The state of the art on tricostate mosses, with description of a new species of Tricostaceae

Abstract A new fossil species in the moss family Tricostaceae, Tricosta angeiophoros sp. nov. (Early Cretaceous, Valanginian, ca. 136 Ma), possesses characters (stem and costa conducting strands) that prompt revision of the familial and generic diagnoses. In turn, discovery of this new species motivates a broader re‐examination of the relationships of tricostate mosses and the significance of the tricostate condition, which we undertake here. Tricostate mosses are an entirely extinct group known exclusively from gametophytes possessing leaves with unique morphology, characterized by a central costa and two symmetric lateral costae that originate separately at the leaf base. The stratigraphic range of tricostate mosses spans the Mesozoic and they are known from eastern Asia and western North America. The group now includes seven species: four species included in the two genera of the family Tricostaceae and three species included in the genus Tricostium . Whereas the Tricostaceae are unequivocally a hypnanaean family, likely belonging to the order Hypnales, the affinities of Tricostium remain obscure, due to a lack of family‐level diagnostic characters. Morphological characters available in the fossils are insufficient for distinguishing Tricostium from the Tricostaceae unequivocally. Uncertainty about the relationships among tricostate mosses and their position in the broader moss systematics leaves unanswered the question of whether tricostate leaves, which are known only in these extinct mosses, have a single origin or have evolved convergently in different groups. The answer to this question also bears on the role of adaptive change in the evolution of tricostate leaves. Taken together, the habitats inferred for tricostate mosses and those that favor their closest living analogs suggest that tricostate leaves may have evolved in response to the selective pressures of variable rheophytic conditions, to minimize leaf damage due to friction and to desiccation‐induced shrinkage. Aside from the evolutionary origins of the tricostate condition and independent tests for the hypothesized drivers of tricostate leaf evolution, another major unanswered question concerns the causes and timing of the post‐Cretaceous extinction of tricostate mosses. The fossil record is likely to provide answers to many of the questions concerning the tricostate condition as additional tricostate mosses are currently under study and fossiliferous strata with good potential for preserving bryophyte fossils have been identified and will be targeted by future studies.