One of the most important innovations in the evolution of plants was the ovule, the structure from which a seed develops. The first seeds evolved in a group of vascular plants sometimes called the pteridosperms, or "seed ferns." This name is misleading since the pteridosperms are not ferns at all, but instead are true seed plants.
At first, the pteridosperms were thought to be intermediates between ferns and modern seed plants, hence the name. H. Potonié first hypothesized the existence of a group of such transitional plants between pteridophytes and seed plants in 1899, but it is now known that ferns themselves are not the direct ancestors of seed plants. Rather, the seed ferns are a paraphyletic group representing the earliest plants with seeds.
Out of this mess of early seed plants are drawn several families. One of the most commonly recognized, yet least well-known families is Lyginopteridaceae (named by Oliver and Scott in 1904), whose members are often called by their common name, lyginopterids. Study of this group is very important for the understanding of one of the major events in plant evolution, namely the origin of the seed.
Lyginopterids are entirely extinct. Their fossils are found in coal balls in deposits ranging from the upper Devonian to Lower Carboniferous periods. The lyginopterids are thought of has vine-like plants, probably less than two meters tall, that probably grew epiphytically on other plants for structural support.
The lyginopterids' most interesting feature is their reproductive organs, for they are what make the first seed plants distinct. Lyginopterids bore cupules, which can be described as tulip shaped structures that enclosed and protected the ovules as they matured into seeds.
In addition to the pages linked above, you can read more about lyginopterids and other fossil plants at the UCMP Virtual Paleobotany Lab with images and notes for a laboratory course.
For information about collections on plants cataloged on-line, or for images, checklists, and databases, try our list of Botanical Collection Catalogs.