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

in Review of Palaeobotany and Palynology (2006), 142(3-4), 123-130

An exceptionally large specimen of the early land plant Cooksonia paranensis Gerrienne et al. has been discovered from the type locality (Jackson de Figueiredo, Parana Basin, Brazil; early Lochkovian ... [more ▼]

An exceptionally large specimen of the early land plant Cooksonia paranensis Gerrienne et al. has been discovered from the type locality (Jackson de Figueiredo, Parana 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 Sciadopkyton-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 Rhymopsida (renamed here Paratracheophyta) is characterised by a more or less isomorphic alternations of generations. (c) 2006 Elsevier B.V. All rights reserved. [less ▲]