Scientists have found that 13 million years ago, as many as seven different species of crocodiles, including three new to science, hunted in the swampy waters of what is now northeastern Peru.
This hyperdiverse assemblage, revealed through more than a decade of work in Amazon bone beds, contains the largest number of crocodile species co-existing in one place at any time in Earth's history, researchers said.
"The modern Amazon River basin contains the world's richest biota, but the origins of this extraordinary diversity are really poorly understood," said John Flynn, Frick Curator of Fossil Mammals at the American Museum of Natural History.
"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," said Flynn, an author of the study published in the journal Proceedings of the Royal Society B.
Before the Amazon basin had its river, which formed about 10.5 million years ago, it contained a massive wetland system, filled with lakes, embayments, swamps, and rivers that drained northward toward the Caribbean.
Since 2002, Flynn has been co-leading prospecting and excavating expeditions with colleagues at fossil outcrops of the Pebas Formation in northeastern Peru.
These outcrops have preserved life from the Miocene, including the seven species of crocodiles.
Three of the species are entirely new to science, the strangest of which is Gnatusuchus pebasensis, a short-faced caiman with globular teeth that is thought to have used its snout to "shovel" mud bottoms, digging for clams and other mollusks.
The new work suggests that the rise of Gnatusuchus and other "durophagous," or shell-crunching, crocodiles is correlated with a peak in mollusk diversity and numbers, which disappeared when the mega-wetlands transformed into the modern Amazon River drainage system.
Besides the blunt-snouted crocodiles like Gnatusuchus, the researchers also recovered the first unambiguous fossil representative of the living smooth-fronted caiman Paleosuchus, which has a longer and higher snout shape suitable for catching a variety of prey, like fish and other active swimming vertebrates.
The new research suggests that with the inception of the Amazon River System, mollusk populations declined and durophagous crocodile species went extinct as caimans with a broader palate diversified into the generalist feeders that dominate modern Amazonian ecosystems.
Today, six species of caimans live in the whole Amazon basin, although only three ever co-exist in the same area and they rarely share the same habitats.
This is in large contrast to their ancient relatives, the seven diverse species that lived together in the same place and time, researchers said.
