Ferns

Most botanists believe that the ferns and fern allies are descendants of the Rhyniopsida, an extinct group of free-sporing plants which originated in the Silurian period (about 430 million years ago) and went extinct in the mid-Devonian period (about 370 million years ago). The Rhyniopsida were primitive vascular plants which were photosynthetic, had branched stems, and produced sporangia at their stem tips, but had no leaves or roots.

The Cladoxylales is a group of plants known colloquially as the "pre-ferns." They also evolved from the Rhyniopsida, but went extinct in the lower Carboniferous period (about 340 million years ago). Some botanists previously considered these as ancestors of the ferns, because they had leaves somewhat similar to fern fronds. However, most botanists now believe the evolutionary line which led to the Cladoxylales went extinct, and that the modern ferns evolved from a separate lineage of the Rhyniopsida.

As a group, the ferns were the first plants to have megaphylls. A megaphyll is a leaf with a complex system of branched veins. Many botanists believe that the ferns evolved megaphylls by developing a flattened and webbed version of the simple, three-dimensional branching system of the Rhyniopsida. The evolution of the megaphyll was a major event in plant evolution, and nearly all ecologically dominant plants in the modern world have megaphylls.