Gymnosperms include all seed-bearing plants whose ovules are exposed such that the pollen goes through the micropyle during pollination.  The seed, one of the most marvelous products of evolution, is so complex that all seed plants must be monophyletic.  The vegetative tissues, particularly the stems support that concept.  Further confirmation comes from Chaw et al. (1999) who examined relationships among all seed plants based on nuclear, chloroplast and mitochondrial similarities.  The seed habit appeared in the upper Devonian in plants that likely were related to Archaeopteris.

Pearson (1995) illustrated the seed plants as forming an early radiation event that gave rise to two main branches.  One gave rise to the rest of the Pteridospermophyta, the Cycadeoidophyta, and the Gnetophyta.  The other branch gave rise to the Coniferophyta, Ginkgoophyta, Cycadophyta, and the Anthophyta.  Curiously, the cycads in Pearson's scheme are closest to the flowering plants.  His whole phylogenetic scheme for the seed plants seems unlikely from morphology, development , and paleontology.

The molecular phylogenetic analyses of Chaw et al. (1999) and Bowe et al (2000) show the flowering plants as sisters to the gymnosperms.  Further, they show that the cycads and ginkgophytes are basal in the gymnosperm line.  An interesting outcome of the analysis is the monophyletic clade, Pinaceae + Gnetophyta.

Tudge (2000) and the Tree of Life Project illustrate similar relationships between seed plant taxa.  Both are summaries based on combinations of molecular, morphological, developmental, and paleontological data.  In these views, the cycads and ginkgophytes are types of seed ferns.  The Coniferophyta is monophyletic and sister to the ginkgophytes.  The gnetophytes are sisters to the flowering plants.

Obviously, much more work is required to sort out relationships of the groups well enough to produce a phylogenetic taxonomy that is consistent with all of the evidence.  Nevertheless, I believe that the system of Bold et al. (1987) is one of the best.  I present a taxonomic system that is a modification of that source.  Those taxa that are marked with an asterisk (*) are extinct (as are all lower taxa within that taxon).   Orders are followed by representative genera.  

PHYLUM PTERIDOSPERMOPHYTA*

CLASS LYGINOPTERIDOPSIDA

CLASS CAYTONIOPSIDA

CLASS GLOSSOPTERIDOPSIDA

PHYLUM CYCADOPHYTA

CLASS CYCADOPSIDA

PHYLUM CYCADEOIDOPHYTA*

CLASS CYCADEOIDOPSIDA

PHYLUM GINKGOPHYTA

CLASS GINKGOOPSIDA

PHYLUM CONIFEROPHYTA

CLASS CORDAITOPSIDA*

CLASS VOLTZIOPSIDA*

ORDER VOLTZIALES: Lebachia

CLASS CONIFEROPSIDA

CLASS TAXOPSIDA

PHYLUM GNETOPHYTA

CLASS GNETOPSIDA

This page maintained by Jack R. Holt.  Last modified: 04/15/03