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.

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 University Press. 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.

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.”

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For a Long Neck, Don’t Chew

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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.”        

Biologists have found a new way to peer back 130 million years in time, illuminating the catastrophic period in which the dinosaurs perished and birds and mammals arose.

The new approach rests on reconstructing the family tree of lice. Vincent S. Smith, a louse taxonomist at the Natural History Museum in London, has found that the tree stretches so far back in time that the host of the first louse would have been a dinosaur, probably one of the theropod dinosaurs that were the ancestors of birds.

Dr. Smith and his colleagues reconstructed the louse family tree by analyzing DNA from present-day louse species that parasitize birds and mammals. Most lice are specialists, feeding on a single species to whose fur or feathers their claws are adapted. The adaptation is so precise that when a louse’s host species evolves into a new one, the louse will diversify into different species, too.

The human head louse, for instance, evolved from the chimpanzee louse when the ancestors of humans and chimps split apart some five million years ago. The human pubic louse, on the other hand, is related to the gorilla louse, from which it parted company some 13 million years ago. Species of human lice thus mirror the splits in the tree of ape and human evolution.

In the same way, species of lice on living animals and birds reflect the splits in animal and bird ancestry back to the time that lice first arose. Family trees based on DNA can be given precise dates at all their branch points if a few datable fossils of the right age are available. But this is difficult to do with lice, for which almost no fossils are known. Dr. Smith was fortunate that two fossil lice discovered in the last few years — one of them 44 million and the other 100 million years old — provided the necessary anchors for his tree.

The assembled family tree shows that lice started to radiate into new species well before the end of the Cretaceous period, Dr. Smith and his colleagues report in the current issue of Biology Letters. The finding implies that their hosts, both mammals and birds, had also begun to flourish and speciate before the reign of the dinosaurs was over.

The new tree bears on a longstanding dispute about the rise of birds and mammals. One school holds that both groups proliferated early in the Cretaceous period, which began 145 million years ago, and that many lineages survived the cataclysm that brought the Cretaceous and the dinosaurs to a sudden end: the strike of a large asteroid 65 million years ago. The opposing view is that mammals and birds did not become successful and radiate into many different species until after the demise of the dinosaurs.

Unfortunately, the new finding will probably not settle the issue of whether birds and mammals speciated before or after the asteroid hit. “I would put this down as an entry in the debate,” Dr. Smith said. “The louse phylogeny adds one more piece of data to this puzzle. It says lice are old, predate the Cretaceous-Paleogene boundary, and must have been living on something.”

The issue is contentious because the fossil record suggests that placental mammals did not expand, in terms of the number of different species, until after 65 million years ago. A plausible reason is that all the dinosaurs had been killed off, except the line that evolved into birds, and the placental mammals speciated into the ecological niches that had been left vacant.

But biologists reconstructing the mammalian tree of evolution from the DNA of living species have come to a different conclusion. Their molecular clock data suggest a much earlier speciation, perhaps prompted by the breakup of the ancient supercontinent of Gondwana around 120 million years ago. In this view, placental mammals evolved into distinct species before the asteroid hit of 65 million years ago, and many would have survived the mass die-out that polished off the dinosaurs.

Michael Novacek, an expert in mammalian paleontology at the American Museum of Natural History in New York, said the louse tree was very interesting and showed that lice were diversifying during the Cretaceous. But the fossil record of placental mammals is reliable and does not record a speciation until later. “The fossil record continues to show that the origin of modern placental mammals postdates or is at the Cretaceous-Paleogene boundary,” he said. “So the contradiction between the fossils and molecular clocks remains.”

In his view, the hosts on which lice were speciating during the Cretaceous could have been a different branch of the mammalian family tree, all of whose species are extinct.

Dr. Smith said Dr. Novacek was correct in saying that there were nonplacental mammals around in the Cretaceous on which lice would doubtless have fed. But these nonplacental mammals, which included several lineages of marsupials, all became extinct, taking their parasites with them. These lice would not show up in his tree, Dr. Smith said, unless they had been able to transfer to the placental mammalian species, and most lice do not regularly switch hosts.

Dr. Smith believes that lice infested birds before mammals, in part because lice are so common on birds. Every bird family but one has lice, and there is a species of bird, the great tinamou, that harbors 18 different species of lice, perhaps because it has many different kinds of feathers, each offering a special niche. Given the ancient root of the louse family tree, it is likely that the first louse infested the feathered dinosaurs that were the birds’ ancestors, he concludes.         

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.

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.

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.        

Lurking in the blood of tropical snails is a single-celled creature called Capsaspora owczarzaki. This tentacled, amoebalike species is so obscure that no one even noticed it until 2002. And yet, in just a few years it has moved from anonymity to the scientific spotlight. It turns out to be one of the closest relatives to animals. As improbable as it might seem, our ancestors a billion years ago probably were a lot like Capsaspora.

The origin of animals was one of the most astonishing and important transformations in the history of life. From single-celled ancestors, they evolved into a riot of complexity and diversity. An estimated seven million species of animals live on earth today, ranging from tubeworms at the bottom of the ocean to elephants lumbering across the African savanna. Their bodies can total trillions of cells, which can develop into muscles, bones and hundreds of other kinds of tissues and cell types.

The dawn of the animal kingdom about 800 million years ago was also an ecological revolution.

Animals devoured the microbial mats that had dominated the oceans for more than two billion years and created their own habitats, like coral reefs.

The origin of animals is also one of the more mysterious episodes in the history of life. Changing from a single-celled organism to a trillion-cell collective demands a huge genetic overhaul. The intermediate species that might show how that transition took place have become extinct.

“We’re just missing the intervening steps,” said Nicole King, an evolutionary biologist at the University of California, Berkeley.

To understand how animals took on this peculiar way of life, scientists are gathering many lines of evidence. Some use rock hammers to push back the fossil record of animals by tens of millions of years. Others are finding chemical signatures of animals in ancient rocks. Still others are peering into the genomes of animals and their relatives like Capsaspora, to reconstruct the evolutionary tree of animals and their closest relatives. Surprisingly, they’ve found that a lot of the genetic equipment for building an animal was in place long before the animal kingdom even existed.

It was only in the past few years that scientists got a firm notion of what the closest relatives to animals actually are. In 2007, the National Human Genome Research Institute started an international project to compare DNA from different species and draw a family tree. The cousins of animals turn out to be a motley crew. Along with the snail-dwelling Capsaspora, our close relatives include choanoflagellates, amoebalike creatures that dwell in fresh water, where they hunt for bacteria.

Now scientists are trying to figure out how a single-celled organism like Capsaspora or choanoflagellates became a multicellular animal. Fortunately, they can get some hints from other cases in which microbes made the same transition. Plants and fungi evolved from single-celled ancestors, as well as dozens of other less familiar lineages, from brown algae seaweed to slime molds.

Primitive multicellularity may have been fairly easy to evolve. “All that has to happen is that the products of cell division stick together,” said Richard E. Michod of the University of Arizona. Once single-celled organisms shifted permanently to colonies, they could start specializing on different tasks. This division of labor made the colonies more efficient. They could grow faster than less specialized colonies.

Eventually, this division of labor could have led many cells in proto-animals to give up their ability to reproduce. Only a small group of cells still made the proteins required to produce offspring. The cells in the rest of the body could then focus on tasks like gathering food and fighting off disease.

“It’s not a hurdle,” said Bernd Schierwater of the University of Veterinary Medicine in Hanover, Germany. “It’s a very good way to be very efficient.”

Yet multicellularity also threw some new challenges at the ancestors of animals.

“When cells die in a group, they can poison each other,” said Dr. Michod. In animals, cells die in an orderly fashion, so that they release relatively few poisons. Instead, the dying cells can be recycled by their living brethren.

Another danger posed by multicellularity is the ability for a single cell to grow at the expense of others. Today that danger still looms large: cancer is the result of some cells refusing to play by the same rules as the other cells in our body.

Even simple multicellular organisms have evolved defenses to these cheaters. A group of green algae called volvox have evolved a limit to the number of times any cell can divide. “That helps reduce the potential for cells to become renegades,” said Dr. Michod.

To figure out the solutions that animals evolved, researchers are now sequencing the genomes of their single-celled relatives. They’re discovering a wealth of genes that were once thought to exist only in animals. Iñaki Ruiz-Trillo of the University of Barcelona and his colleagues searched Capsaspora’s genome for an important group of genes that encode proteins called transcription factors. Transcription factors switch other genes on and off, and some of them are vital for turning a fertilized egg into a complex animal body.

In the current issue of Molecular Biology and Evolution, Dr. Ruiz-Trillo and his colleagues report that Capsaspora shares a number of transcription factors that were once thought to be unique to animals. For example, they found a gene in Capsaspora that’s nearly identical to the animal gene brachyury. In humans and many other animal species, brachyury is essential for embryos to develop, marking a layer of cells that will become the skeleton and muscles.