Fossils reveal that some snakes preyed on baby sauropods.
Ancient snakes lurked in dinosaur nesting grounds in order to gobble up hatchlings as they emerged from eggs, fossils from western India suggest.
Analysis of a clutch of Cretaceous-period dinosaur eggs shows that bones within the nest belong to a 3.5-metre-long predatory snake, coiled around an egg and near the remains of a sauropod hatchling1. The find offers a rare glimpse at the feeding behaviour of Cretaceous snakes and reveals a previously unrecognized threat that hatchling dinosaurs likely faced.
The eggs were first discovered in 1987 by Dhananjay Mohabey, who works at the Geological Survey of India. He identified them as Sauropoda, the long-necked group of dinosaurs to which Brachiosaurus and Diplodocus belong, and assumed that the accompanying bones were those of hatchling sauropods.
Twenty-three years later, Mohabey, palaeontologist Jeffrey Wilson at the University of Michigan and a team of colleagues now reveal that some of the bones belong to a predatory snake, which they call Sanajeh indicus.
Evolutionary ecologist Harry Greene at Cornell University in Ithaca, New York, says the find is "spectacular", as well-preserved snake fossils are so rare, particularly those that preserve substantial parts of the head.
"When Dhananjay first showed me the fossil, I was so stunned when I realized 'this is an articulated Cretaceous snake' that it didn't hit me until a few hours later that 'this is a Cretaceous snake in a dinosaur nest'," says Wilson.
A key part of the investigation came when Wilson and Mohabey uncovered a long-lost piece of the original fossil that provided evidence of the snake's relationship to the dinosaur. "When we found the last piece of fossil matrix and snapped it into place, we could clearly see the snake was coiled around a dinosaur egg — it was incredible," recalls Wilson.
Though Sanajeh was large, the snake's skull structure suggests that it could not open its jaws very far. A wide gape is crucial to large snakes like modern boas and pythons, which crush and eat animals by wrapping their mouths around their prey and swallowing them whole. That Sanajeh could not do this suggests that it was unable to eat the large and hard-shelled eggs that it was coiled around, but instead waited in the nest to eat smaller and softer hatchling dinosaurs.
Palaeontologist Angela Milner at the Natural History Museum in London, who was not involved in the research, says the study is the first to report a snake in association with a dinosaur nesting site. Many dinosaurs nested communally to gain safety in numbers. That would be attractive to a range of nest predators such as snakes, says Milner. "Newly emerged hatchlings would have been sitting ducks before they had time to disperse...so it seems Sanajeh hung around the eggs waiting for an easy meal," she says.
Wilson's team say other nests at the same site in India also show the presence of bones of the same snake, suggesting that they regularly preyed on dinosaurs in this way. "This points to a predation pressure on sauropods that we had not really considered before," says Wilson. The dinosaur hatchlings likely grew fast to get themselves too large to be eaten by Sanajeh, he explains.
Like most of the land-based reptiles on the Indian subcontinent that Wilson and his colleagues have studied, the Sanajeh bones show puzzling links to fauna on southern landmasses, such as Africa, Antarctica, Australia and Madagascar.
India was interlocked with these areas in its remote past, but plate reconstructions show the continent became geographically isolated for a long stretch of its northward drift before it connected with Asia 50 million years ago. Despite this isolation, "India's biota retained a strong southern connection — but how that was accomplished remains a mystery", Wilson adds.
Wilson, J. et al. PLoS Biol. 8, e1000322 (2010).