The seed ferns developed through two different radiation events.  First, they appeared in the upper Devonian and became abundant through the Carboniferous and Permian, but suffered loss of most taxa during the great Permian extinction.  Those that survived radiated through the early Mesozoic and died out in the Cretaceous.  As their name implies, these plants had a fern-like appearance.  However, they bore ovules on the undersides of certain modified leaves.  In fact, the ovule-bearing leaves of Mesozoic seed ferns almost completely enclosed the ovules in  structures that looked remarkably like the ovularies of flowers.  The pollen-bearing structures, too, varied in structural complexity, but generally they resembled inverted umbrellas.  

The Tree of Life Project illustrates the seed ferns as a paraphyletic group that included the cycads and ginkgophytes.  Tudge (2000) illustrates a similar (but more truncate) scheme.  Pearson (1995) shows the seed ferns taking a line that was independent of all other extant groups except the gnetophytes.  I accept the relationships as presented by Bold et al. (1987) and Bierhorst (1971) and have based the following taxonomy on a modification of those sources.

I. PHYLUM CHARACTERISTICS

A. Structure

Habit: The seed ferns are conifer-like with large frond-like leaves

Pollen: Variable; grains with single slits, three slits; some saccate. Details of the microgametophyte are unknown.

Microstrobilus: Absent, pollen-bearing synangia occur on the leaves.

Seeds: Details unknown.

Megastrobilus: Absent, ovules are borne on the leaves.

Stems: Generally woody, secondary xylem with bordered pits on the tracheids; large leaf traces.

Leaves: Large and frond-like.

Roots: Cycad-like.

B. Ecology: They are all extinct.

II. TAXONOMY: As I have defined this phylum, the PTERIDOSPERMOPHYTA has 3 classes and 7 orders. This classification is a modification of Scagel et al. (1984) and Bold et al. (1987).

CLASS LYGINOPTERIDOPSIDA

These plants have a fossil history which ranges from the upper Devonian to the Permian. They have seeds in cupules and synangiate pollen organs. This may be an artificial group.

ORDER LYGINOPTERIDALES

These plants are common in the Carboniferous. They have stems up to 3 cm in diameter which contain pith with vascular bundles. The primary xylem is mesarch. The leaves are up to .5 m long and have dichotomous venation. The seeds are barrel-shaped (about 5mm long) and are surrounded by a single integument. The beak of the nucellus (megasporangium) forms a pollen chamber. The seed occurs in a cupule.

example: Lyginopteris

ORDER MEDULLOSALES

These plants are common in the Carboniferous. They have polystelic stems with a trunk up to 5m tall with a spray of fronds at the top. Leaves have sunken stomata. The seeds have an integument of several layers and form a micropyle. The nucellus ids free from the integuments. The pollen-bearing structures occur in a cup. Pollen wall has a single long suture.

example: Medullosa

ORDER CALLISTOPHYTALES

These plants are found in the Carboniferous and Permian periods. They have stems which are relatively thin (1.8-0.3cm in diameter) with a solid pith. Secondary tissue is well developed. The seeds are small with 3 integument layers (the nucellus is free). Pollen is saccate.

example: Callistophyton

CLASS CAYTONIOPSIDA

These plants occur in the Mesozoic and have recurved cupules in which the seeds occur.

ORDER CORYSTOSPERMALES

These plants occur in the Triassic. They have small fern-like leaves. The pollen is saccate; the pollen organs resemble those of Lyginopteris.

example: Umkomasia

ORDER PELTASPERMALES

These plants occur in the Permian and Triassic. Seeds occur on the lower surface of an umbrella-like organ.

example: Lepidopteris

ORDER CAYTONIALES

These plants occur in the Triassic to the Cretaceous. Seeds occur in circular sacs with slits. Pollen is small and has 2 bladders.

example: Caytonia

CLASS GLOSSOPTERIDOPSIDA

These plants occur from the Permian to the Triassic. They have a cluster of seeds attached to a common receptacle in the axil of certain leaves. This class has a single order: ORDER GLOSSOPTERIDALES

example: Glossopteris

revised: 04/15/2003