Cuticle ultrastructure in Brachyphyllum garciarum sp. nov (Lower Cretaceous, Argentina) reveals its araucarian affinity

Abstract

A detailed and extensive study of a new species, Brachyphyllum garciarum sp. nov., was carried out through the analysis of the gross morphology and the cuticle fine details, structure and ultrastructure characters of its leaves using light microscope and scanning and transmission electron microscope. The fossils consist of compressions of incomplete twigs with well-preserved cuticle, collected from pelitic levels of the Springhill Formation (lower Hauterivian/lower Barremian) at the Río Correntoso locality in the Santa Cruz province, Argentina. The twigs have adpressed scale-like leaves spirally disposed. Leaves have a rhomboidal to pyramidal shape, a width and length always in a 1:1 ratio, margin entire and apex mostly rounded. Leaves are amphistomatic with stomatal apparatuses occurring in groups of narrow-wedge shape along the leaf axis. Stomatal apparatuses are close to each other with subsidiary cells in contact; the guard cells are sunken, with marked polar extensions and thickened mouth. Remnants of hypodermis cells are present in both foliar epidermis. Ultrastructurally, four types of cuticles were observed and evaluated statistically in detail: the ordinary epidermal cell upper and lower cuticles and the subsidiary cell cuticle are compounded of A2 granular layer and a spongy B1 layer somewhat mixed with cell wall remnants, while the guard cell cuticle presents only a spongy B1 layer. An Energy Dispersive Spectroscopy study was also made revealing the presence of 10 characteristic ratios with taxonomic importance, at least at the species level: N/Na, S/Cl, S/Na, Cl/K, K/Na as the most important; followed by N/Ca, S/K, Cl/Ca, Cl/Na; and S/Ca being the least reliable. The combination of morphological and cuticle ultrastructure features of Brachyphyllum garciarum sp. nov. are unique and clearly different from other contemporary species of the genus from Western Gondwana. Moreover, the ultrastructure of the foliar cuticle suggests a highly probable affinity with the Araucariaceae Family, more precisely with the genus Araucaria. Of the four recognized sections among the living species of Araucaria, the leaf morphology of B. garciarum sp. nov. is mostly similar with some species of the Eutacta section.