Crocodiles ruled the Amazon thirteen million years ago
There were seven different species of crocodiles in pre-Amazonian northeastern Peru thirteen million years ago. In no time in Earth’s history have so many crocodile species coexisted in one place, said researchers at the American Museum of Natural History.
This coexistence of so many species, discovered after more than ten years of work in Amazon zone beds, was probably due to an abundant food source, especially of mollusks like snails and clams, animals which today form a small part of a crocodile’s diet.
According to the researchers, who published their findings in the academic journal Proceedings of the Royal Society B (citation below), scientists will now have a better understanding of the history of the Amazon’s incredible rich biodiversity.
A life reconstruction of the head of the mollusk-crunching Gnatusuchus pebasensis, a 13-million-year-old, short-faced crocodile. (Credit: Aldo Benites-Palomino)
Co-author, Professor John Flynn, Frick Curator of Fossil Mammals at the American Museum of Natural History, said:
“The modern Amazon River basin contains the world’s richest biota, but the origins of this extraordinary diversity are really poorly understood. Because it’s a vast rain forest today, our exposure to rocks–and therefore, also to the fossils those rocks may preserve–is extremely limited.”
“So anytime you get a special window like these fossilized “mega-wetland” deposits, with so many new and peculiar species, it can provide novel insights into ancient ecosystems. And what we’ve found isn’t necessarily what you would expect.”
The Amazon basin’s river formed about 10.5 million years ago. Before then, it contained a huge wetland system, filled with swamps, lakes, embayments (recesses in coastlines forming bays), and rivers that drained toward the Caribbean (northward). Today the river flow is eastward to the Atlantic Ocean.
In order to properly understand the history and origins of modern Amazonian diversity, it is important to know what kinds of life existed at that time. Although there is plenty of fossil evidence of invertebrates like crustaceans and mollusks in the region, apart from fish fossils, there are very few of vertebrates.
Prof. Flynn has been co-leading excavating and prospecting expeditions with colleagues at fossil outcrops of the Pebas Formation in the north east of Peru. These outcrops have fossils from the Miocene (from 23.03 to 5.332 million years ago).
Crocodiles in the proto-Amazonian “mega-wetland” swamps 13 million years ago. (Image: American Museum of Natural History)
Three new crocodile species discovered
Three crocodile species are completely new to science. The researchers were fascinated by one of them – Gnatusuchus pebasensis – a caiman with a short snout and globular teeth that is believed to have used its short face to “shovel” mud bottoms, prodding around for clams and other mollusks.
The study suggest that the rise of Gnatusuchus and other shell-crunching crocodiles (durophagous) coincided with a peak in mollusk numbers and diversity, which vanished when the massive wetlands transformed into the modern Amazon River drainage system.
Lead author, Rodolfo Salas-Gismondi, a graduate student at the University of Montpellier, in France, said:
“When we analyzed Gnatusuchus bones and realized that it was probably a head-burrowing and shoveling caiman preying on mollusks living in muddy river and swamp bottoms, we knew it was a milestone for understanding proto-Amazonian wetland feeding dynamics.”
Mr. Rodolfo Salas-Gismond is also a researcher and head of the paleontology department at the National University of San Marcos’ Museum of Natural History in Lima, Peru.
Apart from the blunt-snouted crocodiles, the team also recovered the first fossil representative of Paleosuchus, a smooth-fronted caiman which had a longer and higher snout shape suitable for catching a wider range of prey, including fish and other active swimming vertebrates.
Mr. Salas-Gismondi said:
“We uncovered this special moment in time when the ancient mega-wetland ecosystem reached its peak in size and complexity, just before its demise and the start of the modern Amazon River system.”
“At this moment, most known caiman groups co-existed: ancient lineages bearing unusual blunt snouts and globular teeth along with those more generalized feeders representing the beginning of what was to come.”
The new study findings suggest that when the Amazon River system began, mollusk populations plummeted and durophagous crocodile species became extinct as caimans with broader palate diversified into generalist feeders that today dominate Amazonian ecosystems.
In the Amazon basin today, there are six species of caimans. However, only three ever co-exist in the same area. They rarely share the same habitats. Millions of years ago seven species lived quite happily together in the same place.
Citation: “A Miocene hyperdiverse crocodylian community reveals peculiar trophic dynamics in proto-Amazonian mega-wetlands,” Rodolfo Salas-Gismondi , John J. Flynn , Patrice Baby , Julia V. Tejada-Lara , Frank P. Wesselingh & Pierre-Olivier Antoine. Proceedings of the Royal Society B. February 25, 2015. DOI: 10.1098/rspb.2014.2490.