Paleofloristic Regions

regions of distribution of floras during various periods of the geologic past. In the past, as now, the flora was not homogeneously distributed over the earth’s surface. Even then there existed more or less distinctly demarcated regions of distribution of particular groups of plants, or phytochoria.

The phytochoria of the Devonian, the period in which terrestrial vegetation originated, has not yet been determined. However, by the second half of the Carboniferous period, three distinct regions existed: Angara (Cordaitales predominated), Euramerica (treelike Lycopodiales, calamites, and pterido-sperms were characteristic), and Gondwana (Glossopteris predominated). The Angara region embraced Siberia, eastern Kazakhstan, and Mongolia. The Euramerica region consisted of Europe, North America, North Africa, Anatolia, central Kazakhstan, and Middle Asia. The Gondwana region embraced the continents of the southern hemisphere and the Hindustan Peninsula, which constituted a single continent—Gondwana—during the Late Paleozoic. The Angara region corresponded approximately to the temperate-warm zone of the northern hemisphere, the Euramerica region to the tropical and subtropical zones, and the Gondwana region to the temperate zone of the southern hemisphere.

During the Permian, owing to climatic differentiation and the appearance of an extensive arid zone in the northern hemisphere, the Angara and Euramerica regions were divided into smaller regions. As a result of increased moisture, the paleophytic flora was replaced by the mesophytic flora in the middle of the Triassic. The Angara region was at this time replaced by the Siberian region and later by the Siberian-Canadian region (ginkgoes, Czekanowskia, and Podozamites predominated). In place of the Euramerica region there arose the Indo-European region (characterized by treelike ferns and by various Cycadophytae and conifers). Any differences that existed during the Paleozoic between the flora of the northern hemisphere and the flora of the southern continent and Hindustan became less drastic by the middle of the Mesozoic.

The mesophytic floras of Hindustan, South America, Antarctica, and other parts of the former Gondwana region lack Czekanowskia and conifers and have few specimens of ginkgo. Paleofloras of the temperate zone of the southern hemisphere have not been discovered; apparently these latitudes were occupied by ocean. With the onset of the Tertiary, which was characterized by groups of angiosperms that are extant today and which experienced a gradual general cooling that led to the Quaternary glaciation, the floristic composition and distribution of regions began to approach those of today.