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When pine trees were dwarfs

Fossils of 220 million year old araucarias attest to the transition of this species to large sized trees

For over two hundreds of millions of years, the earth has sheltered and taken care of preserving the vestiges of a rudimentary being. At the end of 2000, through a stroke of luck or the sixth sense of a paleontologist, researchers from the Natural Sciences Museum of the Zoobotanical Foundation and from the museum of the city of Mata, in Rio Grande do Sul, found an outcrop in the region of Faxinal do Soturno, close to Santa Maria, in the center of the state, where there was a petrified plant fragment, circular in format and some 5 centimeters in diameter: a fossilized pine cone.

This organ, in which the seeds of the plant are produced, is exclusive to conifers, one of the oldest groups to arise on the globe showing arboreal stature. At the first moment, however, this original sample did not cause any great enthusiasm. After all, the environs of Santa Maria, 200 kilometers away from Porto Alegre, had already supplied apparently more instigating animal fossils, above all of primitive dinosaurs and early mammals that are situated among the oldest representatives of these groups on the planet. Compared with the remains of these animals, the petrified cone of an old conifer did not seem to be anything so promising.

In the middle of last year, this perception began to change. A new expedition to the paleontological site, this time with personnel from the University of the Valley of the Rio dos Sinos (Unisinos), recovered at the same level of sediments at which the pine cone had been found – a 30 centimeter thick layer belonging to the Caturrita Formation, about 220 million years old – more remnants of conifer organs: branches, twigs, leaves and trunks.

A few months ago, when she began to study in a more detailed manner the this set of plant fossils, a specialist in plant fossils called Tânia Lindner Dutra, from Unisinos, located in the city of São Leopoldo, began to notice the importance of the findings. “They are the best preserved fossil record of conifers ever found in Brazil in such ancient sediments,” Tânia says. “Finding pine cones, trunks and branches from this group of trees in a single outcrop is something that hardly ever happens.” According to the researcher, the conifer fragments recovered at the locality in Rio Grande do Sul show characteristics found today in some species of modern araucarias, but, for sure, they belong to now extinct species of these trees.

Preserved forms
More than its advanced geological age, estimated at between 220 and 209 million years, at the end of the geological period known as Triassic, the fossils called attention for another trait: their state of conservation is surprisingly good. By dint of chance or nature, the sediments that originated from lakes and rivers from the geological layer that protected the plant fossils have kept the outline and shape of these conifers with incredible clarity.

Different from what usually happens with very old fossils, which become flattened by the weight of the geological layers that imprison and preserve them, the conifer parts found in Faxinal do Soturno remain three-dimensional. Some parts, such as the branches, are covered on the outside by iron compounds, a peculiarity that gives them an inflated look. The spiral leaves of the fossilized conifers from the Caturrita Formation are of two basic types: short and square, or elongated in a format that recalls a lance. This does not necessarily mean that they belonged to two distinct species of trees, in view of the fact that even today pine trees may show different patterns of foliage, according to the age of the branch or its stage of growth.

In Faxinal do Soturno, no complete plant was found, with all its organs joined together, and, at the moment, it is impossible to specify how many conifers were made up by the sets of plant fossils taken out of the rocks in Rio Grande do Sul, nor even if all the organs found belong to the same kind of individual. “We have not yet discovered a complete tree, showing the organs joined together,” explains the researcher from Unisinos. “But the conifer parts recovered probably belong to kindred plants that lived in an area very close to each other.”

From the size of their trunks, these specimens of primitive araucarias were more like bushes than large sized trees. “They were about 2 meters high and their trunks were between 15 and 20 centimeters in diameter, and they are anatomically similar to primitive forms of araucaria discovered in India and in Argentina, which lived between the end of the Permian and the Upper Triassic periods (between 260 and 200 million years ago),” Tânia says. The width of the branches varies from 2 to 5 centimeters, and they reach 20 centimeters in length. These are very modest dimensions when compared with modern conifers, amongst which stand out the highest and longer living plants on the planet.

Typical of the Northern Hemisphere, the giant sequoias are, for example, the largest trees on the face of the Earth, sometimes going beyond 100 meters in height. The only species of araucaria present in Brazil, the popular Brazilian pine or Parana pine (Araucaria angustifolia), can reach a height of 50 meters and its trunk show a diameter of up to 2 meters.

There is an explanation for the flagrant contrast in dimensions between the 21st century conifers and their more ancestral forms. In a movement more or less similar to and contemporary with what happened to the dinosaurs, which, from tiny reptiles, gave origin to creatures of good-sized proportions, the conifers, between the end of the Triassic period and the Jurassic period, some 200 million years ago, also took on gigantic proportions. At a given point in History, the conifers, then plants of discrete proportions, started to gain the stature of trees, driven by climatic changes on the planet.

“These fossils of primitive araucarias from Faxinal do Soturno must be representative of the last moment in which the groups of conifers that today display a large stature were still bushes,” comments Tânia, who worked with funding of some R$ 15,000 from Unisinos and the National Council for Scientific and Technological Development (CNPq).

To corroborate this hypothesis, the researcher mentions the petrified remains of large conifer trunks that have been found since 1931 in the Mata region and in other parts of Rio Grande do Sul. Also related to the modern Araucariaceae family, these fossils also had trunks of about 1 meter in diameter. They were discovered at a geological level of the Triassic period a little more recent than the Faxinal outcrop that furnished the remains of bush conifers.

“The conifers from Mata must be older than those from Faxinal,” the researcher reckons. “It is reasonable to believe that the increase in the size of the conifers in various parts of the globe occurred during an interval of 17 million years.” If this change in size amongst conifers really took place at the end of the Triassic period, as the fossil evidence leads one to believe, Rio Grande do Sul is the only Brazilian state, up to the moment, in which signs of this transition are to be found.

Close to the sea
Although there are fossil records that situate the emergence of the first conifers at some 400 million years ago, this group of gymnospermous plants – which are characterized for having naked seeds, unprotected by a fruit or a flower – only came to have any importance in the planet’s flora much later on, coinciding with the development of the large dinosaurs. It is interesting to recall that, at this point in the history of the Earth, the world was very different from what it is today. The climate was extremely hot and dry, and all the continents were joined together in an immense landmass called Pangaea, the center of which was practically a great desert.

Conifers occurred close to the edges of the continents, at places subject to the influence of the sea, supposedly more humid. Their acme in terms of arboreal vegetation practically coincided with the primacy of the dinosaurs on Earth (between 220 and 65 millions of years ago). This parallelism with the zenith of these mythical creatures explains, in part, the great interest in getting to know the evolution of this group of trees. “Some dinosaurs, like the brontosaurs, used to feed of the branches of the conifers”, Tania says.

By virtue of the progressive continental drift, of a series of climatic alterations and of the appearance of plants with flowers (angiosperms) – with a series of adaptive advantages, the conifers (and the gymnosperms) lost their primacy on the planet. Today, the presence of this group of trees – pine trees, sequoias, cedars, cypresses and firs – is restricted to areas with a more temperate climate, usually in places with a high latitude or altitude. The Araucariaceae, a family of gymnosperms with which the organs of the fossilized proto-araucarias from Faxinal do Soturno show some relationship, became limited to the Southern Hemisphere and to three distinct genera: Wollemia, Agathis and Araucaria.

Very visible in Brazil since the last glaciation, which began 5 million years ago and ended 10,000 years ago, araucarias have already occupied an area close to 200,000 square kilometers. They are found today in vast areas of a rainy temperate climate in Rio Grande do Sul, Paraná and Santa Catarina, with sparse patches in the southeast and northeast of São Paulo, the south of Minas Gerais, southwest of Rio de Janeiro and in the east of Misiones Province (Argentina), which, all added up, do not go beyond 4% of the original area. The oldest pine tree in Brazil still alive is to be found in the municipality of Canela, in Rio Grande do Sul. It is 48 meters high and has an estimated age of 700 years.