Stems and leaves of fossil cycads: morphological, anatomical and ultrastructural characters

MARTINEZ, LEANDRO CARLOS ALCIDES; VILLAR DE SEOANE, L.

Salvador

Congreso; XIV International Palynological Congress and X International Organisation of Palaeobotany Conference; 2016

Asociación Latinoamericana de Paleobotánica y Palinología

Extant cycads represent one of the last remnants of antique floras, and their evolutionary history extends back to the mid-Permian. This group was able to survive through several environmental changes and major extinction events across the Paleozoic, Mesozoic, and Cenozoic. Despite this, they have not suffered considerable changes in the morphology and anatomy of their vegetative organs. For those reasons, the features commonly used in systematics to segregate the living genera are few or with subtle differences. Certain characters in living Cycadales are considered as to drought, fire, volcanism and grazing adaptations, developed as ancestral conditions strengthened by the changes occurred in the environments where cycads lived. Therefore, some characters present in leaves and stems suggest a plesiomorphic condition, or homoplasy with respect to other groups of plants (e.g. Pteridosperms, Bennettitales). For these reasons, in several cases the assignment of fossils remains based on livings forms is relatively easy; however, in other occasions doubts arise because of the plasticity of vegetative organs to environmental changes. In this study we describe and compare the cuticle ultrastructure in cycadalean leaves of nine fossil and fourteen extant species. The goal of this research is to increase the knowledge on the morpho-anatomical characters of cycads, with new descriptions and analyses of their leaf cuticles that could help resolving the unclear systematic position of some fossil taxa. Results obtained from a cladistics analysis carried out with the addition of the new data provides different interpretations in the relationships of the Cycadales and their evolution.