In Dinosaur Science, Size Is Just the Beginning

Wednesday, April 20, 2011

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force wielded by such skeletal monsters, portrayed in their very names (like Triceratops: three-horned face), has been superseded.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.

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