Mesozoic Plants by Stan Balducci

Plants, especially non-flowering ones, evolved rapidly during the Mesozoic Era. Some have survived with little change until the present, although many others were overtaken by the flowering plants which appeared toward the end of the era. Ginkgos first appeared 150 million years ago and became common in the Mesozoic Era. They are gymnosperms and produce seeds without a protective covering. One species, Ginkgo biloba, survives as a "living fossil" today.

Certain characteristics enabled early plants to invade and become established on land. Internal vessels called vascular tissue circulated nutrients and water to all parts of the plant. An outer layer of waxy cuticle was developed to prevent drying out. The stomata located on the undersurfaces of leaves open and close to allow a plant to breathe. Roots provide anchorage and nutrient uptake. Finally, spores and seeds ensure the continuation of the species.

Four groups of plants dominated Triassic and Jurassic landscapes. The dominant understory plants were ferns, which included a variety of foliage types. A middle story of plants was quite diverse, including tree ferns, seed ferns, cycads, and cycadeoids. Cycads have a stem or trunk that commonly looks like a large pineapple and is composed of the coalesced bases of large leaves. The leaves break off as the plant grows, leaving a cluster of sturdy bases surrounding the stem. The leaves are large and palm-like, growing in a cluster at the tip of the stem.

Foliage of cycads and cycadeoids is abundant in Mesozoic strata. Because they are among the commonest compression-impression fossils found, the Mesozoic is also known as the "age of cycadophytes" (as well as the "age of reptiles").

The extinct order Cycadeoidales is a mysterious group of Mesozoic gymnosperms that disappeared from the fossil record during the Cretaceous. Like the cycads many cycadeoids have trunk-like stems that are unbranched or sparsely branched and clothed in spirally-arranged persistent leaf bases. The fronds borne by plants of both orders are highly similar in appearance.

The upper story of a Triassic forest was formed by a variety of conifers. These had distinctive patterns of cells in the wood that permit us to ally them with primitive living conifers called araucarians. The Norfolk Island pine is an example that is commonly grown as an indoor house plant.

Although the fossil record of conifers and taxads is difficult to interpret, it is clear that they reached their maximum diversification during the Mesozoic.

The ginkgos also appeared in Mesozoic landscapes. These gymnosperms are small to large and are slow-growing trees. Each individual is either male or female, bearing small reproductive structures of one sex or the other. The leaves are quite distinctive, having a fan shape with parallel veins and the outer margin split or entire. Ginkgos were very common during the Mesozoic all over the world.

The major evolutionary innovation of plant communities during the late Mesozoic was the appearance and rapid radiation of the flowering plants, the angiosperms. By the beginning of the late Cretaceous period, angiosperms were abundant on a worldwide basis. The evolution of petals or distinctive smells and nectar to attract insects can be viewed as part of the selective process that led to the evolution of more complex flowers. The advent of flowering plants during the Cretaceous led directly to the tremendous increase in abundance and variety of living insects. Many species of flies, beetles, bees, wasps, and butterflies have life cycles that are controlled by, and dependent on, specific kinds of flowering plants.

By the late Cretaceous Period, angiosperms had become the most diverse and floristically dominant group as evidenced by the composition of numerous macrofossils and pollen floras.

The following lists some of the common plant genera of the Mesozoic Era:

This Mesozoic flora was the vegetation eaten by dinosaurs, other reptiles, and mammal herbivores during this era.

Bibliography

Ausich, W. I. and N. G. Lane, 1999. Life of the Past, 4th ed. Prentice Hall, 321 pp.

Parker, S. and R. L. Berner, 1990. The Practical Paleontologist. Simon and Schuster, Inc., 159 pp.

Stewart, W. N. and G. W. Rothwell, 1993. Paleobotany and the Evolution of Plants, 2nd ed. Cambridge University Press, 521 pp.

Walker, C. and D. Ward, 1992. Fossils. Dorling Kendersley, 320 pp.