Anatomically preserved plants from the Tournaisian of the Montagne Noire (France): an exceptional window into Early Mississippian floras

International audienceThe Mississippian was an important time in plant evolution, with the diversification of several groups, especially among the ferns s.l. and seed plants. The Montagne Noire localities in southern France include some of the oldest outcrops with anatomically preserved fossil plants from that time. The plant remains occur within the Lydienne Formation, which corresponds to alternating beds of argillaceous rock and radiolarian cherts that contain phosphatic nodules deposited in a shallow sea. The horizons containing plant fossils are considered middle Tournaisian in age based on the conodonts (Galtier et al., 1988).Since the first study of an anatomically preserved Lepidostrobus cone by Schimper in 1870, more than 30 taxa have been identified in the Lydiennes formation (e.g., Böhm, 1935; Bertrand et al., 1935; Galtier, 1970; Meyer-Berthaud, 1984; Galtier et al., 1988; Galtier & Rowe, 1989; Rowe & Galtier, 1989, 1990; Galtier et al., 1993; Decombeix et al., 2006, 2008). Ongoing work on this material aims to provide a better understanding of plant diversity in the Early Mississippian and to elucidate their systematic relationships with better-known Late Devonian and Late Mississippian taxa. Here we present (1) a synthesis of plant diversity in the Montagne Noire and how it compares to Devonian-Mississippian floras from around the world, (2) three examples of recently (re)investigated taxa and how they contribute to our understanding of plant evolution, and (3) remaining questions and future directions. The Montagne Noire anatomically preserved flora is largely dominated by seed plants, which represent more than half of the total number of collected specimens. Zygopteridalean ferns, cladoxylopsids, and lycopsids are also relatively abundant (+/- 10%) while the progymnosperms and sphenopsids are exceptionally rare (1–2%). Reproductive structures are rare (4–5 %) and the majority of the specimens are leaves and stems (Galtier et al., 1988). The latter range from minute axes less than a millimeter in diameter to wood fragments indicating the presence of arborescent taxa. A number of genera found in the Montagne Noire also occur in younger (Late Tournaisian-Viséan) localities (e.g., Stenomyelon, Eristophyton) and a few are known in both Devonian and younger Mississippian localities (e.g., Sphenophyllum, Cladoxylon). Interestingly, there are also some genera that are to date only known in the Montagne Noire such as the seed Coumiasperma, or the seed plant stems Faironia and Lyginopitys. The closest flora in terms of composition is from the Saalfeld area in Thuringia, Germany which is likely coeval. Recent (re)investigations of specimens from the Montagne Noire have provided important information on several key plant genera. One example is Sphenophyllum, a scrambling to lianescent sphenopsid genus known from the Late Devonian to the Triassic and a major component of Pennsylvanian coal swamp floras. The Montagne Noire specimens provide new information on the anatomy of Mississippian representatives of Sphenophyllum, which are poorly known in part due to the small number of permineralized specimens from that time (Terreaux de Félice et al., in revision). The Montagne Noire flora also contains several specimens that can be assigned to the enigmatic progymnosperm Protopitys, an arborescent genus currently known from the Mississippian localities of Canada, Scotland, France, Germany, Australia, and possibly the US (Decombeix et al, 2015). Of particular importance is a branching system bearing sporangia with preserved spores. This specimen represents the third fertile specimen ever reported for the genus and is the best preserved and the oldest one known to date. Ongoing studies will provide new information and insight to assess the affinities of Protopitys. Finally, some plants from the Montagne Noire display a unique combination of characters that challenges our understanding of Devonian-Carboniferous plant relationships and their stratigraphic distribution. This is the case for Stauroxylon (Galtier 1970), an enigmatic genus currently being reexamined that could represent the youngest occurrence of an aneurophytalean progymnosperm, a group typically known in the Middle and Late Devonian and thought to have gone extinct by the end of the Frasnian. Future work will focus on the description of unpublished specimens from the Université de Montpellier collections and collecting of new material. While plant remains are rare in the Lydienne formation, they have the potential to provide key information on plant evolution. As one of the oldest Mississippian deposits with anatomically preserved plants, the Montagne Noire is also a crucial source of information on floral changes around the Devonian-Carboniferous boundary