Babies and very young sauropods – the long-necked, long-tailed plant-eaters that in adulthood were the largest animals to have ever walked on land – were a key food sustaining predators in the Late Jurassic, according to a new study led by a UCL (University College London) researcher.
The study, published in the New Mexico Museum of Natural History and Science Bulletin, used data from fossils laid down 150 million years ago in the Morrison Formation*, in the United States, to map out a "food web" of the time – a gigantic network of who ate what and who ate whom.
The research team found that very young sauropods, relatively defenceless and left to fend for themselves by their enormous parents, were a major food source for multiple meat eaters.
Lead author Dr Cassius Morrison, based at UCL Earth Sciences, said: "Adult sauropods such as the Diplodocus and Brachiosaurus were longer than a blue whale. When they walked the earth would shake. Their eggs, though, were just a foot wide and once hatched their offspring would take many years to grow.
"Size alone would make it difficult for sauropods to look after their eggs without destroying them, and evidence suggests that, much like baby turtles today, young sauropods were not looked after by their parents.
"Life was cheap in this ecosystem and the lives of predators such as the Allosaurus were likely fuelled by the consumption of these baby sauropods."
The research team used fossil records from a single quarry, the Dry Mesa Dinosaur Quarry in the state of Colorado, where a remarkably rich collection of dinosaur fossils was deposited across a time span of up to 10,000 or so years, including at least six species of sauropod (among them a Diplodocus, Brachiosaurus, and Apatosaurus).
To determine who ate what, the researchers used existing data such as dinosaur size, wear and tear on their teeth, the abundance of certain isotopes in the remains, and in some cases the fossilised contents of their stomach revealing their last meal.
They then mapped out the food web of the time – i.e. all the possible links between dinosaurs, other animals and plants – at a higher resolution than has previously been carried out for dinosaurs, with the help of software typically used for modern ecosystems.
The team concluded that sauropods had a key role in this ecosystem, with substantially more links to plants and animals than the other main group of vegetarian dinosaurs, the ornithischians (plant-eaters such as the armoured Stegosaurus who were more dangerous prey).
Dr Morrison explained: "Sauropods had a dramatic impact on their ecosystem. Our study allows us to measure and quantify the role they had for the first time.
"Reconstructing food webs means we can more easily compare dinosaur ecosystems across different periods. It helps us to understand evolutionary pressures and why dinosaurs might have evolved in the way they did."
The researchers noted that 70 million years later, during the time of Tyrannosaurus Rex, fewer sauropods providing easy prey may have helped trigger the evolutionary adaptations (stronger bite force, larger size, better vision) allowing the T. Rex to hunt larger, more dangerous animals, such as a Triceratops, which were armed with three large horns.
William Hart, one of the co-authors from Hofstra University in the United States, said: "The apex predators of the Late Jurassic, such as the Allosaurus or the Torvosaurus, may have had an easier time acquiring food compared to the T. Rex millions of years later.
"Some Allosaurus fossils show signs of quite horrific injuries – for instance caused by the spiked tail of a Stegosaurus – that had healed and some which hadn't. But an abundance of easy prey in the form of young sauropods may have allowed injured allosaurs to survive."
The study involved researchers at institutions in the UK, the United States, Canada and the Netherlands.
* The Morrison Formation is a prominent sequence of Upper Jurassic sedimentary rock (approx. 156–147 million years old) spanning 1.5 million square kilometres across the western United States. Known as North America's most fertile source of dinosaur fossils, it contains massive deposits of mudstone, sandstone, and limestone from ancient rivers and floodplains.
