Psilophyton

Psilophyton Temporal range: Devonian
Reconstruction of Psilophyton forbesii; scale bar = 5 cm
Scientific classification
Kingdom:	Plantae
Class:	†Trimerophytopsida
Order:	†Trimerophytales
Family:	†Trimerophytaceae
Genus:	†Psilophyton Dawson (1859) emend. Hueber & H.P.Banks (1967)
Species
P. burnotense (Gilkinet) Kräusel & Weyland P. charientos Gensel (1979) P. coniculum Trant & Gensel (1985) P. crenulatum Doran (1980) P. dapsile Kasper et al. (1974) P. dawsonii H.P.Banks et al. (1975) P. forbesii Andrews et al. (1968) P. genseliae Gerienne (1997) P. krauselii Obrhel (1959) P. microspinosum Kasper et al. (1974) P. parvulum Gerienne (1995) P. princeps Dawson (1859) P. primitivum Hao & Gensel (1998) P. szaferi Zdebska (1986)

Psilophyton is a genus of extinct vascular plants. Described in 1859, it was one of the first fossil plants to be found which was of Devonian age (about 420 to 360 million years ago). Specimens have been found in northern Maine, USA; Gaspé Bay, Quebec and New Brunswick, Canada; the Czech Republic; and Yunnan, China. Plants lacked leaves or true roots; spore-forming organs or sporangia were borne on the ends of branched clusters. It is significantly more complex than some other plants of comparable age (e.g. Rhynia) and is thought to be part of the group from within which the modern ferns and seed plants evolved.

Almost all the species of Psilophyton have been found in rocks of Emsian age (around 408 to 393 million years ago). One exception is P. krauselii, from the Czech Republic, which is younger, being from the upper part of the Middle Devonian (around 390 to 380 million years ago).

Psilophyton dawsonii is the best-known species. Compressed and mineralized specimens have been found in several locations, particularly in calcareous pebbles on the Gaspé Peninsula, Quebec, Canada. Plants consisted of bare stems (axes) ending in blunt tips. Lower down they repeatedly branched dichotomously; higher up they bore sporangium-bearing 'units' in two rows on opposite sides of the stems. These units branched, also dichotomously, before terminating in sporangia, so that there were clusters of up to 128 paired, downward curved sporangia, oval in shape and about 5 mm long. Spores were released through a longitudinal slit. The first two branching points of the fertile units appear to have consisted of two closely spaced dichotomous branches in which the middle branch did not develop. The trilete spores were between 40 and 75 µm in diameter.

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