Nothofagus

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OLDEST RECORD OF NOTHOFAGUS LEAVES IN NELSON ISLAND, SOUTH ... straight, alternating along midvein, diverging from it in acute angles. (50–60°) ...
OLDEST RECORD OF NOTHOFAGUS LEAVES IN NELSON ISLAND, SOUTH SHETLAND ISLANDS, ANTARCTICA: IMPLICANCES FOR BIOGEOGRAPHY Marcelo Leppe1, Thiers Wilberger1, María Jesus Ortuya2, Héctor Ortiz2, Joaquín Bastias3, Héctor Mansilla1 & Tania Dutra4 1.- Laboratorio de Paleobiología, Instituto Antártico Chileno-INACH, Plaza Muñoz Gamero 1055, Punta Arenas, Chile. [email protected] 2.- Departamento Ciencias de La Tierra, Universidad de Concepción, Concepción, Chile. 3.- Section of Earth and Environmental Sciences, University of Geneva, Genève, Switzerland. 4.- Programa de Pós-Graduação em Geologia, Universidade do Vale do Rio dos Sinos-UNISINOS, Brasil.

INTRODUCTION

The stratigraphic sequence at Rip point is dominated by andesitic and volcanoclastic rocks, but including lapilli-fuff, tuff and ash fossil bearing levels. The material here reported belongs to the lower section with and estimated Lower-Middle Campanian age. The micro and megafossils recovered includes leaf imprints, spores and pollen grains, as well as thin charcoal levels and carbonized woods. Previous authors have stablished a Middle-Upper Campanian age (Rb-Sr 71-77 My) to the upper section at Rip Point through the correlation with the closest locality of Half Three Point in Fildes Peninsula.

The genus Nothofagus Blume has 35 living species that are found in rain forests of the South Pacific (south-east Australia, Tasmania, New Zealand, New Guinea, New Caledonia and southern South America). The recent biogeography of the main four subgenus (Fig. 1) has been frequently used as one of the best examples of the close relation between continental drift and evolution. The fossil record of the southern beech includes Oceania, South America and Antarctica reflecting a close relation with the post-Turonian vicariant events that affected Gondwana.

RESULTS

During the last decade, intensive paleontological studies has been carried out in Patagonia and Antarctica producing a remarkable collection of new fossil records and localities.

The Nothofagus leaves at Rip Point are represented by at least two different morphotypes. The Nothofagus sp.1 type is characterized by ovate leaves, notophyllous (8 x 5 cm) and pinnate, simple craspedodromous venation, entering teeth centrally. Secondary veins are delicate, varying in number from 11 to 13, one to three veins/cm, straight, alternating along midvein, diverging from it in acute angles (50–60°), bifurcating towards margin. Three basal ramifications of secondary veins also diverge at acute angles. Nothofagus sp.2 morphotype are elliptical leaves with membranous and serrated margins, mesophyllous (11 x 4,5 cm). Pinnate venation, simple craspedodromous. Conspicuous secondary veins, varying in number from 10 to 11, one vein/cm, straight. Secondary veins subopposite to alternating, diverging from it at acute angles (40–50). Taphonomically the deposit belongs to a fossil litter in lacustrine environments close to the coast.

METHODS

DISCUSSION

During the 2016 Chilean Antarctic Expedition, fieldwork in a fossiliferous locality at Rip Point, Stansbury Peninsula, Nelson Island, South Shetland Islands, Antarctica, resulted in the discovery of plantbearing beds containing Nothofagus-like leaf imprints, associated with ferns and angiosperm remains.

The biology of Nothofagus support the idea that a land bridge is needed to disperse because its anemochory and anemophily dispersal syndromes. The asynchrony in the presence of Nothofagus leaves could be interpreted as an evidence of land discontinuity between the northern tip of the Antarctic Peninsula and southern South America during an important part of the Campanian and Maastrichtian, that prevented the dispersion of the genus from Antarctica to Patagonia. The biogeographical barrier apparently disappeared during the Lower Maastrichtian, permitting the colonization from Antarctica. The event could be linked with the global record of cooling events that provoked glacioeustacy and consequent fall in the sea levels, recorded by several authors for Antarctica and other continents.

Figure 1: Extant species and distribution of the four subgenera of Nothofagus.

Figure 2: A.- In situ deposit of leaf imprints of Nothofagus, level 3 Rip Point; B.- Nothofagus sp.1 morphotype; C.- Nothofagus sp.2 morphotype. D.- Detail of the leaf margin in Nothofagus sp.1 morphotype; E-H.- Pollen grains of Nothofagidites from levels 3 and 4. bar= 1 cm We are grateful to the crew of Escudero Station and the Chilean Antarctic Institute for their logistic support for our field campaigns to this area. Financial support by the FONDECYT Project N° 1151389 “Paleogeographic patterns v/s climate change in South America and the Antarctic Peninsula during the latest Cretaceous: a possible explanation for the origin of the Austral biota?” are gratefully acknowledged.