The Monsters of Pebas

The Amazon River is the world’s largest river and it has the world’s largest drainage basin—the vast Amazon rainforest, which stretches from the Andes in the west, to the Guiana Highlands to the north and the Brazilian Highlands in the south.   The great river drains east into the Atlantic Ocean….but it was not always so.  Before the Andes Mountains rose, the river drained west into the Pacific.  Throughout the Cenozoic, the mouth of the river moved up around the continent.  Thirteen million years ago, during the Miocene, the river drained north into the Caribbean through a huge tropical swamp–the Pebas mega-wetlands–which covered over one million square kilometers of what is now the Amazon Basin.

An illustration of Pebas Corocodilians–Gnatusuchus is underwater, gobbling clams (art by Javier Herbozo)

Like today’s Amazon Basin, the Pebas mega-wetland was a great riverine rainforest.  And yet the ecosystem was very different from what is there today.  The marshes and swamps were filled with bivalve mollusks that thrived in the oxygen-poor waters.  Predators evolved to feed on these clams and mussels…and what predators!  This is Gnatusuchus, a caiman with spherical teeth for crushing open shellfish. Can you imagine biting through the shell of a clam?  Just thinking about it makes my jaw hurt and my teeth feel broken.  Yet Gnatusuchus bit through heavy shells for every meal!

A life-sized reconstruction of the gigantic Purussaurus

The crocodilian grew to lengths of 1.5 meters (about 5 feet) and had a short round shovel-shaped mouth to focus maximum force on biting through clams.  Life in the Pebas was not all basking and clam feasts for Gnatusuchus.  The reptile was hardly the only reptile in the swamp, but was instead one genus among a hyper-diverse group of crocodilians including giant toothy predators capable of eating Gnatusuchus.  One of these predators, Purussaurus neivensis grew to be 12.5 metres (41 ft) in lengt—making it a rival of the great Mesozoic crocodilians like Phobosuchus (maybe I should have mentioned this horrifying monster first, instead of alluding to him after the clam-eater, but Ferrebeekeeper is interested in mollusks and their predators not in giant crocodiles: this is not Peter Pan, my friend).  There were also piscivorous crocodilians with long scissor snouts foll of hooked teeth (like modern gharials), and even little crocodilians on stilt-like legs that ran around plucking up small prey in the manner of pipers or herons.

Seven million years ago, the Pebas began to change from swamps to channels as Amazonian drainage became spread through an even more enormous basin. Still, the diversity of the creature that lived there became a heritage for the contemporary Amazon, arguably the most diverse ecosystem in the world today.

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