on the presence of a multicellular vs single basal plate of the gyrogonite. This fundamental difference can be assumed representing two alternative types of female gametogenesis. Charophytes with a composite basal plate, the Polyplacata, developed diverse morphologies at the base of the Mesozoic (Triassic-Jurassic), most of them are traditionally ranged within the porocharaceans, characterised by an apical opening. These polyplacate porocharaceans are considered the ancestors of extant Nitella and Tolypella (sensu stricto), recorded already in the Jurassic and Cretaceous respectively. Monoplacata diversified in several steps, the first in the Upper Jurassic and Lower Cretaceous with the appearance of the utricle-bearing clavatoraceans, and the appearance of the first characean genera with a single basal-plate, including the oldest known Sphaerochara (equivalent to Tolypella sect. Rothia). The Clavatoraceae family dominated the wetlands on the Tethyan islands during the Lower Cretaceous relegating the porocharaceans to brackish environments. At the beginning of the Late Cretaceous (Cenomanian and Turonian) there is a gap in the fossil record. After this gap the monoplacate Characeae underwent an important radiation and expanded with the appearance of many genera including modern Chara, Lamprothamnium, and Lychnothamnus. This radiation occurred in parallel with the extinction of a few last species that remained from clavatoraceans and porocharaceans near the Cretaceous-Tertiary boundary. Genus Nitellopsis is documented since the lowermost Tertiary. Characeans reached their maximum of diversification in the Middle Eocene, with numerous genera whose gyrogonites were quite different from the modern ones. These morphological types regressed during the Late Eocene and the Early Oligocene when a global climatic shift occurred. Afterwards the charophytes were progressively reduced to the morphologies of the seven modern genera. Hence the Neogene to recent flora appears as an impoverished remnant of the flourishing of charophytes in the geological past.