Plants of the New Albany Shale. II. Callixylon Arnoldii Sp. Nov.

Abstract

Following the comprehensive studies of Callixylon by Arnold (1930, 1931), this widespread Devonian genus became of unusual interest to morphologists because of the conspicuous similarity of its secondary wood to that of certain gymnosperms. Because of the nature of its wood, the possibility that it was a seed plant has been suggested by some workers although no positively determined seeds have beeln discovered in Devonian sediments. Recently, it has been shown to be a pteridophyte which bore the foliage that comprises the well-known genus Archaeopteris (Beek 1960a, 1960b, 1962). One of the most common genera of the New Albany shale, Callixylon is abundant, in the form of large silicified logs, in the Late Devonian Blackiston formation. It is also abundant as small phosphatized axes in the Falling Run member of the Sanderson formation which, according to Haks (1956) and Cross and Hoskins (1951), is of Early Mississippian age. There have been reports of the occurrence of two species, C. newberryi (Arnold 1931) and C. erianum (Beek 1961), in the Blackiston, and two other species, C. brownii (Hoskins and Cross 1951) and a new species (Beek 1961) in the Fallinig Run. Numerous fragments (100 or more) of secondary wood, possibly representing no more than 4 or 5 different plants, were collected at two localities: Boston, Kentucky, and Junction City, Kentucky. No primary tissues are preserved in any specimen. Thin sections were prepared for study of the wood in three planes. Because pitting is not apparent in thin sections, radial surfaces were exposed, also, by splitting specimens along radial planes, and these were studied and photographed by reflected light. The wood consists solely of tracheids and rays (Figs. 1-6). Definite growth layers are not present. The tracheids are radially storied, alnd possess the bluntly tapered ends typical of gymnosperms (Fig. 4). In tranisy-erse configuration the tracheids appear rectangular (Fig. 1), but they may also be pentagonal or hexagonal. Only their tangential width was measured since it was felt that this would more nearly reflect true genetic variation in cell size than the radial width which, in gymnospermic woods, is usually affected to a greater degree by environmental variation. A large standard deviation usually indicates a broad spread of variation in the population. This suggests, for example, that the mean value of the vertical distance between pit groups (Table I) is, in itself, not very significant as a character. Qualitatively, however, the fact that there is a much wider separation of pit groups (Figs. 2, 4) and, consequently, much less pitting per unlit wall area in this species than in any other, is highly significant, and certainly one of the most striking features of this plant. Equally conspicuous is the predominantly uniseriate arrangemiient of the bordered pits. Occasionally there is very limited biseriation (Fig. 2 at b), and very rarely more extensive biseriation (Fig. 4 at b). Although the pits are