New Insights into the Evolution of Mucilage Cells in Araucariaceae: Araucaria violetae sp. nov. from the Early Cretaceous Araripe Basin (Northeast Brazil)

Abstract

Premise of research. Araucariaceae are currently disjunctively distributed over the Southern Hemisphere, dominating only some restricted habitats. In contrast, fossil representatives of this group from both hemispheres have been reported from the Mesozoic. In the Araripe Basin (northeast Brazil), fossil representatives of Araucariaceae mainly come from the Crato Formation (ca. 120 Ma). They comprise very abundant sterile leafy shoots, rare detached ovuliferous complexes, and dispersed pollen. The morphological variety of the conifer remains showing araucarioid affinities indicates more diversity in the group, which requires investigation. Methodology. Anatomical features were observed using microscopic slides under optical microscopy and with direct observation under SEM. Pivotal results. We describe vegetative shoots of Araucaria violetae sp. nov. on the basis of gross morphological and anatomical characters. The whiplike (pen)ultimate branches bear densely packed helically arranged awl-shaped leaves. Overall, the new species resembles foliage shoots of species of the extant section Eutacta. They could be interpreted either as portions of ultimate shoots of plagiotropic branches or as portions of caducous adventitious shoots. Importantly, we identify mucilage cells in the leaf parenchyma. Conclusions. The new species provides strong evidence for species diversity among Araucaria in the Araripe Basin. Mucilage cells possibly contributed to the water-use efficiency of the fossil plant in a presumed regional hot and semiarid climate. This cell type is present in extant species of the genera Araucaria and Wollemia but absent in extant species of Agathis. However, its presence in Emwadea microcarpa, a fossil stem agathioid, suggests that mucilage cells were lost in stem group agathioids. Thus, our study contributes to the understanding of mucilage cell evolution in Araucariaceae.