'' (
Corystospermales/Umkomasiales, top right), in an Early Triassic Australian landscape The concept of pteridosperms goes back to the late 19th century when
palaeobotanists came to realise that many Carboniferous fossils resembling fern fronds had anatomical features more reminiscent of the modern-day seed plants, the
cycads. In 1899 the German palaeobotanist
Henry Potonié coined the term "
Cycadofilices" ("cycad-ferns") for such fossils, suggesting that they were a group of non-seed plants intermediate between the
ferns and cycads. Shortly afterwards, the British palaeobotanists
Frank Oliver and
Dukinfield Henry Scott (with the assistance of Oliver's student at the time,
Marie Stopes) made the critical discovery that some of these fronds (genus
Lyginopteris) were associated with seeds (genus
Lagenostoma) that had identical and very distinctive glandular hairs, and concluded that both fronds and seeds belonged to the same plant. Soon, additional evidence came to light suggesting that seeds were also attached to the Carboniferous fern-like fronds
Dicksonites,
Neuropteris and
Aneimites. Initially it was still thought that they were "
transitional fossils" intermediate between the ferns and cycads, and especially in the English-speaking world they were referred to as "seed ferns" or "pteridosperms". Today, despite being regarded by most palaeobotanists as only distantly related to ferns, these spurious names have nonetheless established themselves. Nowadays, four orders of Palaeozoic seed plants tend to be referred to as pteridosperms:
Lyginopteridales,
Medullosales,
Callistophytales and
Peltaspermales, with "Mesozoic seed ferns" including the
Petriellales,
Corystospermales and
Caytoniales. Their discovery attracted considerable attention at the time, as the pteridosperms were the first extinct group of vascular plants to be identified solely from the fossil record. In the 19th century the Carboniferous Period was often referred to as the "Age of Ferns" but these discoveries during the first decade of the 20th century made it clear that the "Age of Pteridosperms" was perhaps a better description. During the 20th century the concept of pteridosperms was expanded to include various Mesozoic groups of seed plants with fern-like fronds, such as the
Corystospermaceae. Some palaeobotanists also included seed plant groups with entire leaves such as the
Glossopteridales and
Gigantopteridales, which was stretching the concept. In the context of modern phylogenetic models, the groups often referred to as pteridosperms appear to be liberally spread across a range of clades, and many palaeobotanists today would regard pteridosperms as little more than a
paraphyletic 'grade-group' with no common lineage. One of the few characters that may unify the group is that the
ovules were borne in a
cupule, a group of enclosing branches, but this has not been confirmed for all "pteridosperm" groups. It has been speculated that some seed fern groups may be close to the ancestry of
flowering plants (angiosperms). A 2009 study concluded that "
phylogenetic analysis techniques have surpassed the hard data needed to formulate meaningful phylogenetic hypotheses" regarding the relationships of "seed ferns" to living plant groups. ==Taxonomy==