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Tuesday, December 13, 2011

Worlds largest dinosaurs at american museum of natural history

Before entering the fascinating new exhibition at the American Museum of Natural History, “The World’s Largest Dinosaurs,” which opens on Saturday, walk through the Hall of Saurischian Dinosaurs where the composite skeleton of the Apatosaurus reigns, the core of its 140-million-year-old frame seeming almost incidental to its stupendously sweeping neck and tail. The weighty immensity of these fossilized bones once earned this creature — one of the largest to have plodded the planet — the name of thunder lizard (Brontosaurus).

But over the last generation, we have been living through a revolution in paleontology. The primal force wielded by such skeletal monsters, portrayed in their very names (like Triceratops: three-horned face), has been superseded. Bones, scales and armor are now emphasized less than possibilities of feathers, color and flesh.Birds, not brutish reptilian creatures, are now more often seen as dinosaurs’ closest relations.

In one exhibition at this museum a few years ago, about new discoveries in paleontology, it almost seemed as if dinosaurs’ macho-like archetype had shifted and that these former master hunters of alien prehistoric landscapes were becoming domesticated. And while the new show, devoted to Apatosaurus’s group — long-necked herbivores known as sauropods — might have once evoked a thunderous natural world, red in tooth and claw, now it ushers us into an elegant conceptual terrain, revealing how a field that might have once been vulnerable to fossilization is redefining itself.

In fact, don’t go into the exhibition expecting to view anything like what you see in the museum’s renowned dinosaur halls. There are some specimens here — a six-foot-tall femur of a Camarasaurus, sauropod vertebrae, fossils of skin impressions — yet the focus is not on artifacts but on how these creatures’ bodies worked. What is crucial is not bones but biology.

We don’t look at skeletons, but rather at models and re-creations, and read hypotheses about parts of these creatures that have never been found and never will be: their stomachs, brains, hearts and lungs. How did sauropods eat and digest? What was their circulatory system like and how fast did their hearts beat? How did they breathe and what were their lungs like?

These are intriguing questions because sauropods have been among the most successful land creatures ever; their remains have been found, we learn, on all seven continents in sediments that range over 140 million years. They were also enormous.

When you enter the exhibition, you are led into a hint of a forest primeval into which protrudes a model neck and head of an Argentinosaurus, a dinosaur that, the text tells us, is “currently considered the world’s largest.” Around 95 million years ago, such creatures could weigh 90 tons; the narrowest part of its leg might be about four feet around, and it could be 130 feet long. The main gallery space is dominated by a model of a comparatively miniature species: a 60-foot-long Mamenchisaurus hochuanensis based on a specimen found in China, its midsection serving as a giant movie screen presenting a survey of recent hypotheses about sauropods and their biological processes.

Mark A. Norell, the chairman of the museum’s division of paleontology, was joined in curating the exhibition by P. Martin Sander from the University of Bonn in Germany, who for the last seven years has led a team of German and Swiss scientists, including specialists in nutrition, biomechanics and paleontology. They examined the biology behind the size of sauropods. (Their papers have just been published in “Biology of the Sauropod Dinosaurs: Understanding the Life of Giants.”) The show is a masterly distillation of their findings.

Size, we learn, is accompanied by some distinctive biological tendencies. The exhibition gives some sense of the range in size that exists even among related animals, extending in birds, for example, from the tiny bee hummingbird to the now extinct 880-pound elephant bird of Madagascar. Differences in size are associated with differences in biological processes. Generally bigger animals have slower heart rates; smaller animals breathe faster; bigger animals live longer; small animals produce more offspring.

But the heavier an animal becomes, the larger must be its weight-bearing bones, and the larger such bones are, the heavier they become. Size and weight eventually reach limits, though they lie far beyond contemporary human experience: a replica of a 15-foot-tall Supersaurus hind leg makes a nearby human skeleton seem like a Tinkertoy. A half-foot-long titan beetle here — large enough to inspire creepy sensations — is, we learn, about as large as a beetle can grow because that insect’s cells receive oxygen not through a circulatory system but through diffusion, which becomes more difficult with an increase in size.

In the case of plant-eating sauropods, there was a clear advantage to a long neck (and a light head): we are shown just how much more food becomes available with incrementally longer necks, and how much, too, such necks lessen the need to lug one’s tonnage around from tree to tree. The exhibition points out that an African elephant has to eat 18 hours a day to maintain itself. A sauropod, which could be 10 times the size of an elephant and might require 100,000 calories a day, would have had to devour more and, we read, “get as much down their throats as possible, as fast as they could.”

A 5.5-foot cube filled with foliage here shows the extent of a day’s dining (it would have weighed 1,150 pounds). Chewing would have wasted time, and their jaws suggest that sauropods, like Galápagos tortoises, probably swallowed without chewing; their digestive system may have had a fermentation area, where food could be slowly digested for up to two weeks.



Each aspect of a sauropod poses a kind of physiological puzzle: What size heart could pump enough blood to reach past the elongated neck? What kind of respiration system could maintain a high enough level of oxygen over such a wide expanse? Researchers looked at living animals that might provide biological parallels. Like birds and crocodiles, they propose, sauropods may have had lungs where air movement and gas exchange take place in different areas, delivering oxygen with less effort; CT scans of sauropod bones also reveal air spaces that resemble the hollows in birds’ bones that function as part of the avian respiratory system.

There are also ways that sauropods differ from any known land creature. How could newly hatched sauropods gain the weight and size to reach 90-ton maturity in 23 years? They might have doubled their weight in 5 days and quadrupled it in 12 (something that takes a human infant two and a half years). Some species of sauropod gained about 3,500 pounds a year during adolescence.

Each station in this show deals with a different aspect of the sauropod’s biological life. And all of this is teased out from the spare evidence of fossilized bones, and from analogies with known creatures. There are also “interactive” aspects to these displays, which may interest some visitors: you can pull levers to get a sense of the force required to pump blood; you can look at a kind of zoetrope to see animated images of how such creatures might have walked.

But what is so successful about this exhibition is not found in such installations, but in the way it treats scientific ideas. The show is really a demonstration of deduction, yet nothing about it is abstract or arcane. Its panels are crisp and clear. And they show contemporary paleontology to be an adventurous conceptual enterprise.

This makes the final gallery in which visitors (children, most likely) can pretend to use “tools” to clear aside “dirt” and excavate in a “dig pit” in Wyoming — not unlike the pit where the museum’s Apatosaurus was found at the end of the 19th century — a bit pointless. It has nothing whatsoever to do with the kind of exploration taking place in the exhibition. Perhaps the show was deemed too speculative and this dig pit was meant to remind younger visitors of the profession that lay behind such insights. Instead, it seems a kind of conceptual throwback, reminding me not of the scientists’ labors, but of the images I grew up with: thunder lizards stomping through marsh lands, holding carnivores at bay with their sweeping tails and imposing size.

In the new paleontology, the thunder lizard is gone. The external image of the sauropod is as pastoral as that of a giraffe grazing in the treetops. The thunder is biological.

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