Evolution and fossil record

The gnetophytes have long been believed to be the closest living relatives of the flowering plants (Doyle and Donoghue, 1986, 1992;

Donoghue and Doyle, 2000), although a number of recent molecular studies dispute this relationship (Winter et al, 1999; Pryer et al, 2001; Rydin et al., 2002; Schmidt and Schneider-Poetsch, 2002; Soltis et al., 2002). These studies do not, however, agree on the position of the Gnetophyta in the evolution of the seed plants, and this remains at present an unresolved question. The fossil record is meagre, with records of pollen grains that resemble Ephedra and Welwitschia from the Triassic and Cretaceous. Welwitschia-like fossil cones are known from the Late Triassic (Cornet, 1996). Fossil sporophyll structures associated with ephedroid pollen are known from the Jurassic (van Konijnenburg-van Cittert, 1992), and may represent gneto-phyte progenitors. Leaf fossils with Gnetum-like venation patterns are known from the Early Cretaceous (Crane and Upchurch, 1987). This age is compatible with the placement of the Gnetophyta as sister to the angiosperms rather than in a monophyletic Gymnospermae.

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