Permineralized plants from the Lydiennes Formation (France): a unique record of early Carboniferous plant diversity and biology

International audience

إنجليزي. The earliest Carboniferous (ca 360-330 million years ago) is a pivotal time in plant evolution, with the transition between taxa that characterized the first complex plant communities during the middle to late Devonian and more modern groups that later became major elements of the iconic coal swamps vegetation of the Late Carboniferous. However, localities with well-preserved fossil plants from that time period are rare worldwide, especially anatomically preserved remains that yield important information on the evolution of plant structure and biology. In Southern France, middle Tournaisian anatomically preserved plants were first reported in the Lydiennes Formation near Cabrires (Hrault) in the late 19th century. Since that time, at least 21 genera have been identified, some of them unknown elsewhere. We will present a synthesis of our knowledge of this exceptional fossil flora based on around 450 specimens from the collections of the University of Montpellier, France. The assemblage comprises a mixture of taxa belonging (1) to groups that were abundant and diverse during the Devonian and dwindled during the Carboniferous, such as the cladoxylopsids s.l. and progymnosperms, and (2) to groups that appeared in the Late Devonian and became emblematic members of Late Carboniferous floras such as the Sphenophylllales, Zygopteridalean ferns, and seed plants. In terms of relative abundance, the assemblage is largely dominated by seed plants (60%). Zygopteridalean ferns, cladoxylopsids, and lycopsids are also relatively abundant (+/- 10%), while the progymnosperms and sphenopsids are rare (12%). Anatomically preserved stems and petioles are the most abundant type of remains, while roots and reproductive structures are uncommon. Some of the fossils exhibit an excellent preservation that allows detailed inferences on plant autoecology (e.g., adaptation to aquatic environments), architecture (e.g., the oldest evidence of axillary branching), or development.