Sixty-seven million years ago, when dinosaur hatchlings first scrambled out of their eggs, their first - and last - glimpse of the world might have been the open jaws of a 3.5-metre-long snake named Sanajeh indicus, based on the discovery in India of a nearly complete fossilised skeleton of a primitive snake coiled inside a dinosaur nest.
The snake lacked the wide-jawed gape seen in modern snakes such as pythons and boas, which would have prohibited it from eating rigid dinosaur eggs. But baby dinosaurs would have been just the right prey size for a large snake, says Jason Head, a palaeontologist and assistant professor in the Department of Biology at the University of Toronto Mississauga.
'Living primitive snakes are small animals whose diet is limited by their jaw size, but the evolution of a large body size in Sanajeh would have allowed it to eat a wide range of prey, including dinosaur hatchlings,' says Head. 'This is the first direct evidence of feeding behaviour in a fossil primitive snake, and shows us that the ecology and early evolutionary history of snakes were much more complex than we would think just by looking at modern snakes today.'
The fossils were first found in 1987 by dinosaur egg expert Dhananjay Mohabey from the Geological Survey of India, in rocks of the Lameta Formation in Gujarat, a state in western India known for its rich fossil record of dinosaurs and their eggs. Originally identified as a hatchling dinosaur, the fossils were recognised to include a snake by dinosaur palaeontologist Jeff Wilson from the University of Michigan and Mohabey in 2001.
'I saw the characteristic vertebrae of a snake beside the dinosaur eggshell and larger bones, and I knew it was an extraordinary specimen... even if I couldn't put the whole story together at that point. I just knew we needed to examine it further,' said Wilson. They invited snake specialist Head and geologist Shanan Peters from the University of Wisconsin-Madison, to collaborate on the study of the fossils, including field and lab work in India, the U.S.A., and Canada.
Sanajeh indicus, which means 'ancient gape from India,' is represented by a nearly complete skull and lower jaws along with vertebrae and ribs coiled around a crushed titanosaur egg, next to the remains of a 0.5-metre-long titanosaur hatchling. These dinosaurs, part of a larger group called sauropods, were long-necked, four-legged plant-eaters that grew to weigh up to 100 tonnes, and Wilson says they likely grew quickly in their first year, beyond the reach of predators like Sanajeh.
The findings - along with two other similar snake-egg pairings, suggest that snakes fed on titanosaur hatchlings when they emerged from their eggs. 'The eggs were laid in loose sands and covered by a thin layer of sediment. We think that the hatchling had just exited its egg, and its movement attracted the snake,' explains Mohabey. 'It would have been a smorgasbord,' says Head. 'Hundreds or thousands of defenceless baby sauropods could have supported an ecosystem of predators during the hatching season.'
The remains capture a moment in Cretaceous time. 'Burial was rapid and deep,' says Peters. 'Probably a pulse of slushy sand and mud released during a storm caught them in the act.'