The study, published in the New Mexico Museum of Natural History and Science Bulletin, draws on fossils from the Morrison Formation laid down about 150 million years ago to map a detailed food web for that time. The Morrison Formation is a prominent sequence of Upper Jurassic sedimentary rocks, roughly 156 to 147 million years old, that spans about 1.5 million square kilometres across the western United States and is known as North America s most fertile source of dinosaur fossils. It contains extensive deposits of mudstone, sandstone and limestone from ancient rivers and floodplains that preserved a diverse dinosaur fauna.
Lead author Dr Cassius Morrison of UCL Earth Sciences and colleagues focused their analysis on fossils from the Dry Mesa Dinosaur Quarry in Colorado, a site that preserves a remarkably rich assemblage of dinosaur bones deposited over a period of up to about 10,000 years. The quarry has yielded at least six sauropod species, including Diplodocus, Brachiosaurus and Apatosaurus, alongside a range of carnivorous theropods and other vertebrates. This concentration of material allowed the team to reconstruct interactions among plants, herbivores and predators at an unusually fine scale.
Adult sauropods such as Diplodocus and Brachiosaurus were longer than a blue whale and so massive that their footsteps would have shaken the ground, yet their eggs were only about a foot wide and their offspring needed many years to reach adult size. Dr Morrison noted that size alone would have made it difficult for adults to protect their nests and eggs without crushing them, and evidence suggests that, much like contemporary baby turtles, young sauropods were not cared for by their parents after hatching. As a result, these small, relatively defenceless youngsters had to fend for themselves in landscapes patrolled by large carnivores.
The team concludes that these juvenile sauropods were a major food source for multiple meat eating dinosaurs that shared their environment. Dr Morrison remarked that life was cheap in this ecosystem and that the lives of predators such as Allosaurus were likely fuelled by the consumption of baby sauropods. Because the adults were so large and formidable, predators would have had a much easier time targeting the abundant young than attempting to bring down a full grown sauropod.
To determine who ate what, the researchers combined several independent lines of evidence from the fossil record. They used information on dinosaur body size, wear and tear on teeth, the relative abundance of stable isotopes preserved in bones, and in some cases fossilised stomach contents that reveal an animal s last meal. These data allowed them to infer feeding relationships among herbivores, carnivores and plants with a level of detail usually reserved for studies of modern ecosystems.
The team then used software typically applied to living communities to map out the Late Jurassic food web, charting all the plausible links between plants, invertebrates, vertebrates and their predators. Their analysis shows that sauropods had a central role in this network, with substantially more connections to plants and animals than ornithischian herbivores such as Stegosaurus. Stegosaurs and other ornithischians were heavily armed or otherwise more dangerous prey, while young sauropods offered a plentiful and less risky food source.
Dr Morrison explained that sauropods had a dramatic impact on their ecosystem and that the new work allows researchers to measure and quantify their role for the first time. By reconstructing food webs in this way, scientists can more easily compare dinosaur ecosystems from different times and places. That in turn helps clarify the evolutionary pressures that shaped dinosaur evolution and may explain why particular groups developed certain body plans, feeding strategies or behaviours.
The authors also draw contrasts with the Cretaceous world of Tyrannosaurus rex, which lived around 70 million years later. By that time, sauropods were less abundant in many North American ecosystems, which may have reduced the availability of easy prey for large carnivores. The researchers propose that this scarcity could have contributed to the evolution of traits in tyrannosaurids such as stronger bite forces, larger body sizes and better vision that would have helped them tackle larger and more dangerous prey, including horned dinosaurs like Triceratops.
Co author William Hart of Hofstra University noted that apex predators of the Late Jurassic, such as Allosaurus and Torvosaurus, may have had an easier time finding food than a T. rex did millions of years later. Some Allosaurus fossils show evidence of severe injuries, for example damage likely caused by the spiked tail of a Stegosaurus, that in some cases healed and in others did not. An abundance of juvenile sauropods as relatively safe and plentiful prey may have enabled even heavily injured allosaurs to survive, despite the hazards of hunting large and well defended herbivores.
The study brought together researchers from institutions in the United Kingdom, the United States, Canada and the Netherlands to build this detailed snapshot of a Late Jurassic ecosystem. By combining classic field palaeontology with quantitative food web modelling, the team illustrates how fossil rich formations like the Morrison can reveal not only which dinosaurs lived together but also how energy moved through their communities. This integrative approach promises to refine our understanding of dinosaur palaeoecology and the long term dynamics of ancient ecosystems.
Related Links
Earth Sciences at University College London
Explore The Early Earth at TerraDaily.com
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