This is a major sub-group of the gymnosperms and a full account could occupy an entire volume in itself. There are over 50 genera comprising more than 500 species; of pines alone there are at least 90 species. They are the most conspicuous of all of the gymnosperms, particularly to dwellers in the northern hemisphere. There must be few adults who are unfamiliar with pines, cedars, firs, larches, and spruces. Some of them form huge forests in the higher latitudes. Christmas trees are an important feature of life in midwinter.
The conifers appear to have evolved from what is now an extinct sub-group of gymnosperms, the principal member of which was the genus Cordaites. They first appear in rocks 320 million years old and at their peak they developed into substantial trees more than 30m tall. They became extinct about 250 million years ago. Their large leaves, confined to the upper branches, were strap-shaped and could attain a maximum length of 1m and a width of 15cm.
The reproductive structures of Cordaites are placed in the genus Cordaianthus. The pollen-bearing organs were in the form of loose shoots with, at intervals, groups of reduced leaves some of which bore sporangia. The ovules were borne on separate shoots, each rather flattened ovule having its own separate stalk. In these early ancestral conifers, there were no cones as such, these were to come later as the conifers evolved from the cordaitales. Other cordaitalean seeds are called Cardiocarpus, Kamaraspermum and Mitrospermum. Isolated portions of trunk or branch wood are placed in the genera Dadoxylon and Mesoxylon. Roots are called Amyelon.
The next sub-group of early conifers that we encounter in rocks about 290 million years old are sometimes called ‘transition conifers’, although some botanical authorities are not very enthusiastic about this phrase. Lebachia and Ernestiodendron are typical of this phase of evolution of the conifer sub-group. Members of these genera certainly looked much more like modern conifers than Cordaites did.
Externally they had a strong resemblance to species of the living Araucaria. Their reproductive structures, too, were distinctly cone-like and a number of the features of the cones of modern conifers can be seen in them. Other genera in these so-called ‘transition conifers’ show in varying degree, features that foreshadow the evolution of more advanced conifers as the millions of years passed by.
Pseudovoltzia, Ullmannia, Glyptolepis, Voltzia.
These early conifers became extinct about 175 million years ago and it is evident that by that time they had already given rise to all of the familiar sub-groups of modern conifers. Early members of such sub-groups appeared about 230 million years ago and by the time that the flowering plants were becoming well established all of the modern types of conifers had emerged.
Many genera have been established to classify the diverse types of conifer leaves that are found as fossils. The following selection illustrates how in some cases the names are chosen so as to reflect their resemblance to modern conifer types.
Brachyphyllum, Pagiophyllum, Cyparissidium, Geinitzia, Elatocladus, Cupressinocladus, Pityocladus, Podozamites.
Isolated cones are usually placed in the genus Pityostrobus unless an author considers that a particular cone is so similar to that of a modern genus that it can reasonably be assigned there. Many Pityostrobus cones have features which may appear in the cones of a variety of modern conifers. Presumably, with the passage of time, the genera of modern conifers gradually separated out until at the present day they each have their own characteristic cones.
In our history of plants we are now about 141 million years from the present day. The Earth’s floras consisted largely of ferns, cycads, cycadeoids, conifers and ginkgos, with a few remaining clubmosses and horsetails. Of course, algae still continued to flourish in the salt and freshwater areas of the world; mosses and liverworts lived wherever suitable habitats existed and fungi lived on the dead remains of plants and animals; some fungi, no doubt, were also parasites on living things as they are now.