The “Age of Reptiles” was supposed to have ended long ago. The 170 million year reign of dinosaurs, pterosaurs, plesiosaurs, and their ilk was brought to an close 66 million years ago by an unfortunate combination of earthly and extraterrestrial causes, opening the way for beasts to take over.
But it’s not as if reptiles entirely disappeared or were relegated to being bit players in the new “Age of Mammals”. Scaly survivors thrived in the new world, spinning off strange and superlative species. Titanoboa, for example, earned its title as “Largest Snake of All Time” about 60 million years ago – a reminder that Cenozoic reptiles need not be looked at with a pitying eye. The 13 million year old rock of Peru has now offered another such reminder. There, paleontologists have found a super-rich collection of crocodylians more diverse than anything seen today.
Called the Iquitos bonebeds, and studied by Universidad Nacional Mayor de San Marco paleontologist Rodolfo Salas-Gismondi and colleagues, the two sites have yielded seven different species of prehistoric crocodylians. The crocs range from stout-snouted monsters to long-jawed fish snatchers and blunt-toothed caimans, and five of the species drawn from the bonebeds have never been seen before. The question facing the paleontologists is why so many different species were present in the same place at the same time.
An abundance of hard-shelled prey might hold part of the answer. There were at least 85 species of clams in the same habitats, not to mention other hard-shelled invertebrates like ostracods, and at least three of the new species described by Salas-Gismondi and colleagues had short, reinforced jaws with low, blunt teeth. Paleontologists often follow such anatomical signposts to the conclusion that these animals relied on a diet of hard-shelled prey, or a diet known as durophagy.
Of these crushing caimans, Gnatusuchus was the most extreme. The caiman’s snout was wider than long, and its mouth had a gap between the front and rear teeth. While it’s 13 million years too late to watch the caiman in action, the researchers suspect that Gnatusuchus dug into the Miocene mud with its strong snout to shovel out freshwater clams that the predator busted open with strong snaps of its back teeth.
That paleontologists have found damaged bivalve shells and shed crocodylian teeth worn down to nubs supports the notion that these caimains really were dedicated shell-crushers. And given that many of the delectable invertebrates dug into sediment below oxygen-depleted waters, the air-breathing caimains were able to reach dining spots that shell-busting fish could not access. For the caimans, at least, this glut of food might explain why so many species have turned up at the same sites.
But the crocodylian heyday couldn’t last forever. Around 12 million years ago the proto-Andes started to rise from the land and break up the great South American swamps, replacing lakes and embayments with rivers. For the clam-crushing caimans, feeding grounds became fewer and further between. The new landscape favored sharp-toothed, generalist caimans of the form that inhabit the Amazon today, shadows of an ancient crocodylian paradise.
Salas-Gismondi, R., Flynn, J., Baby, P., Tejada-Lara, J., Wesselingh, F., Antoine, P. 2015. A Miocene hyperdiverse crocodylian community reveals peculiar trophic dynamics in proto-Amazonian mega-wetlands. Proceedings of the Royal Society B. doi: 10.1098/rspb.2014.2490