A New Link to the Origins of Angiosperms
by Christine J. Small • Fall 2002 (Volume 30, no. 3)
The evolutionary history of flowering plants is poorly known, represented by remnant fossils only weakly identified as angiosperms. Until recently, fossil evidence of early angiosperms was based on vegetative materials and pollen. None of these fossils, however, showed the presence of ovules or seeds enclosed in carpels, the true distinction of the angiosperm lineage.
A team of paleontologists and paleobotanists led by Ge Sun of Jilin University, China, and David Dilcher of the Florida Museum of Natural History, University of Florida, recently announced the discovery of a new basal angiosperm family, Archaefructaceae (Ge Sun et al. 2002). The new family is represented only by the two species, Archaefructus liaoningensis and A. sinensis. Five virtually complete fossils of these plants, including flowers, seeds, and fruits, were found in the Yixian Formation in Liaoning, northeastern China. The fossils are believed to be between 125 and 145 million years old, placing them within the Lower Cretaceous or uppermost Upper Jurassic periods. These species are part of a complex basal group likely to be a sister taxon to living angiosperms. Ge Sun et al. (2002), however, do not believe these species to represent the first angiosperms.
The Archaefructaceae are thought to have been weedy aquatic plants due to the presence of long, thin, herbaceous stems and finely dissected compound leaves that were likely to require water for support. Swollen leaf bases, especially on leaves closest to reproductive structures, also suggest that these plants were likely to live in an aquatic habitat, helping to lift the flowers to the water's surface to aid in pollination and seed dispersal. Further, fossils of fish were present in the same rock materials.
These recently discovered fossils suggest that the first flowering plants may have been submerged aquatic plants similar to modern day members of the Nymphaeales (water lily family). This raises questions about the long-standing view that flowering plants arose from woody plants similar to Magnolia. Michael Donoghue of Yale University is quoted in a Paleobotanical article in Science magazine (Stokstad 2002), indicating that "A whole lot depends on whether [Archaefructus] is correctly positioned in the tree." While much is left to be determined regarding the evolutionary ancestry of Angiosperms, these new fossil finds provide greater information on the origins of flowering plants that ever found before.