The lyginopterids have usually been placed into a larger group known as the Pteridospermales, or "seed ferns". The name is misleading since the "seed ferns" were not ferns at all, but an assemblage of the earliest seed plants, who happened to have fern-like foliage. Modern botanists no longer favor the term "pteridopserm" because the group of plants to which it refers is not a monophyletic group, but instead is a mess of several early seed plant groups from which several modern groups of seed plants later evolved.
The most familiar lyginopterids belong to the family Lyginopteridaceae, though there are also species classified in the families Calamopityaceae and Hydraspermaceae. It is likely that these groups form a grade, or paraphyletic group, at the base of the seed plants. The lyginopterids in the broadest sense includes all the early seed plants, including the earliest known seed plant Elkinsia polymorpha. However, because the lyginopterids used in this sense is not a clade, it is likely that in the near future the definition of the group will change again.
There are alternate naming schemes still found in textbooks. For example, there are those who put the lyginopterids within Class Pteridospermidopsida (seed ferns), Order Lyginopteridales. This arrangement is unsatisfactory because "seed ferns" are a series of unrelated clades progressively closer to more modern plants. Pteridospermatopsida does not include these descendent groups. Also, some workers place Lyginopteridaceae within the Class Gymnospermatopsida. However, "gymnosperms" is also a paraphyletic group characterized by a plesiomorphy of all seed plants - the lack of an ovary.
The sister group to the lyginopterids is a clade containing the Medullosceae (another group of former "seed ferns") and all other seed plants. Lyginopterids themselves are characterized by several traits (see Morphology) of the vascular system, ovules, and microgametophyte generation (pollen).
The intrafamilial relationships of the lyginopterids are even more difficult to reconstruct with certainty. The three best-known genera are Lyginopteris (the type genus), Heterangium, and Laceya. However, most of the members of this family are not well known as fossils, making a cladistic evaluation difficult. Another problem is the practice paleobotanists have of giving different generic names to plant parts found separately. For example, when the family was first named in 1904, two workers (Oliver and Scott) based their definition of the group after piecing together the genus Lyginopteris from isolated stems, seeds, pollen, leaves, roots, and ovules, all of which had originally been given different generic names. Without their important work of recognizing the biological relationship of these parts, we might not know of the lyginopterids' existence.
For more information on seed plant relationships, visit the
seed plants page on the
Tree of Life.