Paleoflora da Bacia do Araripe (Aptiano/Albiano): novas perspectivas sobre a taxonomia e paleoecologia de coníferas

Abstract

Based on the Uniformitarian principle and through the analyses of well-preserved fossils it is possible to understand the function of many features of extinct plants and, hence, their importance for the evolution and diversification of this group. A window is also opened for understanding paleoenvironmental and paleoecological settings, as plants present several adaptations to their respective habitats. Gymnosperms are an important group used for interpreting paleoclimates due to its abundance in the fossil record. Among them, the families Araucariaceae and Cheirolepidiaceae (Coniferophyta) stand out, both of which had a widespread geographic distribution during the Mesozoic and are the main components of the paleoflora of the Santana Group (Lower Cretaceous, Araripe Basin), especially the Crato and Romualdo Formation, deposited under stressful conditions, like aridity and salinity. Despite the large number of collected specimens of those taxa and the possible adaptive responses exhibited by them, studies focusing on these issues are still required in order to better depict the paleoecology of the paleoflora of the Santana Group. Thus, this study aimed to contribute with the taxonomy of conifers from that stratigraphic unity, as well as to understand which morphological and anatomical characters were propitious for the adaptive success of Araucariaceae and Cheirolepidiaceae under the conditions that prevailed in the Araripe Basin during the Early Cretaceous. By the detailed analyses of new specimens, two new species of the former family and another one of the latter were described. They all present a varied set of responses to aridity and salinity. Some characters were already reported for other local species, such as reduced leaves, thick cuticle, and sunken stomata. On the other hand, other features are firstly reported here, like mucilaginous cells, lignified hypodermis, and thickened tracheids. These adaptations likely diminished or ou prevented the water loss, stored water during dry seasons, and/or helped to compensate the osmotic potential in saline settings. Furthermore, an exceptionally preserved specimen presented evidence of interaction with microrganisms of the clade Oomycota, indicating that conifers, as the main paleofloristic component, played several ecological roles within that palaeoecoystem. Thus, the paleoflora of the Santana Group was shown to be more diverse than previously thought and new adaptive strategies were revealed, enabling a better comprehension of climatic influence in Northeastern Brazil during the Early Cretaceous.