Upland cordaite-dominated vegetation on calcrete soils
Lowland vegetation dominated by cordaites with minor pteridosperms, sphenopsids, and rare lycopsids
Facies 4 Facies 3
Facies 2 Facies 1
Figure 18.18 Continued
of China, but seen today as a typical urban tree in parts of North America and Europe. Ginkgos were more diverse in the Mesozoic. Leaf shape varies from the fan-shaped structure in the modern form, to deeply dissected leaves in some Mesozoic taxa (Fig. 18.20a, b). Catkin-like pollen organs and bulbous stalked ovules are borne in groups on separate male and female plants. The leaves in the modern Ginkgo are deciduous, that is they are shed in winter, and this may have been a feature of ancient ginkgos.
The cycads, represented today by 305 tropical and subtropical genera, are trees with a stem that ranges in length from a small tuber to a palm-like trunk up to 18 m tall. The leaves are provided with deep-seated leaf traces that partially girdle the stem. Leptocy-cas (Fig. 18.20c) from the Late Triassic of North Carolina has a 1.5 m-tall trunk, showing a few traces of attachment sites of leaves that had been lost as the plant grew, and a set of nine or 10 long fronds near the
top of the trunk. Many other cycads show marked leaf bases along the entire length of the trunk. Cycad fronds are typically composed of numerous parallel-sided leaflets attached to a central axis in a simple frondlike arrangement, but others had undivided leaves.
The bennettitaleans, or cycadeoids, were a Mesozoic group of bushy plants with frondlike leaves very like those of cycads. Some bennettitaleans had a trunk up to 2 m tall, with bunches of long fronds at the top of the trunk and on subsidiary branches. Other bennettitaleans like Cycadeoidea (Fig. 18.20d) had an irregular ball-like trunk covered in leaf bases, representing former attachment sites of fronds, and with a tight tuft of long feathery fronds on top. Some bennettitaleans had flower-like structures (Fig. 18.21a). Classic dinosaur scenes of Jurassic and Cretaceous age often picture one or other of these ben-nettitaleans in the background.
The gnetales have a patchy fossil record, with two Late Triassic examples, a few in the Cretaceous and Tertiary, and three living genera. Gnetales have distinctive pollen that is very abundant in Cretaceous sediments. The group was probably much more diverse at this time. Gnetales gained prominence among botanists because the group is thought by some to be the closest living gymnosperm relative of angiosperms. In particular, gnetales may have their ovules and pollen organs in cones that are rather flower-like (Fig. 18.21b).
Figure 18.20 Diverse gymnosperms: (a) leaves of the modern ginkgo, Ginkgo biloba and (b) of the Jurassic ginkgo, Sphenobaiera paucipartita; (c) reconstruction of the 1.5 m-tall cycad Leptocycas gigas a from the Late Triassic of North America; and (d) reconstruction of the 2 m-tall bennettitalean Cycadeoidea from the Cretaceous of North America. (Based on Delevoryas 1977.)
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