It may have been suicidal for a predator to go after a healthy adult Brachiosaurus, a behemoth weighing perhaps 60 tons that was a member of the long-necked group of dinosaurs called sauropods that included the largest land animals ever on Earth.
But, as new research shows, Brachiosaurus and other sauropod babies appear to have been regular dinner fare for meat-eating dinosaurs 150 million years ago.
Using multiple lines of evidence, scientists reconstructed the food web for a Jurassic Period ecosystem represented by the numerous fossils unearthed at the Dry Mesa Dinosaur Quarry in southwestern Colorado, mapping out who ate what and who ate whom.
The ecosystem was teeming with life, with at least six types of sauropods, known for their long necks, small heads, four pillar-like legs and lengthy tails, and five types of meat-eating dinosaurs.
There were also various other plant-eating dinosaurs as well as flying reptiles called pterosaurs, smaller reptiles, early mammals, crocs, fish and insects.
The researchers concluded that baby and young juvenile sauropods were the most common food source for the meat-eating dinosaurs at the top of the food chain.
“These sauropods would have been high in abundance compared to larger adult sauropods and were relatively defenseless and slow-moving, hence easy to catch and a perfect snack,” said paleontologist Cassius Morrison, a postdoctoral researcher at University College London and lead author of the study published in the New Mexico Museum of Natural History and Science Bulletin.
While Brachiosaurus was the ecosystem’s most massive sauropod, the longest was probably Diplodocus, at roughly 100 feet (30 meters) or more.
Other sauropods, all big, sharing this ecosystem were Supersaurus, Apatosaurus, Camarasaurus and Haplocanthosaurus. But their hatchlings emerged from eggs just a foot (30 cm) wide and needed many years to grow, and evidence suggests they were left to fend for themselves by their parents.
“Adult sauropods relied on their enormous size, long tails and herd behavior for protection,” said ecologist and study co-author Steven Allain, a lecturer in animal science at Anglia Ruskin University Writtle in England.
“Unfortunately, this took time, meaning that the smaller individuals hadn’t reached that ‘too big to mess with’ stage yet. They lacked armor, spikes or heavy plates, making them far easier to subdue than dinosaurs like Stegosaurus, whose thagomizer – tail spikes – could seriously injure or kill a predator,” Allain said.
This ecosystem’s meat-eating dinosaurs were formidable. The largest were Torvosaurus, at around 30 feet (9 meters) long, and Allosaurus, around 26 feet (8 meters) long. There also were Ceratosaurus at around 23 feet (7 meters), Marshosaurus at about 15 feet (4.5 meters) and Stokesosaurus at approximately 12 feet (3.5 meters).
“Hunting a healthy adult Brachiosaurus – or any large sauropod – would have been an extremely daunting, high-risk task for even the largest theropod in the Dry Mesa ecosystem,” Allain said, “with sheer size acting as their primary defense.”
“One well-placed tail swing or a simple sideways step could seriously injure or kill a predator. Even if Allosaurus hunted in groups – which is still debated – bringing down a fully grown, healthy sauropod would have required coordination, stamina and a lot of luck.
“Because of that risk, predators likely focused on safer options such as juveniles, sick or injured adults, individuals stuck in mud or carcasses from drought or floods,” Allain said.
The researchers considered several types of evidence in reconstructing the complex Dry Mesa food web. Among other things, they looked at chemical evidence in tooth enamel that indicated the types of food an animal ate, scratches left on tooth enamel that indicated the types of food eaten, biomechanical models and fossilized stomach remains.
“This deposit was made by a drought, so it’s one of the only places where you get everything, from small lizard-like animals to the largest dinosaurs,” Morrison said.
The environment was dominated by open woodlands of plants such as conifers, cycads, ferns and horsetails growing along rivers and shallow ponds that periodically dried out.
“The reconstructed food web this data has generated contains over 12,000 unique food chains, indicating a richly interconnected system rather than a simple hierarchy of predators and prey. Sauropods emerged from this analysis as central components of this network,” Allain said.