The rhyniophytes are a group of extinct early vascular plants that are considered to be similar to the genus Rhynia, found in the Early Devonian (around 419 to 393 million years ago). Sources vary in the name and rank used for this group, some treating it as the class Rhyniopsida, others as the subdivision Rhyniophytina or the division Rhyniophyta. The first definition of the group, under the name Rhyniophytina, was by Banks,[2]: 8 since when there have been many redefinitions,[1]: 96–97 including by Banks himself. "As a result, the Rhyniophytina have slowly dissolved into a heterogeneous collection of plants ... the group contains only one species on which all authors agree: the type species Rhynia gwynne-vaughanii".[1]: 94 When defined very broadly, the group consists of plants with dichotomously branched, naked aerial axes ("stems") with terminal spore-bearing structures (sporangia).[3]: 227 The rhyniophytes are considered to be stem group tracheophytes (vascular plants).
The group was described as a subdivision of the division Tracheophyta by Harlan Parker Banks in 1968 under the name Rhyniophytina. The original definition was: "plants with naked (lacking emergences), dichotomizing axes bearing sporangia that are terminal, usually fusiform and may dehisce longitudinally; they are diminutive plants and, in so far as is known, have a small terete xylem strand with a central protoxylem."[2]: 8 [4] With this definition, they are polysporangiophytes, since their sporophytes consisted of branched stems bearing sporangia (spore-forming organs). They lacked leaves or true roots but did have simple vascular tissue. Informally, they are often called rhyniophytes or, as mentioned below, rhyniophytoids.
However, as originally circumscribed, the group was found not to be monophyletic since some of its members are now known to lack vascular tissue. The definition that seems to be used most often now is that of D. Edwards and D.S. Edwards: "plants with smooth axes, lacking well-defined spines or leaves, showing a variety of branching patterns that may be isotomous, anisotomous, pseudomonopodial or adventitious. Elongate to globose sporangia were terminal on main axes or on lateral systems showing limited branching. It seems probable that the xylem, comprising a solid strand of tracheids, was centrarch."[5]: 216 However, Edwards and Edwards also decided to include rhyniophytoids, plants which "look like rhyniophytes, but cannot be assigned unequivocally to that group because of inadequate anatomical preservation", but exclude plants like Aglaophyton and Horneophyton which definitely do not possess tracheids.[5]: 214–215
In 1966, slightly before Banks created the subdivision, the group was treated as a division under the name Rhyniophyta.[6] Taylor et al. in their book Paleobotany use Rhyniophyta as a formal taxon,[3]: 1028 but with a loose definition: plants "characterized by dichotomously branched, naked aerial axes with terminal sporangia".[3]: 227 They thus include under "other rhyniophytes" plants apparently without vascular tissue.[3]: 246ff
In 2010, the name paratracheophytes was suggested, to distinguish such plants from 'true' tracheophytes or eutracheophytes.[7]