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Paleobotany

The trees of the desert

Forest of conifer trees thrived in a hot, sandy environment when dinosaurs still populated the earth

Fossils found on a farm in the vicinity of the city of Uberlândia, State of Minas Gerais

PHOTOS EDIVANE CARDOSO/ UFG AND ADRIANO R. DOS SANTOS/ UFUFossils found on a farm in the vicinity of the city of Uberlândia, State of Minas GeraisPHOTOS EDIVANE CARDOSO/ UFG AND ADRIANO R. DOS SANTOS/ UFU

Forty years ago, the owners of Fazenda Sobradinho farm, located in the vicinity of the city of Uberlândia, in the State of Minas Gerais, removed what they believed to be rocks on their plantations and took them to a marble quarry in the region. In 1972, geologists Kenitiro Suguio and Armando Márcio Coimbra analyzed those rocks and concluded that the rocks were actually fossilized tree trunks. Two years later, paleontologist Diana Mussa classified these trunks as being part of a group of primitive pine trees. The scientific community, however, was skeptical about this conclusion, because when these trees were alive, approximately 130 million years ago, that entire region was part of a vast, sandy desert.

The conclusion reached by the Brazilian researchers has only now been confirmed. The most convincing evidence that the fossils found at Fazenda Sobradinho are really the ancestors of conifer trees ? a group that includes pine trees, araucarias and sequoias ? were presented this July in an article published in the Journal of South American Earth Sciences, the result of research work begun 15 years before.

Edivane Cardoso, currently a professor at the Federal University of Goiás, was a biology student at the Federal University of Uberlândia (UFU) in the late 1990s when he decided to analyze the fossils under a microscope, as part of his final term paper. He was amazed when he saw fragments of petrified stems in the samples. More specifically, these were fragments of wood – the tissue that transfers water and nutrients from the roots to the leaves ? of plants that had lived 130 million years ago, during the Cretaceous geological period. Cardoso and his advisor from UFU, geologist Adriano Rodrigues dos Santos, found it difficult to convince other researchers of the discovery. We would send the study to congresses and participants would question our conclusions. They did not believe that the fossils of such ancient plants could be found in the Botucatu geological formation, says Cardoso.

Supercontinent
Up to that time, scientists did not believe that giant plants could have grown in the Botucatu geological formation, as the formation was a sandy desert that extended throughout areas that comprise the Southeast and South regions of Brazil, parts of Paraguay, Argentina, and  Uruguay. Dinosaurs had already populated our planet when those conifers were growing in that region. South America was still connected to Africa and both continents were part of a super continent called Gondwana, whose center was the Botucatu desert. According to geological simulations, this desert was part of an extensive arid region located in the Southern Hemisphere, next to the equator. This region existed during a prolonged cycle, during which very high temperatures prevailed. Scientists used to say that it was impossible for fossils to occur in the Botucatu formation, until dinosaur footprints were discovered there, says paleobotanist Margot Guerra Sommer, from the Federal University of Rio Grande do Sul, who helped describe the conifer fossils in the Journal of South American Earth Sciences.

048-050_coniferas_188The quantity, density, and size of the fossilized trunks allow researchers to presume that a conifer forest existed where a fragment of the cerrado, the tropical savanna, now exists. The fossils are from adult plants that had come down, and many of them have compacted horizontally, a sign that a huge amount of sediment had covered the ancient forest before the plants had petrified. Five big fossils, each of them five meters long, were recovered. Smaller fossilized fragments, including fossilized roots, were also found. We don?t know exactly how many fossils still lie there, says Margot. According to the researchers, these trees probably stood close to each other, thus forming a dense forest. In addition, they were tall trees, going up to a height of 20 meters, and were probably taller than the currently existing pine trees.

When analyzing the microscopic characteristics of the trees, Margot and Etiene Fabbrin Pires, who is currently a professor of the Federal University of Tocantins, verified that the collected examples are anatomically similar. That is, they probably belong to the same species that is part of the protopinaceas group. This is great material, as it is the first paleobotanical record from  the Botucatu formation in the entire Parana River basin, Etiene explains.

In view of this scenario, the researchers asked themselves: how could these trees have developed in the desert? The fossilized roots found in the sandy sediment of Fazenda Sobradinho indicate that the material had not been transported and that the trees had lived in the place where they were later buried. The interesting fact is that these huge trees were sustained by poor soil, says Etiene. According to the researchers, they may have lived on the edge of the desert, in plant formations that grew in the sand dunes, in more humid regions,  environments that resemble those where oases develop.

Although water was available, there was water shortage during certain periods. Tree rings, the patterns on the trunk that indicate the life cycle of the plant, suggest that these trees lived under constant stress. In growth rings, the late wood cells do not have thick walls, which is the case in currently existing trees,? explains Etiene. In these fossils, the size of the cell was smaller. These anatomical characteristics indicate that the growth of these protopinaceae was controlled by alternate dry and humid seasons and not because of more or less sunlight, as is the case with existing conifers, which have adapted to temperate climates.

Perennial Leaves
When analyzed under microscopes, the fossilized wood led the researchers to conclude that the leaves of these trees were probably perennial and did not fall during prolonged droughts. But it is impossible to describe what these trees looked like, as only the trunks and the roots were preserved. They probably resembled conifers today, says Margot. ?Based on the anatomy of the wood, we could say that these trees were the ancestors of trees of the Pinus genus. Cycas, ferns, and other conifers dominated the landscape in different regions of the earth at the time these protopinaceae flourished. There are records of conifers in both hemispheres in previous, contemporary, and later periods,  says Etiene. Flowering plants began to appear at the time the conifers were living in the region of Fazenda Sobradinho. Millions of years later, the flowering plants would adapt to different environments on earth. Nowadays, the conifers are found in a few places in the Southeast and South regions of Brazil.

Researchers still cannot explain how these trees became fossilized. One hypothesis is that this petrification process began when the plant was still alive, which is what happens in many environments nowadays. We are going to investigate the anatomical characteristics that help identify the species, says Margot. Based on the characteristics of the fossils and of the sediment, we also want to get more specific information about the kind of environment in which these plants flourished  in the huge Botucatu desert.

Scientific article
PIRES, E. F. et al. Early Cretaceous coniferous woods from a paleoerg (Paraná Basin, Brazil). Journal of South American Earth Sciences. v. 32 (1), p. 96-109. Jul. 2011.

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