THE GENUS CZEKANOWSKIA HEER - PRINCIPLES OF SYSTEMATICS, RANGE IN-SPACE AND TIME

Study of a unique collection of leaf phytoleims (or compressions) of the genus Czekanowskia from more than 170 localities from the Triassic, Jurassic and Lower Cretaceous of northern Eurasia reveals that species of this genus cannot be identified by morphological characters of leaves. It is suggested that epidermal characteristics should be used for infrageneric systematics. Characteristics of the following taxonomical levels, namely class (order), genus, subgenus and finally species are distinguished. Within the genus Czekanowskia 3 subgenera (Czekanowskia, Harrisella and Vachrameevia) and 72 species have been identified by epidermal structures by the year 1991. Each of the species existed during 1-2 geological stages. The distribution of the genus Czekanowskia was restricted to the Northern Hemisphere and its life history was very dynamic. During successive epochs its distribution was dependent on climatic and paleogeographic changes; these changes were accompanied by migration and speciation of plants. The first undoubted leaf remains of Czekanowskia have been found in Upper Triassic deposits (the Norian-Rhaetic). The systematic diversity of the genus increased gradually from the end of the Triassic up to the Middle Jurassic, then it decreased in a similar manner throughout the Late Jurassic and the Early Cretaceous; at the beginning of the Late Cretaceous, the genus vanished. In the history of the subgenus Czekanowskia two explosive speciations can be noted, in the Middle Jurassic and the Early Cretaceous, which coincide with climatic optima.