PAULO ALVES DE SOUZASpore, alga and pollen: useful for recomposing the floraPAULO ALVES DE SOUZA
On an afternoon in September 1999, Fresia Ricardi Branco, from Chile, found quite a stone in her path. It was a Saturday, a day off her lessons at the State University of Campinas (Unicamp), when, as a specialist in plant fossils and resident in São Paulo, she had decided to make a foray into a very familiar territory: the Bandeirantes highway, which links the state capital to Campinas, along which she would travel three times a week. Alongside her husband, Fábio Branco, a geologist by profession, she had taken the family car and headed off for a stretch of the Bandeirantes being widened. Around 5:30 PM , she was facing a grayish outcrop that she had sighted a few days previously, but had not had the time to explore: a rocky formation some 50 meters wide by 20 meters in height that had sprouted up from the work under way at kilometer 96 of the highway.
Made up by over 20 layers of siltstone, a finely granulated sedimentary rock, the outcrop housed ample vestiges of small plants that had built up about 310 million years ago, or perhaps even before that. “There were so many fossils that you couldn’t help seeing them”, Fresia recalls. “They were of creeping vegetation, from an environment close to glacial, similar to the present-day tundra in the north of Canada and in Siberia.” They were probably part of a green carpet, made up of mosses (bryophytes) and lycophyta of shrub stature (upright plants, with evergreen leaves arranged spirally around the stem), which covered the portions of land in the São Paulo hinterland located between the glaciers and the arm of the sea that penetrated into the continent.
Yes, there were glaciers to the east of the state of São Paulo and an inland sea to the west or southwest. The primitive tundra differed from the present one in at least one important aspect: there was no grass. The angiosperms, plants with flowers, amongst which the gramineae are included, had not yet arisen on Earth.At this moment of the history of the planet, around 300 million years back, between the end of the Carboniferous period and the Permian period, the geography and the climate of the Earth – and not, obviously, just São Paulo’s – were very different from the current conditions.
South America, Africa, Antarctica, India and Australia were joined together in an austral supercontinent, Gondwana, and its southern portions, greatly taken over by glaciers, reached the polar latitudes. While North America and Europe were closer to the Equator and showed a warmer climate, the Southern Hemisphere was living through its last great Ice Age, between 330 and 285 million years ago. The most recent large scale glaciation, the more direct effects of which were concentrated on the Northern Hemisphere, began 2 million years ago, and ended a mere 10,000 to 15,000 years back. But that is another story.
The discovery of remnants of a biome similar to the contemporary tundra by the verge of kilometer 96 of the Bandeirantes in Campinas illustrates well the efforts made (and the results obtained) by a group of researchers from São Paulo that, in the course of four years, went into the field in search of plant fossils imprisoned in sediments originating from the lastmegaglaciation to have occurred in the lands of Gondwana. In their quest for more specimens of primitive plants, the scientists revisited localities in the interior of São Paulo where fossils from the glacial period had already been dug up, such as Monte Mor, Itapeva and Cerquilho, and they also identified new rocky outcrops with vestiges of plants from the Ice Age in other cities – the cases of Salto, Tietê and Campinas.
This work had the objective of reconstituting the succession of flora and plant formations that prospered in the state of São Paulo in the midst of the frequently negative temperature that were in force roughly 300 million years ago. “Besides providing information on the kinds of plants that existed one day, the plant fossils help to understand the climatic changes of the past”, says paleobotanist Mary Elizabeth C. Bernardes de Oliveira, of the University of São Paulo (USP) and of Guarulhos University (UnG), the coordinator of the project, funded from FAPESP. Why carry out the survey, one of those who did it being Fresia, from Chile, just in São Paulo? The main reason is of a scientific nature. On Brazilian soil, it is this unit of the federation that has the thickest – and temporally most extensive – record of sediments and fossils from the ancient austral Ice Age. With an age estimated at between 310 and 285 million years, these layers of glacial origin belong to the Paraná basin and have been giventhe name Itararé Subgroup (see the map on page 50, with the sites studied in the project).
Within this interval of time, the old tundra grassland seems to represent the oldest form of vegetation preserved by the glacial sediments in Itararé. In another three cities, Monte Mor, Itapeva and Buri – whose paleobotanical sites, known for decades, were studied once again -, the fossils found by Mary Elizabeth’s team indicate the existence of a different and slightly older flora than in Campinas. Their age is estimated at between 315 and 305 million years. Amongst the petrified vestiges recovered by the researchers, there were seeds and fronds of pteridosperms (extinct plants akin to present-day ferns), some primitive pine trees, leaves and branches of lycophyta and sphenophyta (plants that are reminiscent of thin bamboos) and some progymnosperms. Almost all of shrub stature.
These plant fossils must have constituted the flora of a climate that was closer to the cold or temperate than actually glacial, typical of a stage when the glaciers had retreated a bit, due to a rise in the temperatures. “In these localities, Itararé provides indications that there was an interglacial phase”, Mary Elizabeth comments. Interglacial is the brief moment of a warmer climate that interrupts the almost polar cold that reigns during glaciation. It is the fleeting “summer” of an Ice Age. It can last many thousands of years, or even a few million. Glaciation, though, extends for dozens of millions of years, sometimes even more than a hundred, and from time to time is allayed by interglacial phases. In Monte Mor, Itapeva and Buri, the Itararé sediments include thin layers of charcoal resulting from peat bogs – marshy areas, from the overflowing of rivers or lakes, characterized by an intense accumulation of vegetable remains in decomposition. This sedimentary landscape and its plant fossils are compatible with the occurrence of an interglacial.
According to the evidence raised by the project, after the temperate vegetation imprisoned in the turf, a formation similar to the tundra resurges – an indication that the cold must have gone back to being acute again and the glacier started advancing over the continent again. It was the return of the Ice Age. This kind of record was found in the municipality of Salto, in an outcrop that, like the site discovered in Campinas, also popped up to the surface during works on a road. On two, actually: at the junction of the SP 75 and SP 308 highways.
This was the place where the theory that one day an almost polar vegetation, made up fundamentally of mosses, had prospered there gained more strength, after the researchers found pebbles of glacial origin associated with the thin fossiliferous sediments, accumulated in a probable glacial lake or sea. The pebbles were released by icebergs, originating from glaciers, which floated and melted in this body of water. “The existence of pebbles along with the fine sediments suggests that the bryophytes did not live very far from the glaciers”, explains paleobotanist Rosemarie Rohn Davies, from the Applied Geology department of the São Paulo State University (Unesp), in Rio Claro, who discovered the Salto outcrop with her student Márcia E. Longhim.
Fossil records of vegetation of the tundra kind are rare, and those salvaged in Campinas and Salto are possibly amongst the oldest from Gondwana. Without any trees and with its permanently frozen subsoil, the monotonous horizontal landscape of the tundra is the coldest biome that there is on the Earth. Its fragile vegetation, usually mosses with less than 10 centimeters in height and sparse dwarf woody stalks, is rarely preserved in layers of rocks. This explains the great difficulty of locating primitive mosses preserved in glacial sediments. Identifying the outcrops in these two cities, which made it possible to salvage and to study scientifically occurrences of primitive tundra, only happened because there were the right people in the right places.
The explanation is this: it was only for a brief period, a few weeks, that the roadwork made the sedimentary rocks with the plant fossils appear. If the researchers had not seen them, the plant remains from the past would now, once again, be buried. The outcrops ended up being grassed over at the end of the engineering works on the roads. Rosemarie recalls that her third and last visit to the site at the junction of the SP 75 and SP 308 highways was not an agreeable experience: “My students and I saw that a tractor was covering the outcrop with earth. So we then collected samples as quickly as possible, while the tractor was gradually coming towards us. It was no use asking the person in charge of the work to postpone the job. The company had a rigid timetable. The last samples were collected when the tractor was practically throwing earth on top of us”.
If the primitive tundra fossils from Salto are reminiscent of those from Campinas, if only because the sediments from both places appear to come from clearly glacial periods, at least one of the forms of plants found in Cerquilho and Tietê is very different from the plants discovered at other sites of the Itararé Subgroup. In these two places, in the midst of sphenophyta and progymnosperms similar to those from Monte Mor and Itapeva, there appear remains of Glossopteridales, an extinct order of plants the size of a tree, with leaves in the format of a spatula, which arose in the Permian period in the lands of Gondwana. The presence of this kind of plant, which develops in warmer climates, permits a few inferences. The sediments from Cerquilho and Tietê must have originated in a phase when the temperatures rose again. “At this moment, there was possibly another interglacial”, claims Mary Elizabeth. The researchers estimate that the rocks from these outcrops have an age of roughly 290 million years.
For Rosemarie, from Unesp, the specimens of Glossopteridales recovered from the banks of the Capivari river, in Tietê, may be the oldest of all Gondwana. “Their leaves show indications of being more primitive than the occurrences of this kind of vegetation discovered abroad and even in Cerquilho”, Rosemarie says. Her theory is supported by a characteristic of the leaf veins of the Glossopteridales found in Tietê. The leaves do not have a very well developed median vascular bundle, a trait usually present in the more “advanced” specimens of this kind of plant. The Glossopteridales are a target for intense scientific debate, because its descendants, plants of the Glossopteris genus, formed the dominant flora all over Gondwana for 40 million years.
Tundra in Campinas 310 million years ago. Afterwards, peat bogs in Monte Mor and Itapeva. Next, tundra, again, in Salto. And, finally, Glossopteridales in Tietê and Cerquilho, 290 million years ago. Talking like this, it seems as if it was simple to determine the succession of floras that took place in the state of São Paulo during the Gondwanan Ice Age. Actually, this order is schematic, didactic, and does not have any pretension to be an absolutely faithful portrait of the past. Some of these floras may have coexisted in time. The tundra from Campinas, for example, seems to be older than the bog in Monte Mor. But this does not necessarily mean to say that the second form of vegetation is the direct successor of the first. “There may have been other kinds of flora that did not get preserved in the sediments studied, or that we have simply not yet managed to find”, Mary Elizabeth ponders.
To determine, albeit in a relatively uncertain manner, the age of each of the Itararé outcrops and to enrich the information about the respective flora, the scientists often availed themselves of pollenology. This deals with the study of fossils of grains of pollen, spores and microalgae, preserved in the rocky sediments. Sometimes, all that is left in a paleontological site are these three elements, seeing that, frequently, the larger parts of the plants do not resist the action of time. Their size is tiny: it varies from 10 to 250 micra (0.01 to 0.25 millimeters).
“With the so-called index fossils, we manage to estimate the age of a package of sediments”, explains researcher Paulo Alves de Souza, who coordinated the project’s pollenological studies and today lectures at the Federal University of Rio Grande do Sul (UFRGS). One of the most important contributions from Souza’s analyses was to discover that the major part of the rocky layers of the Itararé subgroup was older than previously thought. “Before the project, we used to think that the majority of its sediments were from the Permian period, with a small participation of rocks from the Carboniferous period. But we saw that it is precisely the opposite”, says Souza. This means to say that the fossil flora preserved in São Paulo in the Itararé subgroup is older than used to be believed, and is one of the oldest of the austral paleocontinent Gondwana.
Republish