Ferns from the Cerro Negro Formation (Early Cretaceous, Antarctica): diversity, role in the palaeocommunity, and paleoclimatic significance

VERA, EZEQUIEL IGNACIO

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

Conicet Mendoza - International Palaeontological Association

The Cerro Negro Formation is a volcaniclastic non-marine sequence which crops out at Byers Peninsula in Livingston Island, and at President Head in Snow Island (South Shetland Archipelago, Antarctica). Radiometric dating indicate ages of 120.3±2.2, 119.4±0.6, and 119.1±0.8 Ma for deposits near the base, and 120±3 Ma for the youngest exposed levels, restricting the deposition of the unit to the Aptian. A very well preserved palaeoflora has been recovered from this formation. Many studies have been made, focusing on systematics, palaeoecology, and biostratigraphy. The discovery of abundant impression/compression and permineralized fossil plants, allowed a more complete description of the palaeoflora that thrived in this region during the Early Cretaceous. Noteworthy, the new data, when included in previous taxonomic lists, show that ferns represented an important floristic element in these communities. Among this group of plants, Cyatheales are the most diverse, with three fertile fronds (Sergioa austrina Césari, Eocyathea remesaliae Césari and Lophosoria cupulata Cantrill), and four stems (Alienopteris Vera, Yavanna Vera, and two yet-unnamed taxa), as well as three spore species. Other ferns, such as the Osmundaceae (represented by two species of Millerocaulis, sterile fronds, and spores) and Schizaceae (represented only by disperse spores), are also abundant in the unit. The Marattiaceae, a group of ferns with a sparse fossil record after the Jurassic, are also present, by a fragmentary fertile frond, and permineralized pinnules with attached synangia, being both fossils, in account of their comparable morphology and dimensions, most certainly part of a single biological entity. Using Niklas equations for calculating the height of fossil plants, the height for the ferns preserved as permineralized stems was estimated, and the results reveal that some of these forms were probably part of the forest canopy, along with conifers and other gymnosperms. Finally, the presence of abundant and diverse groups of ferns which are nowadays restricted to frostfree climatic conditions (e.g., Cyatheales) support the previously suggested mild climatic conditions for these high latitudes during the Aptian. In addition, the record of Marattiaceae (probably part of the crown-group Marattiaceae due to its similarities with Marattia and Ptisana) may point to warmer climatic conditions, taking into account that extant representatives of this family live in humid tropical or subtropical environments.