Dicroidium (Zuberia) zuberi (Szajnocha) Archangelsky from exceptional Carnian leaf litters of the Ischigualasto Formation, westernmost Gondwana

Abstract

The Carnian Ischigualasto Formation of Argentina is known worldwide by its great abundance of palaeovertebrates, including the oldest dinosaurs and a palaeoflora represented by Onslow-type palynofloras, permineralizations, and compressions of foliage. However, thin deposits composed exclusively by cuticles are also preserved in this unit. This unusual type of preservation, where cuticles have accumulated as thousands of superimposed specimens without sediment between them, represents leaf litters of autochthonous or parauchthonous origin. Leaf litters are preserved into periodically flooded abandoned fluvial channels, which received accumulations of leaves from the riparian vegetation that, combined with pyroclastic inputs, generated ideal diagenetic conditions for their fossilisation. The excellent preservation of the cuticles allows the description of fine epidermal and cuticular features. One of the most common species in the Ischigualasto Formation, Dicroidium (Zuberia) zuberi (Szajnocha) Archangelsky, is described using optical and scanning electronic microscopy. Cuticles are thick, amphistomatic, with stomata sunken and randomly distributed, and with papillae on epidermal and subsidiary cells. Comparisons with previous hypostomatic records of the species suggest phenotypic plasticity of this Umkomasiaceae species. Palynological assemblages recovered from the same deposits indicate humid conditions under a seasonal climate. The vegetation flourished after a biotic turnover dated at 228.91 Ma by radiometric method.