Fossil Plants

plants of past geologic periods, remains of which have been preserved in deposits of the earth’s crust. The study of fossil plants is the subject of paleobotany.

Lower plants (algae and bacteria) are preserved in their entirety; of the higher plants, occasionally mosses and a few small forms are preserved; and of the large higher plants, only separate parts are preserved—stems, bark, leaves, flowers, fruits, seeds, spores, and pollen grains. This fragmentariness of the remains of fossil plants makes it difficult to study them. Usually fossil plants are found in swamp, lake, and river deposits, and in limestone and siliceous deposits of springs; more rarely they are found in amber and lavas and in coastal-marine deposits along with fossil animals. Most often found in the fossil state are remains of woody plants that grew near bodies of water and aquatic plants and herbs that lived in bodies of water or along their shores. The fossil remains of plants that grew far from bodies of water and plants of alpine zones and grassy plains consist mainly of pollen and splinters of wood. Thus, all the accumulated remains of fossil plants can provide only a partial idea of the plant cover of the past, predominantly of woody forms and of plants that lived near bodies of water.

The degree of preservation of the remains depends also on the geologic age of the rocks containing the plant fossils. Some parts of fossil plants from the Anthropogene (wood, fruits, leaves) have almost all the properties of newly dead plants—for example, they can be cut with a knife and they have preserved their original color to a certain degree. Objects that have preserved their anatomic structures are investigated on thin slides or in sections through a microscope. The cuticle, on which epidermal cells are imprinted, small unicellular or colonial algae (diatoms, blue-green algae, flagellated algae), and the spores and pollen of plants are also studied. Larger remains are usually studied with a magnifier or binocular microscope. In studying leaves, much attention is given to details of their structure, such as veining, leaf shape, distribution of stomata, and shape of epidermal cells.

Fossil plants are classified by the same methods as modern plants. However, many fossil plants belong to classes and families that are now extinct. The dissociated occurrence of vegetative parts and reproductive organs of fossil plants, the changes that have taken place in them, and their fragmentary condition often make it impossible to assign a given find to a specific taxonomic plant group and force a temporary artificial classification that unites the plants according to external resemblance to be applied. In such cases plants having different origins may be assigned to the same “genus,” which is usually called the form genus, and, conversely, parts of the same plant may be assigned to different “genera.” For example, in lepidodendrons the various forms of preservation of their trunks are differentiated as the genera Knorria, Bergeria, and Aspidiaria. The peculiar rootlike formations of many lepidophytes (such as Sigillaria and Bothro-dendron) are united in the genus Stigmaria; the casts of the internal cavities of Cordaitales are called Artisia. Spores and pollen from Paleozoic deposits, and to a considerable extent from Mesozoic deposits, are artificially classified. Artificial classification is almost never applied in the USSR to plants of the Paleogene and especially plants of the Neocene and Anthropogene.

Study of fossil plants has established the presence of bacteria and unicellular algae in deposits of the Archean and Proterozoic, that is, more than 3 billion years ago. Terrestrial plants possibly appeared during the Cambrian and Ordovician and undoubtedly during the Silurian. It has been established that the simplest higher spore plants—psilophytes—were dominant at the beginning of the Devonian period. Almost simultaneously there arose such groups of higher spore plants as the Lycopodiales, then the protoferns, and at the end of the Devonian the arthrophytes and protogymnosperms (Archaeopteris) and, finally, the pterido-sperms. The pteridosperms, along with ferns, Lycopodiales, and arthrophytes, attained wide distribution in the Carboniferous when many of them were represented by treelike forms. Conifers, ginkgoes, cycads, and Bennettitales appeared in the Permian or at the end of the Carboniferous but attained maximum development in the Mesozoic. Remains of angiosperms are known as early as from Lower Cretaceous deposits, but they became dominant in the Late Cretaceous.

The majority of higher spore plants and gymnosperms of the Paleozoic and partly of the Mesozoic belong to extinct classes, orders, and families of plants. Among contemporary fern families, the earliest to appear were the Marattiaceae, Osmundaceae, and Schizaeaceae, then Cyatheaceae and Dipteridaceae, while Polypodiaceae appeared much later. Some genera of plants that existed in the Mesozoic still exist today, for example, the horsetail Equisetum, the ferns Osmunda and Gleichenia, and the gymnosperm Ginkgo. At the beginning of the Tertiary almost all presently existing genera of plants were formed. Contemporary species appeared only in the Neocene; in the Pliocene their numbers were already considerable. In the second half of the Tertiary and in the Quaternary there occurred mainly formation of modern species, extinction of old, once widely distributed plants, which are now preserved as relicts (Taxodium, Metasequoia, Sequoia, Ginkgo, Liriodendron, Cercidiphyllum), and, especially, geographic redistribution of the plant cover, which acquired its present appearance after the end of the glacial age.

Lower spore plants evolved at a slower pace; almost all types of algae already existed in the oldest periods of the life of the earth. Apparently only the diatoms and coccolithic unicellular algae appeared in the Jurassic. Various algae of the phylum Charophyta are known from as early as the Devonian (Trochilis-cus, Sycidium). The Mesozoic saw vigorous development of Si-phonineae, whose armor forms massive accumulations in the Triassic deposits of the Tirol and Lombardy, and in the Cenozoic the red algae (Rhodophyceae) developed, which participated with the corals in the construction of reefs. In the Precambrian, reef-forming colonial blue-green algae, whose structures are called stromatolites, and very tiny unicellular forms were widely distributed.

Fossil plants form mineral deposits that are used in industry (beds of peat, lignite, bituminous coal, and fuel shales). Deposits of the shells of various diatoms form beds of diatomites. The study of fossil plants provides an idea of the composition and distribution of the plant cover of the earth in the past and marks the stages of plant evolution. The composition and distribution of the flora in each geologic system and its smaller subdivisions permit (along with data on fossil animals and even when these are absent) the age of geologic deposits to be determined, which is of particular significance for continental formations.