Gnetidae

Gnetidae, commonly known as gnetophytes, is a subclass of gymnosperms within the class Pinopsida. The group was previously considered the distinct class Gnetopsida within the distinct division Gnetophyta. However, nuclear phylogenomic studies recover gnetophytes as the sister group of the Pinaceae, which means that gnetophytes are nested within Pinopsida; separating them renders Pinopsida paraphyletic. Accordingly, Yang et al. (2022) treat Gnetidae as one of three subclasses of Pinopsida, together with Pinidae and Cupressidae. The group comprises three morphologically distinct extant genera, each the sole living genus of its family, Ephedra (Ephedraceae), Gnetum (Gnetaceae), and Welwitschia (Welwitschiaceae). It has also been treated as a broadly circumscribed order, Gnetales, though Yang et al. (2022) and WFO Plant List divide Gnetidae into three orders (Ephedrales, Gnetales sensu stricto, and Welwitschiales) each containing a single extant family and genus. Within the subclass, Ephedra is sister to a clade formed by Gnetum and Welwitschia. The earliest unambiguous fossils attributable to gnetophytes date to the Late Jurassic, and the group reached its greatest known diversity during the Early Cretaceous. Gnetophytes are distinguished from other living gymnosperms by the presence of vessel elements in the secondary xylem. This and other angiosperm-like features once supported hypotheses of a close relationship with flowering plants, but recent phylogenomic analyses indicate that these similarities evolved independently.