About 150 million years ago, the land that is now the western United States was alive with dinosaurs. New research shows that the smallest members of the biggest dinosaurs played a huge role in keeping that world alive.
Researchers from University College London and Hofstra University recreated a complex food chain from the Late Jurassic. Their investigations were primarily located at a paleontological site known as the Morrison Formation, and a location in Colorado identified as the Dry Mesa Dinosaur Quarry. Their findings will be published in the New Mexico Museum of Natural History and Science Bulletin.
Under the direction of Dr. Cassius Morrison at University College London, scientists learned that during the age of the sauropods, the babies and small juvenile sauropods provided the main source of food for the meat-eating dinosaurs. The sauropods, the long-tailed or long-necked vegetarians, eventually grew to larger sizes than any other land animal.
While Diplodocus and Brachiosaurus, adult sauropods, were both enormous animals, Dr. Morrison explains, “When an adult sauropod like this walked, it would shake the earth beneath us.”
More importantly, however, the size of the juveniles was the issue. These juvenile sauropods began life in egg-size, about one foot in diameter, and would grow to full size over a very long period of time. The evidence indicates that the parent sauropods would not remain to protect their eggs or young.
“Given the weight of the parent sauropods, it is very likely that they would not be able to care for the eggs without crushing them,” adds Dr. Morrison. He further noted that juvenile sauropods were most likely abandoned at hatching, similar to baby turtles today.
This made the juveniles an easy target for predation. “Life in this ecosystem was very cheap, and the existence of a predator such as an Allosaurus would have been dependent on the consumption of baby sauropods for nourishment,” said Morrison.
The fossils used in the study were found in Dry Mesa, where the remains were deposited during a very short period of time, 10,000 years or less. They therefore represent a period when all the animals lived in the same ecosystem rather than coming from mixed periods of time.
The quarry contained fossils of at least six different sauropod species, along with other predators, smaller mammals and reptiles, as well as plants.
To determine which dinosaurs ate which, researchers compiled several different types of clues. They looked at the size of dinosaurs, the wear patterns on their teeth, the bone structure of dinosaurs, and chemical analysis of body fluids preserved in fossils. Occasionally, they were able to determine what the last meal of a dinosaur was from the fossilized contents of the dinosaur’s stomach.
These various clues were then entered into computer software that is typically used for studying modern ecosystems. In this software, each individual animal was represented by a number of nodes on the food web. As the dinosaurs grew and evolved, each of them developed their own unique dietary requirements and risk factors, which resulted in their presence as separate nodes within the food web.
Plants were also grouped into height categories, ranging from ground level to the tops of tall trees. Insects and other smaller animals were also added to this food web, even though there were no complete fossils. Their presence was considered highly likely, given the fossil record of these ecosystems.
The final result of this study was a very complex food web that included just under 100 different ecological nodes. All of these nodes were connected through more than 700 different feeding connections. Food chains that existed during this period in Earth’s history were typically short. They began with plants, which were then eaten by herbivores, and then by carnivores.
The rapid succession of these food chains indicates how quickly energy travelled through the different levels within this system.
Throughout this ancient ecosystem, sauropods played a very central role in many of the Earth’s plant and animal communities. Their feeding had more significant impacts on the growth of plants and predators in the environment than did other types of herbivores, such as Stegosaurus, which were better protected due to their armoured skin and spikes.
As young sauropods matured, there were many species of carnivores actively hunting them. Adults, however, had no natural predators. As the young developed into adults, their diet shifted from lower-level plants to taller trees. As such, the growth and feeding of one species affected various aspects of the ecosystem during its life.
Researchers have presented data to suggest that sauropods were true engineers of their ecosystems. In addition to shaping plant growth through their feeding, they disturbed the land through their movements and provided food for predators while they were young.
According to Dr. William Hart, co-author of the study from Hofstra University, the vast quantities of prey that were easily accessible to many predators made it possible for them to survive over long periods of time. “Apex predators during the Late Jurassic period, such as Allosaurus or Torvosaurus, were likely able to obtain enough food more readily than T. rex millions of years later,” he states.
Some fossils of Allosaurus indicate serious injury. Studies have shown that many injuries were inflicted by Stegosaurus tails. Dr. Hart believes that many young sauropods would have provided so many opportunities to eat that injured Allosaurus were often capable of surviving despite being injured.
The research team was able to compare the food webs from the Jurassic period to the food webs of different dinosaur ecosystems that have existed throughout history. According to the research, the number of sauropod species present at the end of the Mesozoic Age decreased. As a result, dinosaurs such as Tyrannosaurus rex evolved larger bodies and stronger bites to hunt prey like Triceratops.
Allosaurus was most likely able to survive and thrive as an apex predator during this time because the abundant supply of young sauropods provided many opportunities for injured individuals to find food to support their growth and development.
According to Dr. Morrison, “when constructing food webs, it allows one to make a direct comparison of dinosaur ecosystems between different times, thus providing insight into the types of pressures that drove dinosaurs to evolve in their specific ways.”
The Dry Mesa ecosystem also follows patterns typically seen in living systems today. It is similar to modern open habitats such as African savannahs, which show distinct size gaps between smaller animals, larger herbivores, and the world’s largest carnivores.
The findings of this study should lead to a more complete understanding of the role of dinosaurs within their environments and how they functioned as parts of living systems rather than as isolated species. Understanding how energy moved through the various levels of ancient ecosystems will give researchers a greater understanding of how energy moves through modern ecosystems and how they respond to change.
This method allows for the reconstruction of fossil ecosystems from locations throughout the world and will lead to a better understanding of how the Earth reacted to changes in climate, mass extinctions, and the emergence of new species over time.
The insight gained from studying how ancient ecosystems maintained their stability may ultimately aid in protecting today’s ecosystems from the forces of change.
Research findings are available online in the New Mexico Museum of Natural History and Science Bulletin.
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