The Sphenophyllales are known from fossils dating as early as the Devonian and through the Triassic, though most fossils come from Carboniferous strata.
In a number of ways, Sphenophyllum (pictured at right) is an atypical sphenophyte. Several characters of Sphenophyllum, such as its bifacial vascular cambium, trilobed protostele, broad wedge-shaped leaves with "I-beam" reinforcements in cross-section, and lack of peltate sporangiophores, are not found in any other known sphenophytes. These characters are more typical of other plant groups. It has been suggested that the Sphenophyllales are related to the Lycophyta (Stewart and Rothwell, 1993) or represent an independent offshoot of the trimerophytes (Stein et al., 1984).
Stewart, W.N. and Rothwell, G.W. 1993. Paleobotany and the Evolution of Plants. Second edition. Cambridge University Press, Cambridge.