Gnetidae

Gnetidae, called gnetophytes, is a subclass of gymnosperms under the class Pinopsida. The group includes three living genera that look very different from one another: *Ephedra*, *Gnetum*, and *Welwitschia*. Each is the only surviving genus in its own family. Studies of nuclear genomes show that gnetophytes are the sister group of the pine family (Pinaceae), meaning they sit inside Pinopsida; if you separate them out, Pinopsida becomes paraphyletic. Because of this, Yang et al. (2022) placed Gnetidae as one of three subclasses of Pinopsida, alongside Pinidae and Cupressidae. The group has also been treated as a single order, Gnetales, but Yang et al. (2022) and the WFO Plant List split it into three orders—Ephedrales, Gnetales *sensu stricto*, and Welwitschiales—each with one living family and genus. Within the subclass, *Ephedra* is the sister group to a clade made up of *Gnetum* and *Welwitschia*.

The oldest clear gnetophyte fossils come from the Late Jurassic, and the group was most diverse in the Early Cretaceous. Unlike other living gymnosperms, gnetophytes have vessel elements in their secondary xylem. This trait, along with other features that resemble those of flowering plants, once led scientists to think gnetophytes were closely related to angiosperms, but recent phylogenomic work shows these similarities evolved independently.

Finding common traits across all gnetophytes is hard. Two features often used are the presence of enveloping bracts around both ovules and microsporangia, and a micropylar projection of the ovule’s outer membrane that produces a pollination droplet—though these are quite specific. L. M. Bowe called the three genera a “bizarre and enigmatic” trio because each is so specialized to its environment that they barely look alike. *Gnetum* species are mostly woody vines in tropical forests, except *Gnetum gnemon*, a tree from western Malesia. *Welwitschia mirabilis*, found only in the dry deserts of Namibia and Angola, is a ground-hugging plant with just two large strap-like leaves that keep growing from the base throughout its life. *Ephedra* species, widespread in dry climates, have long slender branches with tiny scale-like leaves at the nodes.

Classification has been tricky with only three well-defined genera in the whole group. Early in the 20th century, Pearson discussed the class Gnetales rather than the order. G. H. M. Lawrence called them an order but noted the three families were distinct enough to be separate orders. Foster and Gifford agreed and placed the three orders together in a class they named Gnetopsida. The evolutionary relationships among seed plants remain unresolved, and gnetophytes have been central to phylogenetic debates. Molecular studies of living gymnosperms have clashed with morphological evidence over whether gymnosperms (including gnetophytes) are monophyletic or paraphyletic and gave rise to angiosperms. The key question is whether gnetophytes are the sister group of angiosperms, or whether they are sister to, or nested within, other living gymnosperms. Many fossil gymnosperm groups—like Bennettitales, Caytonia, and glossopterids—were as morphologically distinct as the four living groups, and including them makes the picture even more complex. Some morphological studies have linked gnetophytes with Bennettitales and Erdtmanithecales. Other authors have argued that gnetophytes are the sister group to all other gymnosperms, contradicting the anthophyte hypothesis, which held that gnetophytes were sister to flowering plants.

Under the gnetifer hypothesis, gnetophytes are sister to conifers, and gymnosperms form a monophyletic group that is sister to angiosperms. This idea first appeared in the mid-20th century, when the vessel elements in gnetophytes were interpreted as having evolved from tracheids with circular bordered pits, like those in conifers. It gained strong support only with molecular data in the late 1990s. While most obvious morphological evidence still backs the anthophyte hypothesis, some less obvious traits shared by gnetophytes and conifers support the gnetifer hypothesis: tracheids with scalariform pits and tori mixed with annular thickenings, no scalariform pitting in primary xylem, the scale-like and strap-shaped leaves of *Ephedra* and *Welwitschia*, and reduced sporophylls.

From the early 20th century, the anthophyte hypothesis was the leading explanation for seed plant evolution, based on shared morphological features between gnetophytes and angiosperms. In this view, gnetophytes and the extinct Bennettitales are sister to the angiosperms.