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Mega alligators inhabited Amazonian giant swamp millions of years ago

Unprecedented chemical analysis of minerals in the region made it possible to discover the precise age of the fossilized fauna

Unprecedented chemical analysis of minerals in the region made it possible to discover the precise age of the fossilized fauna

Prospection work on Solimões Formation deposits on the Purus River, in Acre – Photo: Divulgation/Paleontology Laboratory, Ribeirão Preto

A new research by the Department of Biology of the Faculty of Philosophy, Sciences and Letters of Ribeirão Preto, USP, performed dating of rocks from the Amazon region, indicating its geological age with precision. Such analyzes provide greater understanding about the formation of the region and its fauna. The work is written by Marcos César Bissaro Junior, who was guided by Professor Annie Schmaltz Hsiou. It is the first time that the absolute dating in these sedimentary layers is performed, which made it possible to discover the exact time in which fossilized animals lived in the region.

Marcos César Bissaro Júnior – Photo: Reproduction

The analysis of these rocks from Talismã and Niterói sites, in the region between the states of Acre and Amazonas, revealed the phenomena that allowed a mega fauna to inhabit the region millions of years ago. These giant animals populated the marshy region that is now called the Pebas system, and could be more than ten meters long, ranging from reptiles to giant rodents. “We have seen that the diversity boom happened in the Neogene, the period where the Miocene is,” Marcos said.

According to Annie, dating can help to understand the origin of Amazonian biodiversity, the impact of the emergence of the Andes, which would have reversed the course of the Amazon River some 23 million years ago. This fact caused an accumulation of water in the studied region, forming a mega swamp, propitious for the proliferation of a rich mega-sized fauna. “All the water arms that underwent this change flowed into the Pebas system, creating this great lake. That’s the story the sediments in that region tell us.”

One of the species of Purussaurus was described: P. brasiliensis (illustrative image), the largest of all, from Solimões Formation (Late Miocene, Acre) – Illustration: Ornitholestes via Wikimedia Commons/Public domain

This large lake most notably housed the largest crocodilians of the Cenozoic period in South America, Purussaurus brasiliensis, or Purussauro, which reached about 12 meters long. Another animal was Neopliblema, which would be a kind of giant capybara. These species, along with others like snakes, turtles and sloths, indicate, according to Annie, an environment extremely rich in water.

Annie Schmaltz Hsiou – Photo: Reproduction

The sediments analyzed date from the Miocene period, about 8 to 10 million years ago. The dates were made by measuring the presence of uranium and lead in the rocks. Uranium, which undergoes chemical decay, turns into lead over time. By measuring the amount of these elements in the minerals, it is possible to verify the age of the rocks.

Until the study, what was used to date the fossils was an estimate based on discoveries from other places in South America. According to the dating of these other fossils, there was attribution of the same estimated date to the discoveries in the Amazon. “A certain species lived in the Amazon, but it was also seen in the Miocene of Argentina, so it was assumed that they lived at the same time,” Annie explained.

“Now, with the absolute date, we can see not only when this Amazonian biome was formed, but also the fossils of the animals that lived in it.”

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