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For a predator, hunting down a healthy adult Brachiosaurus, a behemoth weighing around 60 tons, could be suicidal. It belongs to the group of long-necked dinosaurs, the sauropods, which include the largest land animals on Earth.
But, as new research shows, 150 million years ago, the young of Brachiosaurus and other sauropods seemed to have been a regular dinner for carnivorous dinosaurs.
Use multiple sources of evidenceScientists have reconstructed the food web of a Jurassic-era ecosystem, represented by a trove of fossils unearthed at the Dry Mesa Dinosaur Quarry in southwestern Colorado, to map who ate what and who ate whom.
The ecosystem teems with life, with at least six species of sauropods, known for their long necks, small heads, four columnar legs and long tails, and five species of carnivorous dinosaurs.
There were various other plant-eating dinosaurs as well as flying reptiles called pterosaurs, smaller reptiles, early mammals, crocodiles, fish, and insects.
The researchers concluded that young and young sauropods were the most common food source for the carnivorous dinosaurs at the top of the food chain.
“These sauropods were much more numerous than larger adult sauropods and were relatively defenseless and slow-moving, making them easy to capture and perfect snacks,” said paleontologist Cassius Morrison, a postdoctoral fellow at University College London and lead author of the study published in Nature. New Mexico Museum of Natural History and Science Announcements.
Although Brachiosaurus is the largest sauropod in the ecosystem, the longest was probably Diplodocus, which was approximately 100 feet (30 meters) or more.
Other large sauropods that shared this ecosystem include Supersaurus, Apatosaurus, Camarasaurus, and Semisaurus. But they are only a foot (30 centimeters) wide when they hatch from eggs, take many years to grow, and there is evidence they are left to fend for themselves by their parents.
“Adult sauropods relied on their large size, long tails and group behavior for protection,” said ecologist Steven Allan, study co-author and lecturer in animal sciences at Anglia Ruskin University, Rittel, UK.
“Unfortunately, this takes time, which means smaller individuals haven’t reached the ‘too big to bother’ stage yet. Their lack of armor, spikes, or thick plates makes them easier to subdue than dinosaurs like Stegosaurus, whose tail spikes could seriously injure or kill a predator,” Alain said.
The carnivorous dinosaurs in this ecosystem were very powerful. The largest was Tortosaurus, which was about 30 feet (9 meters) long, and Allosaurus was about 26 feet (8 meters) long. There was also Ceratosaurus, about 23 feet (7 meters), Massosaurus about 15 feet (4.5 meters), and Storkosaurus about 12 feet (3.5 meters).
“Even for the largest theropods in dry mesa ecosystems, hunting a healthy adult Brachiosaurus, or any large sauropod for that matter, is an extremely difficult and high-risk task, as their huge size is their primary defense,” Alain said.
“A well-placed tail swipe or a simple side step could seriously injure or kill a predator. Even if Allosaurus hunted in packs – which remains controversial – taking down a fully grown, healthy sauropod would have required coordination, stamina and luck.
“Because of this risk, predators may focus on safer options such as juveniles, sick or injured adults, individuals stuck in mud, or carcasses that died from drought or flooding,” Alain said.
The researchers considered several types of evidence when reconstructing the complex dry mesa food web. Among other things, they looked at chemical evidence in tooth enamel that indicates the type of food the animal ate, scratches left on tooth enamel that indicate the type of food eaten, biomechanical models and fossil stomach remains.
“This deposit was formed by drought, so it’s one of the only places that has everything from small lizard-like animals to the largest dinosaurs,” Morrison said.
The environment is dominated by open woodlands with plants such as conifers, cycads, ferns and horsetails that grow along rivers and shallow ponds that periodically dry out.
“The reconstructed food web generated by this data contains more than 12,000 unique food chains, suggesting that this was a tightly connected system rather than a simple hierarchy of predators and prey. Sauropods emerged from this analysis as a core component of the network,” Alain said.