Pachytesta crenulata, a new medullosan ovule from the Pennsylvanian of Iowa

Abstract

A new species of medullosan ovule, Pachytesta crenulata, is described from the latest Atokan–earliest Desmoinesian (early Moscovian) of Iowa. The ovule is approximately 5.5–6 cm long and 2.4–3 cm wide at the mid-point with three commissured primary ribs, three secondary ribs expressed at the apex, and a large number of sclerotesta ridges (18–24 between each primary rib-secondary rib pair), formed from interwoven bands of fibers. The micropyle, defined by the epidermal cuticle of the endotesta, is flared at the tip and triangular in cross section. A collar of sclerotesta and sarcotesta, 10 mm tall surrounds the micropyle. The space enclosed by the collar is a triangular prism, with a primary rib at each edge and sides approximately 2.3 mm in length. The space between the micropyle and the sclerotesta–sarcotesta collar appears to have been filled with thin-walled endotesta cells. At the ovule apex, lobes of sarcotesta associated with the primary ribs partially occlude the top of the collar, forming a triangular opening with the flared tip of the micropyle in the center. Pachytesta crenulata appears most similar to P. composita and P. illinoensis. All three ovules have tertiary ribs or ridges and secretory fibers in the sclerotesta; in both P. illinoensis and P. crenulata a collar of sclerotesta and sarcotesta surrounds the micropyle. Based on these similarities, we place P. crenulata in Taylor's [Taylor, T.N., 1965. Paleozoic seed studies: a monograph of the American species of Pachytesta. Palaeontographica, Stuttgart] P. composita– P. illinoensis group . The condition of the sclerotesta and nucellar stalk of P. crenulata may indicate its developmental stage. Ovules having hollow fibers in the sclerotesta and nucellar stalks with a convex profile appear immature relative to ovules with filled fibers in the sclerotesta and deeply invaginated nucellar stalks. Although the pollination and fertilization status of most P. crenulata ovules remains equivocal, one mature ovule probably was pollinated. All of the immature ovules showed signs of seed predation.