FOSSIL PLANTS/Gymnosperms 453 Figure 17 Stout, barrel shaped trunk of the extinct Bennettita lean gymnosperm Cycadeoidea microphylla (Jurassic, Isle of Port land, Dorset, England) Ovules were borne in clusters of compact cones Pollen organs were borne in clusters on a receptacle These plants were probably small trees or shrubs Figure 18 Leaves of the extinct cycad like Bennettitalean gym nosperm Zamites gigas (Jurassic, Yorkshire, England) Bennettitales (Triassic to Upper Cretaceous) This extinct group had a cosmopolitan distribution In habit, Bennettitales were shrubs or small trees with stout, sparsely branched trunks or squat, unbranched, and barrel-shaped trunks (Figure 17) The cones of Bennettitales were highly distinctive flower-like structures Cones in the genus Williamsoniella comprised a central ovule bearing receptacle subtended by a whorl of rather broad, leaf-like pollen-bearing segments Whorls of leaf-like bracts subtend the pollen and ovule-bearing parts Cones were borne either along branches (Williamsoniaceae) or along the squat trunks embedded and partly protruding from a thick layer of old leaf bases (Cycadeoidaceae) In some, pollen- and ovule-bearing parts were in separate cones (e.g., Williamsonia), whereas in others they were borne within the same cone (e.g., Williamsoniella) Leaves were large and either pinnate or flat bladed (Figure 18) The striking superficial similarity to modern cycads masks fundamental differences in anatomy and especially the structure of the cones groups of gymnosperms, the fossil record of gnetales is most poorly understood Macrofossil remains are sparse, but there is a lengthy pollen record that begins in the Late Triassic Fossil pollen resembles that of living Ephedraceae and Welwitschiaceae The record of dispersed pollen indicates that living Gnetales are relicts of a group that was once more widespread and much more diverse Maximum diversity was achieved during the mid-Cretaceous Some putative early Gnetales macrofossils resemble conifers, raising the possibility that the poor fossil record might in part be explained by misidentification (Crane, 1996) See Also Dendrochronology Fossil Plants: Angiosperms Mesozoic: Triassic; Jurassic; Cretaceous Microfossils: Palynology Palaeozoic: Devonian; Carboniferous Tertiary To Present: Oligocene Gnetales (Late Triassic to Recent) This group of gymnosperms contains approximately 70 living species classified into three genera and families (Ephedraceae, Gnetaceae, Welwitschiaceae) Living members are predominantly shrubs or vines, but the group contains some small trees Of the living Further Reading Beck CB (ed.) (1988) Origin and Evolution of Gymno sperms New York: Columbia University Press Crane PR (1996) The fossil history of the Gnetales Inter national Journal of Plant Sciences 157: S50 S57