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Big! The Life of Sauropod Dinosaurs

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.).

Reconstructed skeleton of the Late Jurassic sauropod Brachiosaurus brancai on display at the Museum für Naturkunde in Berlin (Germany). Height about 12 m (39.4 ft.). Some researchers would place this dinosaur in a separate genus, Giraffatitan. Courtesy of Axel Mauruszat/Wikipedia.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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.

Comments

  1. [...] the movie Jurassic Park, a tree extinct for millions of years delights the paleobotanist. Then a sauropod eats its leaves. This movie later shows us how to re-create the dinosaur but not how to grow the [...]

  2. Steven Black
    Michigan State University
    October 29, 2012, 1:04 am

    Stuart Pimm,

    The CO2 levels were about 5 times greater during this time as was the plant growth rate. Food was more plentiful and able to sustain dense populations, thus making gigantic life possible. Interestingly, many humans consume more calories a day than a sauropod when you convert energy and resource usage to calories.

    Steven Black

  3. [...] As for how these approximately 20-ton beasts—the largest of all known dinosaurs—expelled their methane, Wilkinson said, “we don’t have any strong view on what happened with sauropods.” [...]

  4. [...] (Read blog post: “The Life of Sauropod Dinosaurs.”) [...]

  5. [...] (Read blog post: “The Life of Sauropod Dinosaurs.”) [...]

  6. Roger Seymour
    University of Adelaide, Australia
    June 1, 2011, 8:59 pm

    There are considerable cardiovascular problems associated with a circulatory system that requires the heart to pump blood up an 8-meter neck, because the heart would have to develop at least 650 mm Hg blood pressure to overcome the hydrostatic pressure and to get the blood moving. A 200 kg heart would be insufficient to do this. The number 200 comes from the assumption that the blood pressure is about the normal mammalian value of about 100 mm Hg. At 650 mm Hg, the muscle would have to increase to about 2000 kg, yet pump the same amount of blood. Furthermore to circulate the blood at 650 mm Hg would require about half of the animal’s energy, compared to about 10% if the blood pressure were 100 mm Hg. In view of this problem, I think that sauropods did not browse in trees like giraffes, but kept their necks approximately horizontal or lower.

  7. Tara
    NJ
    May 24, 2011, 4:41 pm

    Dr. Sues,
    I am an honors seventh grade student and have been assigned to do a project on you. I have a few questions regarding where you carried out your fieldwork, what dinosaurs you have discovered, what theories you support, and what technology aided you in your work. Please email me if you have a chance. Thank you!
    Tara

  8. Stuart Pimm
    Key Largo
    May 12, 2011, 9:08 pm

    Hans: others will have surely thought about this, but eating all that stuff must have imposed some stiff ecological constraints. The nearest thing today is elephants. They have to be close to water— and I’m sure you’ve calculated how much water these monsters must have needed each day. Elephants must be within 10km (usually less) — a real problem over much of their range in the dry season. Only in the wet can they wander.

    Then there are severe limits on plant production. 10 tons of biomass per hectare per year is a good value, 20 is pushing it even in warm, moist productive places. A kg gives you about 10,000 dietary (kilo) calories. You can readily calculate how much space these animals would need even under the most optimistic circumstances. Elephants quickly eat many places close to water out of house and home.

    S