446 FOSSIL PLANTS/Gymnosperms Figure Ovule (seed) bearing cupules of the early gymno sperm Xenotheca devonica (Upper Devonian, Devon, England) This fern-like theme in gymnosperm evolution continued into the Carboniferous period Much of the fern-like foliage of coal swamp forests is actually attributable to extinct gymnosperms These are the so-called ‘seed ferns’ or Pteridosperms (Figure 5) As the name implies, these were seed-bearing plants that possessed fern-like leaves, which is a combination of characteristics seen only in extinct gymnosperms Groups common but not exclusive to the Carboniferous period include Calamopityales, Hydraspermales, Medullosales, Lyginopteridales, and Callistophytales For the most part, these were understory shrubs and small trees Some are also known to have been vinelike climbers The earliest gymnosperms of more modern aspect were an extinct group of conifers called the Voltziales (Figure 6) This group first appeared in the Late Carboniferous and the plants are thought to have resembled in general habit and leaf morphology living conifers such as Araucaria heterophylla (Norfolk Island Pine) During the Late Palaeozoic, gymnosperms were an important component of terrestrial floras, but there was a significant increase in species diversity during the Early Mesozoic Numerous petrifactions of stem and trunk show that most Mesozoic gymnosperms, like their modern relatives, were woody shrubs and trees Evidence from fossil forests deposits reveal landscapes dominated by gymnosperms, which formed the main canopy element (Figure 7) In particular, the conifers underwent a major radiation during the Triassic period Studies of the pollen Figure Reconstruction of the early gymnosperm Elkinsia poly morpha (Upper Devonian, USA) After Serbet and Rothwell (1992) International Journal of Plant Sciences 153: 602 621 record show that the living Gnetales diversified alongside angiosperms during the mid-Cretaceous, but then underwent a decline towards the end of this period Molecular phylogenetic studies provide an alternative perspective on the fossil history of some groups of gymnosperms These are beginning to yield an unprecedented level of phylogenetic resolution in the most species-rich living groups, and they are providing additional insights into long-standing phylogenetic conundrums Molecular data support the hypothesis that conifers had a single origin, clearing up a longstanding controversy over the phylogenetic position of the Taxaceae (Yews) (Figure 8) Also, within conifers, the pines (Pinaceae) come out as sister group to all other living species As long suspected, the monkey puzzle family Araucariaceae is closely related to the Yellow-Wood family, Podocarpaceae In addition to