Gymnospermae ('naked seeds') include four classes, the cycads, ginkgos, conifers and Gnetales. They are plants that produce ovules on an open sporophyll (usually in a cone) rather than enclosed within a carpel as in angiosperms. This is a plesiomorphic character for seed plants, and traditionally the gymnosperms have been thought to be paraphyletic with regard to the angiosperms (flowering plants) with the Gnetales as sister to angiosperms. However, recent molecular trees unexpectedly recover the Recent gymnosperms as a monophyletic group, as shown below. Cantino et al. (2007) suggested the use of the name "Acrogymnospermae" for this clade, to distinguish it from the traditional usage of the name Gymnospermae. This has significant implications for our understanding of reproductive evolution in the seed plants. See Friedman and Floyd (2001) for a review of the alternative phylogenies. Notably, double fertilisation (previously believed a uniting character of Gnetales and angiosperms) may be more widespread among seed plants than previously believed, highlighting the potential dangers of extrapolating observations on a few exemplars to cover larger taxa.
A number of fossil taxa have been included in the crown seed plants in the past (see Doyle  for a review), such as corystosperms, Bennettitales, Glossopteridales and Caytonia. It is uncertain whether these fossil 'gymnosperms' would fall within a monophyletic clade of modern gymnosperms. The large gap in the fossil record between the earliest definite crown Acrogymnospermae (in the Palaeozoic) and the earliest angiosperms (in the early Cretaceous - Sun et al., 2002) suggests that at least some of the fossil 'gymnosperms' might be expected to lie on the angiosperm stem. In particular, Caytonia and the Bennettitales have been nominated by different authors as angiosperm ancestors.
Cantino, P. D., J. A. Doyle, S. W. Graham, W. S. Judd, R. G. Olmstead, D. E. Soltis, P. S. Soltis & M. J. Donoghue. 2007. Towards a phylogenetic nomenclature of Tracheophyta. Taxon 56 (3): E1-E44.
Doyle, J. A. 1998. Phylogeny of vascular plants. Annual Review of Ecology and Systematics 29: 567-599.
Friedman, W. E., & S. K. Floyd. 2001. Perspective: The origin of flowering plants and their reproductive biology – a tale of two phylogenies. Evolution 55 (2): 217-231.
Sun, G., Q. Ji, D. L. Dilcher, S. Zheng, K. C. Nixon & X. Wang. 2002. Archaefructaceae, a new basal angiosperm family. Science 296: 899-904.