Before the Amazon River formed as we know it today, some 10.5 million years ago, water in the Amazon basin flowed northward to the Caribbean. Instead of thick rainforest, the habitat was a massive wetland system -- a system researchers say was dominated by crocodiles.

Some 13 million years ago, northeastern Peru hosted seven species of crocodiles, a diverse co-existing assemblage not seen before or since. The arrival of the Amazon, flowing toward the Atlantic, flooded the habitat and diminished mollusk populations -- the main food source for several of the thriving croc species.

"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," the lead study author Rodolfo Salas-Gismondi, a researcher at the University of Montpellier, in France, said in a press release.

"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," Salas-Gismondi added.

Species of crocodiles with more a more generalized approach to feeding were more likely to survive the changes. Today, six caiman crocodile species live in the Amazon basin, but no more than three exist in the same habitat.

Clams and snails (and the crocs that ate them) weren't the only ones hard done by the formation of the modern Amazon River. It also made paleontology exceptionally difficult.

"The modern Amazon River basin contains the world's richest biota, but the origins of this extraordinary diversity are really poorly understood," said co-author John Flynn, an paleontologist and curatory with 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," Flynn explained. "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."

The new research was published this week in the journal Proceedings of the Royal Society B.