Jump to content Jump to navigation

Paradise of the crocodiles

Posted on 26-02-2015 by Frank Wesselingh

Suppose you are taking a swim in a tropical lake. Through your goggles you see a scenery teeming with life, plants, fish, is that a stingray?  

At a distance you spot a silhouette of a crocodile with a round snout. His position is odd, with nose down shoveling into the dysoxic lake floor.

If you had the guts to go swimming in Lake Pebas, a lake twice the size of France that existed 13 million years ago in western Amazonia, you may have spotted several other crocodylian species as well. Three of them were adapted to crack and eat shells. But also a large long-snouted gavial or the 12 meter long Purussaurus would be swimming around you.

At the time at least seven crocodylian species lived in the lake, possibly even more. Researchers have now described three of them and asked the question why crocodylian diversity was so high in this lake. A clear indication is found in the shells that once lived in the lake and that have been extensively studied by Naturalis researcher Frank Wesselingh. 

Reconstruction of proto-Amazonian swamps from the late Middle Miocene (approximately 13 million years ago) and three new blunt-snouted caimans with crushing teeth specialized for feeding on clams, snails and other molluscs. (Javier Herbozo)

In its 12 million history, Lake Pebas hosted a remarkable evolutionary radiation of endemic bivalves. These so-called pachydontines adapted to live in the dysoxic lake floors. They formed an ideal and abundant food source for caimanine crocodiles who adapted very specific nutcracker teeth and shovel type of mandibles to harvest the pachydontines.

If you swim today in the Amazon or one of its lakes you may encounter a caiman, but none of the bizarre morphs of 13 million years ago. These all went extinct with the demise of the mega lakes around 7 million years ago.