FOSSIL PLANTS/Gymnosperms 449 They are also closely similar in overall appearance to extinct Mesozoic gymnosperms in the Bennettitales Significant differences in the structure of the seed and pollen-bearing organs indicate that the two are only distant relatives Cycads probably originated in the Late Palaeozoic Plants such as Archaeocycas are known from the Lower Permian, and the ancestors of modern cycads are thought to extend back into the Carboniferous The group reached its maximum diversity during the Mesozoic era Callistophytales (Late Carboniferous) Figure Large nutlike seeds (about 2.5 cm long) of Trigonocar pus parkinsoni (Upper Carboniferous, England) This small extinct group was first characterized in detail in 1975 The best known species is Callistophyton poroxyloides The stem anatomy of the group resembles that of Lyginopteridales, whereas the reproductive structures are closer to those of Cordaitales Callistophyton is portrayed as a small shrubby plant with stems up to cm diameter, bipinnate fronds, and a scrambling habit The leaves are similar to fossil foliage of Medullopteris type Ovules and pollen organs were borne on the underside of unmodified leaves Pollen was monosulcate with a bladder, resembling the pollen of conifers Cordaitales (Late Carboniferous to Early Permian) This distinctive extinct group had a cosmopolitan distribution Some species were large trees that attained heights of as much as 30 m, whereas at least one species is known to have been a prostrate scrambler some m in height The leaves of many species were large and strap-like, bearing a striking resemblance to the leaves of flowering plant monocotyledons such as Hippeastrum (Amaryllis) or living conifers in the genus Agathis Pollen- and ovuleforming structures were borne on shoots scattered among the leaves The wood was similar in structure to that of Araucarioxylon The centre of the stems contained a large pith cavity with transverse septa Fossilized infillings of this pith (Artisia) are commonly found The reproductive structures are compound fructifications comprising cones subtended by bracts borne along a central axis Figure 10 Living cycad in the Fairchild Botanical Garden, Flor ida, USA The group has a predominantly tropical to subtropical distribution Some species reach 18 m in height, whereas others are small with tuberous stems Cycads bear one or more crowns of large compound leaves Seeds and pollen are borne in cone-like structures on separate plants Large species of cycads bear a superficial resemblance to palm trees with stout stems Voltziales (Late Carboniferous to Early Jurassic) This extinct group of conifer-like plants comprises three families: Utrechtiaceae (Lebachiaceae), Emporiaceae, and Majoniaceae (Figure 6) On the whole, these are thought to resemble in general habit and leaf morphology living conifers such as Araucaria heterophylla (Norfolk Island Pine) Some species had bisaccate pollen like that of living Pinaceae The structure of the wood also resembles that of living conifers in the Araucariaceae The fructifications of Voltziales