The progymnosperms are an extinct group of woody, spore-bearing plants that is presumed to have evolved from the trimerophytes, and eventually gave rise to the gymnosperms. They have been treated formally at the rank of division Progymnospermophyta or class Progymnospermopsida (as opposite). The stratigraphically oldest known examples belong to the Middle Devonian order the Aneurophytales, with forms such as Protopteridium, in which the vegetative organs consisted of relatively loose clusters of axes.Tetraxylopteris is another example of a genus lacking leaves. In more advanced aneurophytaleans such as Aneurophyton these vegetative organs started to look rather more like fronds, and eventually during Late Devonian times the aneurophytaleans are presumed to have given rise to the pteridosperm order, the Lyginopteridales. In Late Devonian times, another group of progymnosperms gave rise to the first really large trees known as Archaeopteris.
^Stewart, W.N.; Rothwell, G.W. (1993). Paleobiology and the evolution of plants. Cambridge University Press. p. 521pp.
^Lang, W. H. (1925). "Contributions to the study of the Old Red Sandstone flora of Scotland. I. On plant-remains from the fish-beds of Cromarty. II. On a sporangium-bearing branch-system from the Stromness Beds." Transactions of the Royal Society of Edinburgh,54: 253-279.
^Serlin, B. S. & Banks, H. P. (1978). "Morphology and anatomy of Aneurophyton, a progymnosperm from the Late Devonian of New York. Palaeontographica Americana,8: 343-359.