An exceptional specimen of the early land plant Cooksonia paranensis, and a hypothesis on the life cycle of the earliest eutracheophytes

An exceptionally large specimen of the early land plant Cooksonia paranensis Gerrienne et al. has been discovered from the type locality (Jackson de Figueiredo, Paraná Basin, Brazil; early Lochkovian, Early Devonian). This nearly complete specimen consists of five dichotomous axes attached at their base to a small thalloid(?) structure. Each terminal axis segment ends in an expanded, cup-like, empty tip. Three interpretations of the specimen are proposed. (1) The whole specimen is a gametophyte of the Sciadophyton-type, with a central area from which five axes depart; under this interpretation, the terminal cups are gametangiophores. (2) The basal structure represents the remains of a rhizome bearing five upright aerial axes, in which case the whole plant is a sporophyte. (3; our favoured hypothesis) The specimen is a cluster of five individual sporophytes still attached to the remains of a small female or bisexual gametophyte. In the latter case, this fossil is evidence that reduced thalloid gametophytes and branched axial sporophytes are plesiomorphic among the earliest eutracheophytes. We suggest that a major difference in life cycle defines a basal dichotomy in tracheophytes. Eutracheophyta, including all living vascular plants, have a heteromorphic sporophyte dominant alternation of generations, whereas their extinct sister-group Rhyniopsida (renamed here Paratracheophyta) is characterised by a more or less isomorphic alternations of generations.