Late Mid Devonian Sawdonia (Zosterophyllopsida) from Venezuela

Abstract

Premise of research. Fossil plants from the Late Middle to early Late Devonian age Campo Chico Formation, Sierra de Perijá, western Venezuela, are determined to represent a new species of the zosterophyll genus Sawdonia. While mentioned in previous publications, this plant has not been previously described or formally named. A conclusive taxonomic assignment was not possible until clarification of the sporangial morphology of the type species of Sawdonia, S. ornata (Gensel and Berry 2016), from the Early Devonian of Canada.Methodology. The adpressions were prepared using dégagement, including serial dégagement and photography at each stage. Permineralized remains were prepared using established techniques for pyrite sectioning and polishing. Taphonomic analysis included preparing models and subjecting them to compression to aid in understanding their shape and orientation in life.Pivotal results. The Venezuelan plants are adpressions of spiny axes that exhibit two types of branching (especially in vegetative ones), circinate tips, anatomy with G-type tracheids, and lateral sporangia with unequal-sized sporangial valves, with the abaxial valve being spinous as is characteristic of the genus Sawdonia Hueber emend Gensel and Berry 2016. Sporangia are irregularly arranged along some axes and appear to be located in medial to subdistal regions. Spores were not obtained.Conclusions. Differences in length and density of spines, the presence of two types of branching related to its growth habit, and details of sporangial morphology distinguish the Venezuelan plants from those of the type species and S. deblondii, thus leading to establishing a new species. This discovery demonstrates the longevity of the genus (ca. 20 Myr) during a time of profound vegetational change, including the rise of forests, and is the first record of a fertile zosterophyll recorded from the Middle Devonian of South America.