A new species of Psilophyton from the Lower Devonian of northern New Brunswick, Canada

Abstract

A new spiny trimerophyte is described from early to middle Emsian (Lower Devonian) strata of Atholville, New Brunswick, Canada, where the specimens occur in a secondarily silicified tuff. Psilophyton crenulatum sp.nov. is characterized by vegetative branches which are 2 mm to 8.3 cm long, twice to seven times dichotomous and terminated by slender recurved tips. Some vegetative divisions are two closely spaced dichotomies (double dichotomies) which appear as trifurcations. The central axis of the trifurcation varies from about 1 mm to 2.5 cm in length. Fertile units terminate a dichotomous axis, and are alternate and distichous or are helically arranged. They are five to seven times dichotomous in three dimensions and consistently pendulous after the second or third dichotomy. These units are covered with crenulations on the first few orders of branching and terminate in sporangia 3–5 mm long by 1–2 mm wide. Sporangia are consistently twisted around each other. In situ spores are 48–102 μm in diameter and are compared with the dispersed spore genus Apiculiretusispora. Stomata are reported but an epidermis is usually lacking in most specimens. Spines are terete in cross section, multicellular, up to 6 mm long, and undivided, bifurcate, or trifurcate. Crenulations are semicircular to oval and 100–200 μm in diameter. Vascular strands up to 1 mm in diameter are centrarch with metaxylem tracheids that have scalariform- to circular-bordered pits. Psilophyton crenulatum represents the oldest known bonafide species of Psilophyton. A reconstruction is included along with a discussion on the evolution of megaphyllous leaves, the origin of adventitious roots, and branching similarities between P. crenulatum and younger ferns.