Ferns are homosporous, they produce one asexual spore. After it lands on a suitable moist medium, it produces both male and female gametes of similar genetic makeup. After they combine, the new fern that is produced from the prothallus is for the most part a clone of the parent plant. Sexual reproduction is a huge advantage because it allows considerable genetic variation in characteristics. 

In times of severe environmental change, this can mean the difference between survival and extinction. The Cycadofilicales (=Pteridospermales) were the first to transition from asexual reproduction to producing true seeds. The seeds of Lyginopteris were small and formed at the tips of short stalks at the ends of the leaves. Neuropteris was closer to the cycad pattern of reproduction. Its seeds were large and carried in a terminal crown of leaves. In appearance, this species resembled a tree fern and, judging from its numbers in fossil deposits, was very successful in its time.

In the painting above, a Carboniferous- period meadow is dominated by a forest of araucarians and Neuropteris, a seed fern and a distant precursor to the modern cycad. Horsetails and other fern allies form the undergrowth. The large horned animals approaching the water hole are Estemmosuchus. Members of the therapsids, these herbivores were probably warm-blooded. Dinosaurs would not arise until after the therapsids had vanished in a series of worldwide extinctions, which brought forth a whole new set of plant and animal players of the Mesozoic Era.

Ptilophylum, a compound fern leaf fossil

The Carboniferous, known variously as the "Age of Ferns", and the "Coal Age", was a time of lowlands and immense, swampy forests of simple vascular plants like horsetails and tree ferns. For plants, it was a time of experimentation in reproductive structures. Seed-bearing plants had just appeared.