About 13 million years ago, up to seven different crocodile species resided in the same swamps in what is now northeastern Peru, a study has found. The Amazon River is incredibly rich in biodiversity, but the research shows that biodiversity was even more remarkable back then.

John Flynn, American Museum of Natural History's Frick Curator of Fossil Mammals and one of the authors of the study, has been co-heading excavating and prospecting expeditions at fossil outcrops in northeastern Peru's Pebas Formation. He and his colleagues found that the Amazon bone beds are home to the most number of species of crocodiles at any given time, which likely occurred because of an abundance in food source.

The study was able to record seven crocodile species co-existing in one place at the same time. Today, the most number of crocodile species converging in the same area at the same time is three.

Before the Amazon River was formed 10.5 million years ago, it was a massive wetland system with swamps, rivers, embayments and lakes draining toward the Caribbean. Knowing the kind of life that was in existence millions of years ago may help in understanding how the Amazon became as diverse as it is now.

Three of the species Flynn and colleagues discovered in the Pebas Formation are completely new. One of them is the Gnatusuchus pebasensis, a short-faced caiman with round teeth thought to be fond of shoveling mud with its snout to dig for clams as well as other mollusks. The study suggests that the peak in the number of crocodile species, particularly shell-crunchers like the Gnatusuchus, coincided with the highest level of mollusk diversity.

Aside from blunt-nosed species like the Gnatusuchus, researchers also uncovered the first unambiguous fossil representative of the Paleosuchus, a living smooth-fronted caiman. The Paleosuchus' higher and longer snout shape makes it capable of catching various prey, such as fish and other swimming vertebrates.

Today, there are six caiman species in the Amazon basin. While just three of them only ever co-existed in the same area, it is very rare for the species to agree to sharing the same habitats.

The study received funding support from NASA, a doctoral grant to Rodolfo Salas-Gismondi (the study's lead author), the Institute of Research for Development, the National Center for Scientific Research, France's ECLIPSE program, the AMNH Frick Fund and The Field Museum.

Other authors include Patrice Baby, Julia Tejada-Lara, Frank Wesselingh and Pierre-Olivier Antoine.