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." 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." 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. In 2013, Hao and Xue returned to the earlier definition. Their class Rhyniopsida (rhyniopsids) is defined by the presence of sporangia that terminate isotomous branching systems (i.e. the plants have branching patterns in which the branches are equally sized, rather than one branch dominating, like the trunk of a tree). The shape and symmetry of the sporangia was then used to divide up the group. Rhynialeans (order Rhyniales), such as
Rhynia gwynne-vaughanii,
Stockmansella and
Huvenia, had radially symmetrical sporangia that were longer than wide and possessed vascular tissue with S-type tracheids. Cooksonioids, such as
Cooksonia pertoni,
C. paranensis and
C. hemisphaerica, had radially symmetrical or trumpet-shaped sporangia, without clear evidence of vascular tissue. Renalioids, such as
Aberlemnia,
Cooksonia crassiparietilis and
Renalia had bilaterally symmetrical sporangia and
protosteles. ==Taxonomy==