Giants Who Scarfed Down Fast-Food Feasts
Denis Finnin/AMNH
CENTERPIECE A life-size model of a 60-foot female Mamenchisaurus, whose fossilized bones were discovered in China, was close to ready at the American Museum of Natural History in New York.
By JOHN NOBLE WILFORD
Published: April 11, 2011
Nothing in the dinosaur world was quite like the sauropods. They were huge, some unbelievably gigantic, the biggest animals ever to lumber across the land, consuming everything in sight. Their necks were much longer than a giraffe’s, their tails just about as long and their bodies like an elephant’s, only much more so.
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Wide-eyed first graders are not the only ones fascinated by sauropods, particularly those outsize friends Apatosaurus (formerly known as Brontosaurus), Brachiosaurus and Diplodocus. Scientists are redoubling their study of the unusual biology of these amazing plant-eaters. They are asking questions not unlike, in spirit, those of schoolchildren.
By what physiological strategy of heart, lungs and metabolism were the largest of sauropod species able to thrive over a span of 140 million years? How did they possibly get enough to eat to grow so hefty, to lengths of 15 to 150 feet and estimated weights of up to 70 tons? A mere elephant has to eat 18 hours a day to get its fill. Even in the Mesozoic era, there were only 24 hours in a day.
For more than seven years, a group of German and Swiss scientists has made a concerted effort to test the limits of body size in terrestrial vertebrates and, in the process, try to answer these and other questions related to the enigma of sauropod gigantism. Findings by many other scientists have been reviewed and analyzed, then tested with new experiments and more observations.
“We actually have been re-engineering a sauropod,” said P. Martin Sander, a paleontologist at the University of Bonn and leader of the research team. “We are looking for physical advantages it had over other large animals and assessing various hypotheses.”
One clear explanation has emerged: These were the ultimate fast-food gourmands. Reaching all around with their long necks, these giants gulped down enormous meals. With no molars in their relatively small heads, they were unequipped for serious chewing. They let the digestive juices of their capacious bodies break down their heaping intake while they just kept packing away more chow.
This was seemingly the only efficient way for sauropods to satisfy their appetites and to diversify into some 120 genera, beginning more than 200 million years ago. They eventually dominated the landscape for a long run through the Cretaceous, only to die out with all nonavian dinosaurs 65 million years ago.
The German-Swiss team of paleontologists, biologists and other scientists, financed by the German Research Foundation, has now weighed in with its comprehensive report “Biology of the Sauropod Dinosaurs,” a book published last month by Indiana UniversityPress. Dr. Sander is one of the book’s editors and also guest curator of a major exhibition, “The World’s Largest Dinosaurs,” opening Saturday at the American Museum of Natural History in Manhattan and scheduled to run until Jan. 2, 2012.
A centerpiece of the show will be a life-size model of a 60-foot female Mamenchisaurus, whose fossilized bones were discovered in China. An early and not especially large sauropod, it lived 160 million years ago, laid eggs and possibly lived in a herd. It weighed 13 tons and ate 1,150 pounds of vegetation a day. The model focuses attention on the animal’s 30-foot neck and small skull and jaws to illustrate the remarkable biology and behavior of sauropods.
Early in their investigations, material scientists in the German-Swiss group proposed that sauropod bone had superior mechanical properties compared with large mammal bone, which would have given these dinosaurs stronger skeletons to support heftier bodies. The hypothesis was tossed aside after tests showed that sauropod and cow bone tissue had the same strength.
Then the investigators found no evidence that availability of food and the physical and chemical conditions in the Mesozoic era were sufficiently different to have accounted for sauropod gigantism. If anything, the environment then was probably less favorable for plant and animal life than it is today. So the researchers directed their efforts to a detailed examination of the biological makeup of these giants.
Dr. Sander noted in the book that the new study was one of the few dinosaur projects in which paleontologists were outnumbered by nonpaleontologists, mainly biologists. Mark A. Norell, a dinosaur paleontologist at the American Museum and principal curator of the exhibition, remarked, “This shows how biological our field has become.”
In a recent interview televised from his office in Bonn, Dr. Sander pointed to an illustration of the dinosaur’s anatomy. “What makes a sauropod a sauropod is its most conspicuous feature, its enormously long neck,” he said.
The animals had the longest necks for their body size of any dinosaur known. Dr. Sander and his colleagues think that two of the sauropod’s primitive inheritances probably account for this. One was the absence of mastication, and the other its egg-laying reproduction.
Denis Finnin/AMNH
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By not chewing their food, the animals had no need for a full set of large teeth or strong jaws and associated muscles. They had only incisors up front for cropping and cutting vegetation. As a result, their heads remained small and lightweight. A plant-chewing African elephant, for example, has a 1,000-pound head; a Mamenchisaurus head weighed 45 pounds.
A small head, of course, took a load off the sauropod neck, presumably allowing it to grow longer. Even so, the neck had to be bolstered with more vertebrae than mammals have. These bones are light for their large size, because they are hollowed out with many air pockets. Mammals, even the giraffe with a six-foot neck, are limited to no more than seven neck vertebrae; the Mamenchisaurus neck had 19.
The sauropod’s neck became what the hook-and-ladder is to a firefighter: a means of extended reach that could be critical. It gave these animals an ability to graze a much wider radius of ground vegetation without moving a step. Dr. Norell said that biomechanical studies indicated that the long necks may not have able to stretch higher to browse in trees, as giraffes do.
In any event, sauropods could outcompete other plant eaters and over time, as one scientist wrote, “enter the niche of giants.” And their consequent gigantism was perhaps their best defense against predators, intimidating even the neighborhood T. rex.
Sauropods took a long while evolving their body plan, which, in silhouette, became the ubiquitous logo of Sinclair oil back in the mid-20th century. But the retention of another of its primitive features, egg-laying, increases the number of offspring and thus improves the chances of long-term survival of a family of species — and time enough to innovate.
In a 2008 summary in the journal Science of the project’s preliminary findings, Dr. Sander and Marcus Clauss, a dinosaur specialist at the University of Zurich, wrote that sauropods gradually evolved what appeared to be a high growth rate, a birdlike respiratory system and a flexible metabolic rate.
One conclusion is that their very young grew rapidly: A human baby doubles in weight in about five months, a sauropod in only five days; and an adolescent sauropod put on 3,500 pounds a year. These are growth rates higher than in today’s reptiles. They enabled these dinosaurs to reach sexual maturity in their second decade of life and full size in their third.
Stopping at an exhibit being readied for the new museum show, Dr. Norell pointed to an illustration of how heart rates are related to an organism’s size. The heart of a mouse beats 700 times a minute; a human, 72; an elephant, 28; a sauropod, less than 10.
Dr. Sander cited the bird-lung model as an important innovation. If correct, he said in the interview, this and other evidence suggests that sauropods were warm-blooded to some extent. “If an elephant had birdlike lungs, it would grow even bigger,” he speculated.
The fact that dinosaurs’ distant relative the crocodile has a respiratory system somewhat like a bird’s suggested to scientists that it might also have been true of sauropods. All the air-sac cavities in their long neck and torso resemble those in birds. Also, it might explain how animals with such long windpipes managed to draw in and absorb sufficient oxygen.
In time, however, sauropods seemed to feast on their enormous size. Writing in the project’s book, Dr. Clauss said that these giants “might represent a rare example of herbivores that actually benefit from an increase in body size, in terms of a larger gut and a longer retention of food in that gut.”
The bigger they got, in other words, the greater their capacity to store vast food intake in digestive chambers. Galapagos tortoises, which eat and don’t chew, have stomach chambers that hold food for up to 11 days, giving microbes time to break it down and extract the nourishment.
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Dr. Clauss of Zurich and Jürgen Hummel of the University of Bonn conducted fermentation experiments mixing micro-organisms with contents of sheep stomachs and various plants, including horsetail plants, cycads, pine needles and ginkgo leaves known to have been growing when sauropods foraged. From this and other evidence, they estimate that the giants probably took two weeks to digest an all-day dinner.
Other scientists, who are not involved in the study, said the experiments and analysis by the German-Swiss group provide an impressive body of knowledge about how some dinosaurs grew so big and why sauropods, in evolutionary terms, were so successful over a span of 140 million years and a global range.
“I’m not sure they’ve hit the nail on the head, always,” Peter Dodson, a University of Pennsylvania paleontologist, said of Dr. Sander’s team. “But they have certainly a number of important insights.”
Dr. Dodson agreed with the researchers on the long neck’s critical place in sauropod biology and the growth rates of sauropod bones that appear to show the animals had metabolic rates closer to those of mammals than those of reptiles. But this does not necessarily mean, he said, that sauropods were fully warm-blooded.
In the team’s book, Dr. Clauss conceded that there was debate on the metabolic rates and a lack of consensus on the nature of the sauropod cardiovascular system. He noted that among many scientists a direct link between the sauropod respiratory system and gigantism “is not yet compelling.”
The research, Dr. Dodson added, raises the related question of why mammals have never approached in size the larger sauropods. Some extinct Asian rhinoceros species that reached weights of 15 tons were the closest mammals came. “They were to mammals what sauropods were to dinosaurs,” he said. “But it was not a successful body plan in their time, an idea that went nowhere.”
Paul Sereno, a dinosaur fossil hunter at the University of Chicago, said the new research “is very valuable,” but he doubted there was enough hard evidence to support the bird-lung hypothesis. Still, he said, the sauropod “is an incredible animal, one of the best land animals that’s been invented.”
