2011-04-18 11:30:34
Among dinosaurs, the biggest of the big is Argentinosaurus. This long-necked,
puny-headed creature is a member of a group of giants called sauropods. This
particular extinct creature measured as much as 140 feet (43 meters) long and
weighed up to 90 tons (82 metric tons).
Beyond inspiring awe, a creature of these proportions inspires all sorts of
questions: Why and how did these dinosaurs, which started out relatively small,
become so big? How did they feed and maintain their large bodies? A new exhibit
at the American Museum of Natural History in New York City explores the
mysteries surrounding their enormity.
The exhibit is dominated by a model of Mamenchisaurus, a smaller relative to
Argentinosaurus. The skin is peeled back on half of the model to allow a peek
into the animal's physiology. On the left side of the abdomen, projected images
show body processes, such as the path of fresh air into the lungs and a
birdlike system of air-storage sacs.
Large sauropods needed more air and more food than smaller creatures. A young
adult Mamenchisaurus, for example, needed to consume 100,000 calories a day to
maintain itself. This nutrition came from leaves and other plant material they
snipped and stripped before gulping their meal down.
The long neck for Mamenchisaurus it was 30 feet (9 meters) made their
enormous body size workable, allowing them to the reach to feed themselves
efficiently. Sauropods used their necks to reach food without moving their more
lumbering bodies. And in turn, the long neck itself very light was possible
because of the dinosaur's small head, with the largest of sauropod brains
weighing only 4 ounces (113 grams) as compared with the 48-ounce (1.4
kilograms) human brain.
The sauropod habit of gulping down its dinner is also crucial, according to P.
Martin Sander, a co-curator of the exhibit and professor of vertebrate
paleontology at the University of Bonn.
"Chewing limits body size," Sander said, explaining that animals that rely on
chewing, like many mammals, depend on big molars and muscles to grind up their
food to make nutrients available. As an animal gets larger, it needs
progressively more energy, and eventually this cumbersome system can't provide
it with enough calories.
Sauropods, meanwhile, had teeth, but they did not chew. The earliest sauropods
had spoon-shaped teeth that made for a powerful bite, and later, some evolved
pencil-tip-shaped teeth that functioned like rakes, stripping away leaves and
needles from trees.
Another factor has kept mammals from reaching sauropod size: reproduction.
Sauropods could lay 150 eggs in a year, making them more capable of bouncing
back from a given catastrophe than mammals, which invest much more in fewer
offspring. This means sauropods could exist at a much lower density in a given
area than mammals without being wiped out, according to Sander. Lower density
living makes larger body size possible because each individual has access to
more resources.
"It's pretty clear what the limiting factors are in body size, that's what
we've learned from studying dinosaurs," he said.
Even sauropods did not reach the theoretical maximum for size for land animals,
which has been estimated at 150 to 200 tons, Sander said.
Baby sauropods did not start out large. They emerged from eggs smaller than a
soccer ball and grew at astonishing rates of up to 12 pounds (5.4 kg) per day,
according to Michael Novacek, a senior vice president at the museum. This rapid
growth rate implies a high rate of metabolism, and therefore, warm-
bloodedness, for at least a period during the sauropods' life span, according
to Novacek.
There is no question that sauropods were successful: They walked the Earth for
140 million years.
Exhibit Details:
The World's Largest Dinosaurs
April 16, 2011 through Jan. 2, 2012
American Museum of Natural History in New York City