EVOLUTION OF PLANTS

In the coming posts, I shall present examples of certain specific groups of flowering plants. First, however, I need to "set the stage" and briefly discuss in this current post, the earliest history of the various kinds of flowering plants. In the following text, I mention geologic time periods. To see a quick visual summary of these intervals, please see one of my earliest posts; namely, "GEO TIME SCALE," which was posted in November, 2014. Or, you can look at any historical geology textbook (or go to the online source "Wikipedia") to view the succession of the various periods of geologic times.

Paleozoic Time: Based on fossil spores, the first plants occurred on land about 470-460 mya (million years ago) during middle Ordovician time. By Silurian time, the first vascular land plants occurred, and most of them lived in the Northern Hemisphere. During Devonian time, colonization of the land was well underway. Early Devonian plants, however, had no leaves. By the Late Devonian, forests of large primitive plants (including ferns) appeared.

Mesozoic Time: Lycophytes, cycads, ginkgos appeared; with conifers and many new ferns evolving during Jurassic time. 

FORMATION OF GONDWANALAND (TECTONIC PLATES                IN ACTION=ON THE MOVE!)

During mid early Jurassic time (about 190 m.y.a. [million years ago], most of the continents were in a land mass called Gondwanaland, which lasted until middle Cretaceous time (which was about 100 m.y.a.). During subsequent younger geologic time intervals, Gondwanaland broke apart and eventually Earth’s Southern Hemisphere looked similar to that of today.                 

Figure 1. Jurassic paleogeographic map of the world, when the supercontinent Gondwanaland formed.

During the 19^th century, late Paleozoic coal deposits of India, South Africa, Australia, South America, Antarctica, and New Zealand, as well as the Falkland Islands (in the Atlantic Ocean, east of Argentina), were found to contain fossil plants collectively designated as the “Glossopteris Flora,” a name of the most conspicuous genus, which was a variety of seed fern, called Glossopteris (about 265 million years old = late Permian age.) The presence of this fossil flora, which is found nowadays on widely separated land masses, was one of the major facts that lead to the concept that all of these land masses once were connected in what is now referred to as “Gondwanaland.”  This concept has now been well established as fact via the discovery of other land fossils (including mammal-like reptiles) and paleomagnetism (Stanley, 1999).

The oldest known lineages of flowering plants [= angiosperms] evolved about 140 to 125 m.y.a. [middle Early Cretaceous time]. Flowering plants “bloomed” with the appearance of pollinating insects about 100 m.y.a. [at the beginning of Late Cretaceous time]. In sum, plant evolution was driven by plate tectonics—firstly by continents colliding together and secondly by continents moving apart. When Gondwanaland separated into four continents---[South America, Africa, Australia, and Antarctica]---each of these areas became increasingly isolated, and each took on its own types of genetic diversification caused by these isolations. 

Figure 2. Outline of a Glossopteris late Permian seed-fern leaf (2 inches long = 5 cm].  These leaves were thick, fleshy fibrous, and had a minute-network of parallel lines and groves on their surface. Their presence in the southern continents confirm that these lands were once united into a single supercontinent.

REFERENCE

Stanley, S.M. 1999. Earth System History. W.H. Freeman and Company. New York. 615 pp.