Crocodilians: crocodiles, alligators, caiman and gavials; are commonly considered to be relicts, leftovers from the time of the dinosaurs and unchanged for millions of years. This is of course preposterous and there is no better example of this than research out in the Proceedings of the Royal Society B on the Pebas mega-wetland system. Around 13 million years ago a massive wetland – swamp – lake complex extended over large areas of modern Colombia, Peru and Brazil. This massive wetlands habitat eventually drained north into the Caribbean Sea. In this extensive and biologically productive habitat were abundant food sources including 85 endemic species of molluscs. Many of these have anti-predatory shells that were either very thick or heavily ornamented (extra ridges or spikes), but collections of these mollusc fossils still show large amounts of predatory damage. This damage had been previously attributed to fish or crustacean damage, although fossils of these types of predators had not been found. What have been found at Pebas are seven crocodilians, including six caiman species and one gavial. Such a high diversity of predatory animals is usually possible through niche partitioning and the fossil evidence shows this. Whilst apex predators (like the modern alligator) are present in the form of the massive Purussaurus neivensis (estimated total body length of 8 metres (about half the length of an articulated lorry)) and Mourasuchus atopus, four other species have conical shaped teeth that would be ideal for crushing hard shell material. What the Pebas environment demonstrates is the crocodilians ability to evolve and adapt to exploit a resource, making them a dominant feature of this extinct habitat. Unfortunately for the Pebas fauna nothing lasts forever. Continued uplift of the Andes Mountains eventually modified the topography of the Pebas area and created a river system flowing to the east: the Amazon River was born and shell-crunching caiman gradually went extinct.
Amazonia contains one of the world's richest biotas, but origins of this diversity remain obscure. Onset of the Amazon River drainage at approximately 10.5 Ma represented a major shift in Neotropical ecosystems, and proto-Amazonian biotas just prior to this pivotal episode are integral to understanding origins of Amazonian biodiversity, yet vertebrate fossil evidence is extraordinarily rare.