Reconstruction of the frond of Laceya hibernica, a Lyginopterid pteridosperm from the uppermost Devonian of Ireland

Abstract

The frond of Laceya hibernica May et Matten was studied using anatomical/morphological features and statistical analyses. Quantitative and qualitative characters were used to identify and determine position in the frond of isolated fragments. Features examined were: cross-sectional diameter and height; cross-sectional areas of the entire axis, vascular tissues and sclerenchymatous tissue; number of protoxylem groups; and shape of vascular tissue. The frond was at least twice compound with a single dichotomous forking of the primary rachis. Alternate pinnae occurred prior to bifurcation of the primary rachis and along the secondary rachises. Anatomically, the primary rachis had three regions: the base—with a V-shaped vascular strand with 8–12 protoxylem groups; the mid-section—with a W-shaped strand with 10–16 protoxylem groups; and immediately below the bifurcation—with two V- or U-shaped vascular strands with 3–8 protoxylem groups each. Secondary rachises were characterized by a V- or J-shaped vascular strand with 4–8 protoxylem groups. Pinna traces were bilobed to arcuate with 2–4 protoxylem groups. Protoxylem maturation was mesarch throughout the frond. A well-developed sparganum outer cortex occurred in the rachises and was reduced or lacking in pinnae and pinnules. Reconstructions of the frond were assembled. Comparisons were made between the frond of Laceya and the fronds of members of the Lyginopteridales sensu lato, Stenokoleales and Aneurophytales.