Protosalviniahas a large thallus on whichP. arnoldiiandP. ravennaforms develop. Light and scanning electron microscopy of Upper Devonian compression forms ofProtosalvinia arnoldii, P. ravennaandP. furcatafrom the Ohio Shale of Kentucky and Ohio, statistical analyses of their cellular dimensions and patterns, and mathematical constructs of thallus shapes and sizes indicate thatP. ravennaandP. furcataforms could be ontogenetically derived from aP. arnoldiistage. Tetrads, interpreted as the meiotic sporangial products, are morphologically identical for the three forms; tetrads contain a sporopollenin fraction and occur singly in closed, hypodermal conceptacles. Solitary, multicellular, stalked clavate structures occur in similar closed chambers. Fossil thalli are composed of hydrocarbons with some lignin‐like compounds interpretable as cuticular in nature.P. ravennaandP. furcataare morphological forms of a single species showing phenotypic plasticity within an onshore‐offshore paleoecological gradient. The form of thallus and the resistant spores are interpreted as adaptations to a littoral environment. The morphology and mode of reproduction ofProtosalviniasuggest a heteromorphic diplobiontic life‐cycle, similar to the Heterogeneratae (Phaeophyta).