Researchers excavating 13-million-year-old bonebeds in Peru reveal that seven different croc species were living together in the mega-wetlands of proto-Amazonia. From caimans with blunt snouts to duck-faced caimans, this is the largest number of crocodylian species coexisting in one place ever. The findings, published in Proceedings of the Royal Society B this week, suggests that the unusually high abundance of clams and snails led to specialized feeding strategies and this awesome hyperdiversity.
The transcontinental Amazon River system as we know it (that is, draining west to east into the Atlantic), formed around 10.5 million years ago. But before then, the area was a massive wetland network of lakes, swamps, and rivers that drained northward toward the Caribbean. And as those unique habitats declined, selection began favoring our modern generalist-feeding caimans. While previous work has unearthed lots of fossil invertebrates like mollusks and crustaceans, vertebrate fossils from before that pivotal switch are rare.
"We uncovered this special moment in time when the ancient mega-wetland ecosystem reached its peak in size and complexity, just before its demise," study leader Rodolfo Salas-Gismondi of Université de Montpellier says in a news 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."
An international team spent over a decade exploring this hyperdiverse crocodylian assemblage in Miocene fossil outcroppings of the Pebas Formation in northeastern Peru. Of those seven coexisting crocs, three are never-before-seen species (pictured above): “small nosed” Gnatusuchus pebasensis, the “crushing machine” Kuttanacaiman iquitosensis, and Caiman wannlangstoni. All three were small, blunt-snouted caimans with teeth built for crushing. This shell-crunching (or durophagous) feeding strategy matches up well with the observed peak in mollusk diversity and abundance.
For example, short-faced Gnatusuchus pebasensis (right) would shovel mud with its snouth, and after digging up clams, they simply chomped down with their globular teeth. "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," Salas-Gismondi adds.
But with the inception of the modern Amazon River system, mollusk populations declined and specialist durophagous caimans went extinct. Those that were able to capture a variety of prey diversified into generalist feeders—such as the smooth-fronted caiman Paleosuchus, fossils of which the team had also found. This living species had longer, higher snouts well suited for catching swimming prey, like fish.
Nowadays, there are only six species of caimans in the whole Amazon basin, and only three ever occupy the same area at once (though they rarely share the same habitats). “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,” says study co-author John Flynn from the American Museum of Natural History.
Here are the skulls and jaws from all seven different species that used to live together in what’s now the Amazon Basin of northeastern Peru. Notice how diverse their snouts are: (1) Gnatusuchus pebasensis, (2) Kuttanacaiman iquitosensis, (3) Caiman wannlangstoni, (4) Purussaurus neivensis, (5) Mourasuchus atopus, (6) Pebas Paleosuchus, and (7) Pebas gavialoid. The three new species (1-3) are shown in illustrations below their respective fossils.
Images: Javier Herbozo (top), Model by Kevin Montalbán-Rivera & Aldo Benites-Palomino (middle), Javier Herbozo & Rodolfo Salas-Gismondi (bottom)