For example, "Lycopodiophyta" and the shorter "Lycophyta" as well as the informal "lycophyte" may be used to include the extinct zosterophylls or to exclude them.

They are defined by two synapomorphies: lateral rather than terminal sporangia (often kidney-shaped or reniform), and exarch protosteles, in which the protoxylem is outside the metaxylem rather than vice versa.

The extinct genus Asteroxylon represents a transition between these two groups: it has a vascular trace leaving the central protostele, but this extends only to the base of the enation.

Zosterophylls and extant lycophytes are all relatively small plants, but some extinct species, such as the Lepidodendrales, were tree-like, and formed extensive forests that dominated the landscape and contributed to the formation of coal.

Their cladogram for the lycophytes is reproduced below (with some branches collapsed into 'basal groups' to reduce the size of the diagram).

[14] Cooksonia cambrensis, Renalia, Sartilmania, Uskiella, Yunia † Hicklingia Adoketophyton, Discalis, Distichophytum (=Rebuchia), Gumuia, Huia, Zosterophyllum myretonianum, Z. llanoveranum, Z. fertile Zosterophyllum divaricatum, Tarella, Oricilla, Gosslingia, Hsua, Thrinkophyton, Protobarinophyton, Barinophyton obscurum, B. citrulliforme, Sawdonia, Deheubarthia, Konioria, Anisophyton, Serrulacaulis, Crenaticaulis Nothia, Zosterophyllum deciduum extant and extinct members In this view, the "zosterophylls" comprise a paraphyletic group, ranging from forms like Hicklingia, which had bare stems,[15] to forms like Sawdonia and Nothia, whose stems are covered with unvascularized spines or enations.

Their preferred cladogram shows the zosterophylls and associated genera basal to both the lycopodiopsids and the euphyllophytes, so that there is no clade corresponding to the broadly defined group of lycophytes used by other authors.

Asteroxylon, identified as an early lycopodiopsid, had vascular traces that extended to the base of the enations.