Plant palaeoecology of the latest Eocene flora from the Saint-Chaptes and Alès basins of Gard, southern France

Abstract

Rapid cooling during the Eocene-Oligocene transition resulted in a vegetation shift in southern France, from warm, humid vegetation to open savannah-like vegetation; however, the precise nature of this shift is uncertain. In this paper, we study the latest Eocene flora from the Saint-Chaptes and Alès basins (southern France) and compare it with other well-known southern European localities to evaluate vegetation changes near the Eocene-Oligocene boundary. Five plant communities are recognised: 1) hydrophytic vegetation dominated by water lilies and a Limnobiophyllum-like plant; 2) reeds that formed a palustrine belt around the lake; 3) ferns with Osmunda aff. parschlugiana and Pronephrium stiriacum, conifers with Doliostrobus taxiformis, a Sequoia-like plant, angiosperms with Daphnogene sp., Byttneriopsis sp., Myrica sp., Comptonia difformis and palms, which inhabited the lake shores and constituted the riparian vegetation; 4) Tetraclinis, which grew just behind the riparian vegetation but not far from watercourses, forming open landscapes and 5) Zizyphus ziziphoides, Fabaceae, and probably Ailanthus sp. and Tetraclinis forming an open woodland distal to the lake. The flora of the Saint-Chaptes and Alès basins resembles coeval floras from Sarral (Catalonia, Spain) that are characteristic of Southern Europe, although riparian vegetation is more diverse in the Saint-Chaptes and Alès basins, and Fabaceae diversity is higher at Sarral. We hypothesise that vegetation composition was mainly controlled by (1) basin architecture, allowing for a variety of wetland habitats and (2) climate. The coexistence of genera that are restricted to Central Europe (e.g., Byttneriopsis or Doliostrobus), together with genera more typical of Southern Europe (e.g., diversity of Fabaceae, C. difformis and Z. ziziphoides) suggests that the Saint-Chaptes and Alès basins contain latest Priabonian floras that are transitional between these two regions.