Seen from a height, the crowns of the trees merge together and turn the countryside of the Amazon into a wide green carpet that holds the largest variety of plants and animals in the world. But the hundreds of thousands – perhaps millions – of species of insects, fish, birds and other beings that today live here are only what remains of a fauna that was much richer and dominated between 13 million and 6 million years ago over an area of South America that stretched from Venezuela, in the north, to Uruguay and Argentina, in the south.
At that time the continents had the form and position they currently have and the South American scenery was very different: magnificent rivers of calm waters cut across a plain of almost 1,000 kilometers in width that extended itself for 6,000 kilometers in a southerly direction, dotted with lakes, swamplands and grasslands, as well as sparse forests. This area that corresponds to one third of South America – or the same as all of Europe – was an immense wetland, possibly 20 times greater that the wetlands in the state of Matto Grosso, today the largest water logged flatlands on the planet.
“A spectacular variety of animal species lived there, probably extinct because of climate alterations and in the relief of the continent over the last 5 million years” says the paleontologist Mario Alberto Cozzuol, from the Catholic Pontificate University of Rio Grande do Sul. During the next six months an article by paleontologist Cozzuol will come out in the Journal of South American Earth Sciences, which will have one of the most widely covering re-constructions of this scenario and part of the South American fauna at the end of the Miocene geological period, between 13 million and 6 million years ago. The variety of forms that the animals exhibited in the midst of those lakes, swamplands and forests is impressive.
Fossils of alligators and crocodiles found in Brazil, Venezuela, Colombia, Peru and Argentina give an idea of this diversity. Only in the region of current state of Acre there must have been living 17 species of these reptiles with thick skin covered by hard plates – today there are only four around there. In the rivers and lakes of the South American wetlands there were alligators such as the Caiman brevirostris, an extinct species of around 2 meters in length and with a large flattened cranium of 30 centimeters. Around there, much larger predators also hunted, an example being the Purusaurus brasiliensis, an alligator of almost 15 meters in length that, with a jaw of more than 1 meter full of razor sharp teeth, could chew up at the one go a capybara that was inattentive while drinking water.
But one family especially calls the attention for their appearance and behavior: the Nettosuchidae, with crocodiles with a flattened cranium, fragile teeth and a long snout. They differentiated themselves from the others by feeding in a passive manner: instead of pursuing fish, turtles and even small mammals, the crocodiles of this family – such as the Mourasuchus amazonensis, discovered in 1964 by the Rio Grande do Sul paleontologist Llewellyn Ivor Price – opened up their mouth of almost 1 meter and filled their bag with water similar that that done by the pelican. Next they closed their teeth and expelled the water, retaining mollusks, crustaceans and small fish. “The three known species of Mourasuchus lived exclusively in South America, between 15 million to 6 million years ago” explains Cozzuol.
Such a variety of predators, in the opinion of the paleontologist, could only have survived in an environment with an abundance of food – and apparently there was no lack of food in the South American wetlands. Over the last two decades paleontologists working in the southwest of the Brazilian Amazon, in Venezuela, Peru, Colombia and Argentina have identified almost 200 genres of reptiles, birds and mammals that lived between 15 million and 5 million years ago. As in taxonomy, the science of the classification of living beings, the genre is the level of organization that groups together species with common characteristics, the 200 genres of the South American Miocene fauna indicate the existence of an even greater variety. “This diversity” explained paleontologist Cozzuol, “suggests that this wetland was a stable environment for a long time, capable of producing sufficient food to maintain this fauna for millions of years.”
The terrestrial fauna of the region was complex, with groups of animals at all levels of the food chain, from those that ate only vegetation to those that fed on other animals, and the most varied forms and dimensions. Among the mammals, there were carnivores of the Cyonasua genre, distant kin of the coati with long sharp teeth, and small rodents such as rabbits or the true rats, which prime example is the Phoberomys pattersoni. An extinct relation of the paca and pacarana found in the Amazon, the Phoberomys was the largest rodent in the world: it weighed 700 kilograms and was twice the size of the tapir, currently the largest South American land mammal.
During this period there was a grand diversification of primates throughout the world, including the emergence of the first ancestors of human beings. In South America, Cozzuol and the American anthropologist Richard Kay, from Duke University, have identified two new primate species: the monkey Solimoea acrensis, of little more than half a meter in height and similar to the Spider Monkey, and the Acrecebus fraileyi, a distant relative of the Black-capped Capuchin.
The explanation for the close relationship among such different animals is the heterogeneity of the landscape of rivers and lakes interlaced by grasslands and forests. “Only this varied scenery would allow for the springing up of such distinct species” says Cozzuol. “This landscape must have remained stable for a few million years, sufficient time for the diversification of the species” he explains. Until 13 million years ago the area upon which the South American wetlands was seated was an immense flatland that stretched through part of Venezuela, the Brazilian Amazon, Colombia, Peru, Bolivia, Uruguay and Argentina, with a hydrographic system very different from that of today.
At that time the Amazon river had not formed and the highest lands, located to the west of Manaus, close to the course of the Purus river, had formed a natural barrier and had impeded the run-off of the rain waters and of the Andes mountains to the east. The dulled water of this plateau ran to the ocean by only two narrow channels. Through the hydrographic basin of the Orinoco river, the waters of the South American lowlands escaped via the north and arrived at the Maracaibo bay, on the Venezuelan coast. To the south, they reached the Atlantic ocean by way of the hydrographic network that gave rise to the Prata river basin millions of years later.
Mountains under growth
This network of rivers and lakes began to be modified some 11 million years ago, when the Andes mountains began to rise in the east of Colombia. Almost at the same time the Atlantic ocean, which had extended from the current Prata river basin in the south of Bolivia and occupied the center-south of Brazil, drew back. The sediments from the Colombian and Peruvian mountain chains fed the vegetation of the plateau and helped to alter the course of the rivers and lakes, which in only a few dozen years had transformed themselves into wetlands. The wetlands, for their part, little by little turned into dry lands in which grass germinated and forests grew. “This continuous deposition of sediment impeded the rivers from flowing out into a stable channel and form the formation of major forests” explains Cozzuol. At the same time this hydrographic network, in constant change, maintained the connection between the north and south of the continent.
Polished up in cooperation with the geologist Edgardo Latrubesse, from the Federal University of Goiás, this scenario began to be mounted some 20 years ago, when Cozzuol, while at the National University of La Plata, studied aquatic mammals that had lived in the north of Argentina between 9 million and 6 million years ago. This was when this Argentinean paleontologist, who ten years ago exchanged his native land for Brazil, found examples of pink river dolphins and sea dolphins distinct from those that had been found not very far from there in Patagonia. These animals might have migrated from the north of South America, but it was not known if these two regions had been connected by rivers or lakes. “There was a lack of knowledge about the fauna that had lived in the north of South America during the Miocene era, between 13 million and 6 million years ago” says Cozzuol. The golden opportunity came about with the announcement of a research position at the Federal University of Rondonia in 1995.
From then until now Cozzuol has traveled to the interior of the state of Acre during the period when the rivers are low. For one obvious reason: this is when the mountains of sediment that preserve the Miocene fossils under the forest are exposed in their mountainous cliffs. He also visited paleontology sites in Venezuela and Peru and analyzed fossils guarded in the museums Bernardino Rivadávia and La Plata, in Argentina, and at the paleontology collection of the Federal University of Acre, brought together by Alceu Ranzi, Jean Boquentin-Villanueva and Jonas de Souza Filho. The comparison between the examples found in these countries revealed that the fauna that lived in Acre during the Miocene era, of which 40 genres were known, is very similar to that of Argentina, of which 130 genres have been identified. “The fauna found in Acre only appears to be poorer” says Cozzuol. “The search for more fossils should show that this fauna is even more diverse than that of Argentina.”Republish