So far we have considered the very first primitive plants that existed on Earth many hundreds of millions of years ago, and we have seen when they first appeared on land and the effects that this had. Now perhaps, we should look at the more advanced forms including most of those plants that we are so familiar with.
The group of ancient plants that forms a link between advanced and primitive plants is known as the Cordaitales which first appear in the Lower Carboniferous rocks some 300 to 350 million years ago.
This group reached the height of its development in terms of both numbers of individuals and species at the time when the last of the important coal deposits were being laid down, that is, in the Late Carboniferous times about 270 million years ago.
They could be thought of as being intermediate in type between epidodendrons mentioned earlier and modern conifers. (Remember that conifers are trees that bear cones, such as larch or Scots pine.) All the members of the Cordaitales were tree-like, and were able to withstand very dry conditions.
They had leaves that were not flattened so that less water was lost though them. The outer skin of the leaf became much thickened, and the shape of the tree was such that air becomes trapped within the branches.
In middle Permian times, about 250 million years ago, true conifers were becoming quite abundant, and almost 100 million years later in the Jurassic period they had reached their peak. It can be shown that conifers evolved directly from the Cordaitales. Evolution of the conifers occurred in two different directions. Members of the genus pinups become more and more able to withstand drying, indeed, would not survive in damp conditions.
Another group, known as the Taxus-Macrocarpa group, continued to retain the more primitive type of flattened leaf. Members of this group could not withstand drying but could survive in very poor soils where there were strong winds and high rainfall or snow-that is, mountain conditions. All conifers must live in association with a root fungus because they do not have root hairs.
There are two other groups of plants that seem to have developed from the early types, but this is not certain. They are both palm-like types, but one group, the Bennetitales became extinct during the Cretaceous period, about 100 million years ago.
The plants that you are probably most familiar with are the flowering plants. These probably evolved indirectly from the conifers at some point during the Jurassic, but they did not become important until the Tertiary, between 50 and 70 million years ago. They have not yet reached their maximum development.
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