ARCHAEOPTERIS

fossil genus of plants originallyand sometimes stillconsidered early tree ferns in the order Protopteridales but now generally and tentatively assigned to a transitional fern group ancestral to both seed ferns and conifers. It was common in the Late Devonian Epoch (374 to 360 million years ago). Callixylon, a genus reserved for fossil stem fragments of a plant in the order Cordaitales, is now also known to contain many stem fragments associated closely with the fronds of Archaeopteris and is judged to belong to that genus.