Seed ferns (Pteridospermatophyta) range from the Devonian to the Cretaceous. They flourished from the Carboniferous to the Permian. Pteridosperms had fern-like foliage, but reproduced with seeds (Selmeier, 1996, p. 142). Seed ferns exhibited both vine-like and arborescent forms. The term pteridosperm ("seed fern") is descriptive but, misleading as these plants are really early gymnosperms (Cleal & Thomas, 2009, p. 139). Seed ferns actually represent several distinct gymnosperm groups.

Seed Fern Characteristics

Without the knowledge of reproductive structures the foliage of seed ferns is difficult to distinguish from ferns. Fronds are pinnately divided as in the ferns. However, fertile fronds possess pollen organs or ovules instead of spore producing structures. Fern spores are wind dispersed. Medullosans pollen grains, at 0.3 mm in diameter, were five times larger than average. This has led some paleontologists to suspect that medullosans were pollinated by insects (Kenrick & Davis, 2004, p. 93). Seeds were grouped on special branches or positioned on leaves along the midrib or margins. The trunks of many seed ferns consisted of separate vascular segments in the form of wedges (polystele). In some species the wedges produced secondary xylem (wood) only to the inside, others produced wood towards the inside and outside, still others produced wood all the way around the wedge. There is a distinct evolutionary trend in the number of vascular bundles embedded in the pith along with the position of peripheral vascular bundles. Over time, a single, deeply divided vascular bundle evolved into three or more. Outer vascular bundles became fused, forming rings of secondary wood by the Lower Permian (Jung, 1996, p. 158). Glossopteris is a seed fern with a eustele vascular bundle (concentric vascular bundles with enclosed pith), which is characteristic of conifers and angiosperms. Both the polystele and eustele wood of seed ferns was composed of conifer-like wood.

Seed Fern Trees

Among the arborescent seed ferns Medullosa was the largest at 10m (Willis & McElwain, 2002, p. 112). Medullosa had a crown of large fronds. Fronds were enormous reaching a length of 7 meters. Fronds were pinnately divided, dichotomously branched, and spirally arranged around the stem. The trunk was composed of vascular segments. Secondary xylem formed towards the inside of primary xylem (opposite of living plants). The secondary xylem was surrounded by the vascular cambium, secondary phloem, and cortical tissues. The external part of the lower trunk was clothed in large prop roots. Medullosa seemed to prefer mineral rich soils to peaty substrates. Medullosa lived in fire prone parts of the mire where its resin rich wood and abundance of foliage acted as a good source of fuel (Kenrick & Davis, 2004, p. 93).

Neuropteris and Alethopteris are common seed fern fronds. Whittleseya, Dolerotheca, and Aulacotheca are pollen blossoms. Trigonocarpus is the cast of large pteridosperm seeds. Pachytesta is the permineralized form of these seeds. Myeloxylon is the frond base of Medullosa and is often found attached to the exterior of the lower trunk (Jung, 1996, p. 158).

Science Olmpiad Fossil Event

The 2016 Science Olympiad Fossil List includes the genus Glossopteris within the phylum Pteridospermatophyta (Seed Ferns).

Cleal C.J. & Thomas, B.A. (2009). Introduction to Plant Fossils. United Kingdom: Cambridge University Press.

Jung, W. (1996). Ferns, Cycads, or What? In Dernbach, U. Petrified Forest: The World's 31 Most Beautiful Petrified Forests (pp. 155-159). Germany: D’ORO Publishers.

Kenrick, P. and Davis, P. (2004). Fossil Plants. Smithsonian Books: Washington.

Selmeier, A. (1996). Identification of Petrified Wood Made Easy. In Dernbach, U. Petrified Forest: The World's 31 Most Beautiful Petrified Forests (pp. 136-147). Germany: D’ORO Publishers.

Willis, K.J. & McElwain, J.C. (2002). The Evolution of Plants. New York: Oxford University Press.