The last four decades have witnessed a revolution in the study of dinosaurs. Scientists no longer examine just the structure of the skeletons and the relationships of these fascinating animals, but have started probing issues of their biology. How did dinosaurs move? How did they feed? What was their circulatory system like? How did they breathe? How did they reproduce? How fast did they grow? Many of these questions involve organ systems that are never preserved in fossils, so paleontologists have to draw on other scientific disciplines to interpret the evidence they do have.
Sauropods were one the most successful groups of land animals of all time. The fossil remains of scores of species have been found worldwide in sedimentary rocks representing about 135 million years. Their jaws and teeth show that these dinosaurs were plant eaters. Some sauropods were the largest land animals that ever lived, weighing as much as 80 metric tons (176,370 pounds) and attaining lengths of up to 35 m (115 ft.).
Size matters. Large size confers protection against predators. It helps in competing for food resources. Large animals live longer. However, the longer and heavier a backboned animal grows, the larger and heavier its bones and the larger and thicker its muscles must become. Eventually, body size reaches physical limits.
Over the last seven years, P. Martin Sander (University of Bonn) has headed a large team of German and Swiss scientists, including experts on biomechanics, nutrition, and physiology, studying the biology of sauropod dinosaurs. This work has yielded a wealth of new scientific information, which helps fill in many of the gaps in our understanding of the life and times of these extinct giants.
Sander’s team has just published a volume summarizing the principal scientific results from this research project, “Biology of the Sauropod Dinosaurs: Understanding the Life of Giants” (Indiana University Press), and a new exhibition based on this work recently opened at the American Museum of Natural History in New York.
Most sauropods have a long neck and a rather small head. A long neck greatly extends reach during foraging, both horizontally and vertically, and reduces the energetically costly need to move a bulky body around the terrain. Field observations have revealed that African elephants spend 80 percent of their time foraging to gather enough to eat. At a certain weight, it actually becomes impossible for plant-eating mammals to gather and chew enough food to sustain themselves. A sauropod dinosaur weighing as much as 10 African elephants may have required some 100,000 calories a day. It would have had to eat as much as it could as fast as it could. In the exhibition at the American Museum of Natural History, a cube filled with over 1,000 pounds of foliage beautifully visualizes the staggering amount of required daily forage.
The jaws and teeth of sauropod dinosaurs show that these animals did not chew their food. Much like present-day plant-eating reptiles, such as the Galápagos giant tortoise, sauropods simply gathered and swallowed plant fodder. It was long thought that these dinosaurs employed rocks in a muscular gizzard to break up ingested plant material, but Sander’s team found little evidence to support this hypothesis. Vertebrates lack the enzymes required to break down cellulose and hemicelluloses, the structural carbohydrates that comprise most of available plant food. However, a variety of microorganisms can process these carbohydrates and have entered into symbiotic relationships with plant-eating animals. Residing in the digestive tracts of their hosts, these symbionts turn cellulose and hemicelluloses into sugars and fatty acids that can then be used by their hosts. The huge guts of sauropod dinosaurs probably formed vast fermentation chambers, where large amounts of plant fodder could be retained for processing over periods of perhaps as much as two weeks.
Paleontologists had long wondered how sauropods could feed effectively as they only had access to plant fodder thought to be of poor nutritional quality compared to the flowering plants (angiosperms) that sustain plant-eating mammals and reptiles today. Angiosperms became common only during the later part of the Cretaceous Period, in the twilight of the Age of Dinosaurs. Thus, most sauropod dinosaurs had to subsist on conifers, ferns, horsetails, and other groups of land plants. Sander’s team, using a method employed to assess the nutritional value of forage for animal husbandry, has now established that present-day representatives of many of these plant groups, such as horsetails and certain ferns, contain as much energy as extant grasses and other flowering plants. Thus, a diversity of good-quality forage was available to sauropods and other plant-eating dinosaurs.
Every aspect of sauropod bodies poses intriguing biological issues: How large would the heart have to be to pump enough blood all the way up to the brain of Brachiosaurus, located almost 8 m (26 ft.) above the heart? Based on the body mass of this dinosaur, Sander’s team calculated that the heart would have weighed about 200 kg (441 pounds). What kind of respiratory system could have supplied enough oxygen to sustain so large a body? Sander’s team studied present-day animals to explore potential biological parallels. Like birds and crocodilians, sauropod dinosaurs probably had complex respiratory systems where airflow and gas exchange occurred in different areas. Sauropod vertebrae contain cavities that closely resemble the hollow spaces in bird bones. These spaces suggest that the respiratory systems of sauropods included large air sacs in addition to the lungs, much like the lung-air sac system in birds. Without such air sacs, sauropods could not have breathed because their long windpipes would have contained a greater volume of air than their lungs.
Another fascinating issue in sauropod biology is development and growth. Much like other dinosaurs, sauropods laid eggs, producing a number of small offspring each breeding season. The size of the hatchlings was constrained by the physical limits of egg size. Due to the tremendous size difference between adult and baby sauropods, the latter likely had to fend for themselves as soon as they emerged from the eggs. By contrast, large plant-eating land mammals such as elephants give birth to only few offspring that are well cared for.
How could a newly hatched Argentinosaurus have attained a mature weight of 80 metric tons (176,370 pounds) in a mere 23 years? (Paleontologists can determine age from growth features in the microscopic structure of dinosaurian limb bones.) It might have doubled its body mass in less than a week. By comparison, it takes a human infant two and a half years to double its weight. Some species of sauropod dinosaur may have gained up to two metric tons (4,400 pounds) per year during adolescence.
Sauropod dinosaurs offer unique insights into the evolutionary challenges and consequences of attaining very large body size. Researchers once considered gigantism a sign of “decadence” preceding the demise of evolutionary lineages. However, the great diversity and ecological success of sauropods worldwide over 135 million years clearly contradict that notion.
Hans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time. A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.