Cooksonia (classified in Rhyniophytina) is the oldest and most primitive known vascular land plant (specimens in catalog accompanying this main page), found in the Upper Silurian of Wales, Scotland, Ireland, Czechoslovakia, Kazakhstan, Siberia, New York, and Ontario. This plant has a slender dichotomously branched stem with terminal sporangia. The flora that existed in the Upper Silurian to Lower Devonian was probably an assemblage of marsh inhabitants with structural feature transitional between aquatic non-vascular and land vascular plants.

The first lycopods (club mosses) appeared during the Sieganian Stage of the Middle Lower Devonian Period (e.g. Drepanophycus and Baragwanathia). Other groups of primitive vascular land plants that made their first appearance in the Lower Devonian are: (1) Trimerophytes including Psylophyton, Dawsonites, and Pertica; (2) Rhyniophytes including Rhynia and Taeniocrada; and (3) Zosterophyllophytes including Sawdonia and Rebuchia. By the Upper Devonian the first sphenopsids (horsetails) appeared including the genera Eviostachya, Sphenophyllum, Pseudobornia, and possibly Archaeocalamites. Also in the Upper Devonian, the first Pterophytina appeared. The pterophytes include five classes that evolved all around the same time: (1) Cladoxylopsida; (2) Coenopteridopsida; (3) Filicopsida; (4) Progymnospermopsida; and (5) Pteridospermopsida. Although the true ferns (Filicales) may have evolved from the coenopterids or cladoxylopsids, none of the Devonian types should be referred to as “ferns”, which is common a mistake (Gensel & Andrews, 1984). The coenopterids become extremely abundant in the Carboniferous, although they are represented in the Upper Devonian by Rhacophyton, and some less well known types. The cladoxylopsids were short-lived, geologically speaking, becoming extinct during the Carboniferous Period.

The progymnosperms have a pteridophytic reproduction type (seed reproduction type) and gymnosperm-like stem. The fern-like genus Archaeopteris and the gymnospermous stem Callixylon were often found closely associated. In 1960, Charles Beck discovered that these two genera were one in the same, and established the new class Progymnospermopsida. These plants, which grew to al large size, are probably the precursors to many groups of gymnosperms.

One of the most important phases in plant evolution was the development of the seed (ovule) and early diversification in the class Pteridospermopsida. This class became highly diverse in the Lower Carboniferous. These plants grew to very large sizes, with evidence shown by tree stumps found in northeastern England reach a trunk diameter of 8 feet at the base. Coal swamp forests were a big feature of the Carboniferous landscape. Giant lycopods, like Lepidodendron grew as high as 100 feet or more, and were sparsely branched. The trunk was covered with linear leaves on pads of tissue called leaf cushions. These trees also had many cones covered with sporangia. Another giant was the horsetail, Calamites, which reached heights over 50 feet (a stem section can be seen in this plant catalog). Coenopterids, pteridosperms, Cordaitales, and the first conifers (include modern fir, pine, spruce, and cedar to name a few) inhabited these swampy forests.

The giant lycopods (Lepidodendron) and horsetails became less abundant toward the end of the Carboniferous and extinct in the Permian. Only one small genera of club moss is known to have survived the Permian Period and continued the club moss line in a very unspectacular way to the present. Coenopterids, Cordaitales, and Sphenophyllales became extinct at, or just before the end of the Permian Period. A major mass extinction, probably caused by several factors that included glaciation, a severe drop in sea level, and upper oceanic waters becoming very brackish. This eliminated an enormous number of marine and terrestrial animals. Where or not these events caused the extinction of many types of plants is unclear, as most were on the decline in any case. In the Permian Period three new groups of gymnospermous seed plants evolved: Cycadales; Ginkgoales; and Cycadeoidales (Bennettitales). The gymnosperms dominated the Triassic and Jurassic periods, and were adapted for survival in the very arid conditions that existed over much of the planet. Toward the end of the Jurassic Period, seed ferns were approaching extinction, and many of the modern Coniferales had become established.

The conifers were well represented in the Cretaceous by modern families, and Cycadeoidales were very abundant. The cycadeoids disappeared at the end of the Cretaceous, as they were displaced by angiosperms which became the dominant plant group. The Angiospermae appeared in the Lower Cretaceous, but their precursors are still unknown. By the Upper Cretaceous the angiosperms were very abundant and of great diversity. These plants have flowers as their reproductive structure, and the product of reproduction is a seed, like in gymnosperms. The difference between the two is that angiosperms have a protective fruit surrounding the seed, rather than on the surface of a cone scale like the gymnosperms.

In the Lower Tertiary, all modern types of conifers were existing with an overall geographic distribution much different from today. Cycadophytes and Ginkgophytes slowly became of lesser importance as the Tertiary Period progressed, and many of the modern genera of angiosperms had evolved by the Eocene Epoch. The Pleistocene glaciations led to by a trend of cooler temperate climates resulted in the final modernization of terrestrial plants.

Araucaria mirabilis

Cerro Cuadrato, Patagonia, Argentina
Triassic, about 280 million years old.
Specimen is about 10 mm (4 inches) long.