Early Cretaceous Mesochara-rich assemblages from central Patagonia, Argentina, predate the origin of homogenous Charoidean floras by about 30 million years.

Abstract

During the Early Cretaceous charophyte assemblages were dominated by the family Clavatoraceae, especially in low latitudinal lakes from the Peri-Tethyan basins. In higher latitudes, however, charophyte assemblages were mainly formed by early characeans associated to a reduced diversity of clavatoraceans. The charophyte studied assemblages from the Los Adobes Formation in the Ca~nad on Asfalto Basin (central Patagonia) are unique as they are exclusively composed of several species belonging to the early characean genus Mesochara, which is considered the ancestor of modern species of the subfamily Charoideae. These assemblages show well-preserved gyrogonites and unusual species richness for this genus, including Mesochara maruchoensis, aff. Mesochara harrisii, Mesochara stipitata and Mesochara adobensis sp. nov. Two of these species display a worldwide distribution (M.harrisii and M. stipitata), while two (M. maruchoensis and M. adobensis sp. nov.) were only recorded in Patagonia, Argentina, being considered endemic to this region. The charophyte assemblages of the Los Adobes Formation represent autochthonous floras of alkaline ponds or small lakes in distal fluvial floodplains, developed under dry climatic conditions, as indicated by sedimentary facies analysis of the studied beds, and taphonomic and palaeoecological analyses of charophytes and associated fossil groups, such as palynomorphs and ostracods. This type of charophyte assemblage predates by about 30 million years the first floras composed exclusively of members of the subfamily Charoideae, which radiated worldwide in the Late Cretaceous and continued to dominate the charophyte floras during the Cenozoic until the present.