1. Laboratoire de Palobotanique, ISEM, Université Montpellier 2, Place Bataillon, F-34095 Montpellier Cedex 5, France
2. Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0406, USA
3. Universitt Tbingen, Geologisch-Palontologisches Institut, Sigwartstrasse 10, D-72076 Tbingen, Germany

Correspondence to: Brigitte Meyer-Berthaud¹ Correspondence and requests for materials should be sent to B.M.-B. (e-mail: Email: meyerber@isem.univ-montp2.fr).

Abstract

Archaeopteris is an extinct plant which is of botanical interest for two reasons. It was the main component of the earliest forests until its extinction around the Devonian/Carboniferous boundary^{1, 3}, and phylogenetically, it is the free-sporing taxon that shares the most characteristics with the seed plants^{4, 5}. Here we describe the largest group of anatomically preserved Archaeopteris remains ever found, from the Famennian marine beds of south-eastern Morocco⁶, and provide the first evidence that, in terms of development and branching strategies, these 370-million-year-old plants were the earliest known modern trees. This modernization involved the evolution of four characteristics: a lateral branching syndrome similar to the axillary branching of early seed plants; adventitious latent primordia similar to those produced by living trees, which eventually develop into roots on stem cuttings; nodal zones as important sites for the subsequent development of lateral organs; and wood anatomy strategies that minimize the mechanical stresses caused by perennial branch growth.