Seeds, ovules containing a fertilized egg and ready to be shed from the plant, are reproductive organs characteristic of both gymnospermous and angiospermous plants. In angiosperms (Magnoliophyta) an additional structure, the matured ovary, encloses one or more seeds to form a fruit. Seeds and fruits are less commonly found as fossils than are vegetative remains. They may be preserved structurally as casts, or as compressions which are sometimes found with leaf compressions. Seeds and fruits often occur in lignites. See also Lignite; Magnoliophyta.
The oldest known seed plants are of Mississippian age. Carboniferous seed plants include the extinct Cordaitales, probable conifer ancestors, and Pteriodospermae, seed plants with fernlike foliage. During the Mesozoic Era, all major modern groups of seed plants were represented, along with members of the declining cordaitalean and pteridospermous stocks. Among the most completely known Mesozoic seeds are those of the cycadeoids, extinct cycad relatives. See also Cordaitales; Cycadeoidales; Pteridosperms.
Upper Cretaceous fruits are known from northern Africa, Long Island, New York, and elsewhere. Tertiary fruits and seeds have been found in numbers in the United States in the Brandon lignite of Vermont and in the Clarno Formation of central Oregon. The best-known European Tertiary fruits and seeds are from the brown coals of Germany and the Eocene London Clay Formation of England.
Important paleobotanical findings resulting from the study of fossil seeds and fruits include the knowledge obtained of the independent evolution of the seed habit in unrelated groups; the discovery that much Carboniferous fernlike foliage was borne on seed plants rather than on ferns; and the discovery that Glossopteris, an important plant in widespread Permian floras of the Southern Hemisphere, was a seed plant. Pyritized fruits from the London Clay Formation reveal the presence of many extinct genera along with modern genera in early Tertiary time. Morphological changes in herbaceous angiosperm seeds from sequences of Tertiary beds furnish data on rates of evolution in plants. Because plant classification is based primarily upon reproductive structures, fossil seeds and fruits provide highly reliable evidence for identification and interpretation of fossil plants. See also Paleobotany.
