Functional morphology and homologies of gymnospermous ovules: evidence from a new species of Stephanospermum (Medullosales)

Abstract

Fifteen specimens of a new species of medullosan ovules, Stephanospermum tridentatum sp.nov., have been discovered in Upper Pennsylvanian coal balls collected near Steubenville, Ohio. These ovules are most similar to S. elongatum, but consistent differences in sarcotestal vascularization, ovule shape, number of buttresses that connect the crown to the micropylar beak, and number of teeth at the apex of the crown demonstrate that they represent a new species, S. tridentatum. Our knowledge of reproductive biology in medullosan seed ferns is enhanced by features of the pollen chamber and micropyle in the new species. The pollen chamber is campanulate and displays a cutinized epidermal layer, a nucellar beak, and a membranous floor. A mass of tissue in the position of a central column is preserved in one specimen. In contrast with ovules of living gymnosperms, the micropyle remains open following pollination. These specimens provide new data for characterizing postpollination closing mechanisms in early seed plant ovules and for interpreting sequences of character originations that were associated with the evolution of modern spermatophyte reproduction. Key words: medullosan, Stephanospermum, ovule, pollination biology.