Taxonomy
Leafless, dichotomously branching fossils bearing spines and possessing vascular tissue from the Devonian of Gaspé Peninsula, Canada, were thought by Dawson in 1859 to resemble the modern whiskfern, Psilotum. Accordingly, he named his new genus Psilophyton, the type species being P. princeps. Unfortunately, it later turned out that his description and subsequent reconstruction was based on fragments of three different unrelated plants, which caused confusion for many years. The sporangia were from Psilophyton, but some aerial stems were from what is now Sawdonia, and the rhizomes were from Taeniocrada.
In 1871, Dawson described specimens which had strong spines as P. princeps var. ornatum. He considered that groups of paired terminal sporangia found with these were part of the same plant, although no actual connection was found. Much later, in 1967, it was shown that fossils called "Psilophyton princeps" had two very different patterns of xylem development: from the centre outwards (centrarch) in P. princeps and from the outside inwards (exarch) in P. princeps var. ornatum. Hueber and Banks selected new specimens as the type for the species P. princeps, and Hueber later transferred P. princeps var. ornatum to a new genus as Sawdonia ornata. Only P. princeps had paired terminal sporangia; those of S. ornata were borne on the sides of the stems. Later additions of species to the genus have been based on the description by Hueber and Banks.
Dawson named another specimen Psilophyton robustius. On the grounds that this had side branches which divided into three as well as dividing into two, in 1956 Hopping moved it to a new genus Trimerophyton.
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