OSAKA They were the biggest of the big. With their long necks and tails, the sauropod subgroup of dinosaurs comprised the largest animals ever to roam on land, scientists say. Many grew to more than 20 meters in length and weighed over 10 tons. A few years ago, paleontologists discovered the fossil of a beast that was an estimated 40 meters long and weighed around 80 tons.
The staggering size of these dinosaurs raises a question: How did they grow so big in the first place? Researchers say the answer lies in a number of special characteristics, including unique, fast-growing bones.
The oldest dinosaur fossil ever found was discovered in Argentina, in a stratum dating from the late Triassic Period -- around 230 million years ago. That animal was small, measuring just 1-3 meters long. As the world entered the Jurassic Period, animals became progressively larger, evolving in response to various environmental changes. As herbivorous dinosaurs grew, so too did carnivorous ones.
For many, the word "dinosaur" likely brings to mind the Tyrannosaurus rex, which had the strongest bite of any land animal. Others will picture the Triceratops, with its three horns, or maybe the Stegosaurus, with the bone plates on its back. All of these dinosaurs were around 10 meters long -- considerably bigger than an elephant or giraffe, but on the small side compared with the colossal sauropods.
In August, a team led by professor Shinobu Ishigaki of the Okayama University of Science discovered fossil footprints left by sauropods in Mongolia. They measure 106cm in length, and even the toenail prints are preserved. The dinosaur that left them behind is thought to have been 25-30 meters long.
Examples of sauropods include dinosaurs in the Titanosaurus genus, such as a Tambatitanis discovered in Japan's Hyogo Prefecture, as well as the Diplodocus, a type discovered thanks to funding by U.S. industrialist Andrew Carnegie. These great beasts first appeared some 210 million years ago.
Sauropod hatchlings were fairly small. The fossilized eggs found to date are up to 30cm in length -- roughly three times the size of an ostrich egg. Immediately after hatching, the young dinosaur would have been 50cm long and weighed less than 10kg. From that point, though, it would have grown rapidly, adding more than 10kg each day during its peak growth phase.
The animals are thought to have continued growing until death.
Scientists unlocked the secrets of this process, in part, by peering into fossilized bones. When the bones were sliced very thinly and examined with a microscope, researchers saw layers with scattered crystal structures. Between them were layers in which the crystals were lined up in one direction. Shoji Hayashi, curator of the Osaka Museum of Natural History, explained what this means in terms of growth.
"A large framework was quickly created with hollow bones," he said. "Later, the gaps were filled in with finer bone to make them sturdy."
Normally, examinations of dinosaur bones turn up rings akin to those that reveal the age of a tree. This is because the animals grew at a slower pace in the winter, when food was scarce. This is not the case with sauropod bones, however.
Hayashi reckons that their "growth didn't stop at all during the year, so annual rings were less likely to form."
The conventional wisdom used to be that sauropods lived in the water to help support their massive bodies. Fossils have nixed that theory. Though dinosaurs bear similarities to reptiles, their leg bones are very different. Reptiles have legs on the sides of their bodies, forcing them to crawl. But dinosaurs' legs extended from the bottom of their bodies, and the bone structure was capable of supporting even large amounts of weight.
BREATHING AND EATING Bones, though, are just part of the answer to the size riddle. Sauropods also had a unique way of breathing. Their lungs were combined with air sacs that functioned like pumps.
The larger an animal is, the more oxygen it needs to live. When mammals and reptiles inhale, air is drawn into their lungs, oxygen is taken into the blood, and then the animal exhales. The same airway is used for inhaling and exhaling, and incoming air is mixed with outgoing air.
In contrast, when a sauropod breathed, air was sent to the lungs while some was stockpiled in the air sacs. When the dinosaur exhaled, two of the sacs would deflate, with the air they held being sent to the lungs.
As fresh air entered the lungs when the animal was both inhaling and exhaling, its body took in oxygen more efficiently. This respiratory mechanism can be found in today's birds. One reason migratory birds are able to fly such long distances is that they have a high capacity for oxygen, thanks to such air sacs.
Sauropods' feeding habits, meanwhile, also contributed to their growth. The dinosaurs swallowed leaves and stems from plants -- say, ferns and sago palms -- without chewing them. Back then, the planet had plenty of volcanic activity, and the concentration of carbon dioxide in the atmosphere was high. This allowed vegetation to grow very quickly, but as the plants had relatively low nutritional content, enormous quantities had to be eaten.
Since they swallowed their food whole, sauropods did not need muscles to grind up the leaves and stems, so their heads did not have to be particularly large. Instead, they evolved to have long necks.
Toru Sekiya of the Fukui Prefectural Dinosaur Museum theorizes that "this enabled them to eat while moving only their long necks, thereby limiting their energy consumption."
With their size advantage, adult sauropods had little to fear from even carnivorous dinosaurs. Environmental changes were another story.
The creatures went into decline in the latter half of the Cretaceous Period, which ended about 65 million years ago. As temperatures dropped, angiosperms -- flowering plants -- began to grow explosively in place of sauropods' staple gymnosperms. Since eating angiosperms required grinding them up, the Triceratops and other herbivorous dinosaurs with large jaws flourished.