Abstract:
Reexamination of fossilized plant material from the westernmost Pennsylvanian-age wetland flora in North America reveals that material of Pecopteris oregonensis Arnold represents a filicalean fern frond with annulate sporangia and anatomically preserved vascular tissues of the rachis. The frond, which is redescribed as Senftenbergia oregonensis (Arnold) Hillier et Rothwell comb. nov., is pinnately compound, with five orders of catadromous dissection, and it appears to have reached a length of 80 cm or more. The rachis displays an anchor-shaped xylem bundle, and dichotomizing aphlebiae are produced at the base of primary pinnae. The rachis, primary pinnae, and secondary pinnae have a dense covering of coarse trichomes. Secondary and tertiary pinnae display a length:width ratio of 2.5:1. The pecopterid pinnules measure 1.5-6 mm in length and 1.0-2.0 mm in width, with dichotomous venation, smooth-to-undulating margins, and a rounded tip. Sporangia are marginal and recurved under the abaxial pinnule surface to form two rows that are parallel to the midvein. Each sporangium has a narrow stalk, longitudinal dehiscence, and a terminal multiseriate annulus. The fern is assigned to the Tedeleaceae, and a combination of characters for distinguishing genera within the family is proposed.
