February 2 0 , 2017:

byTAKURO KUSASHIO

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.

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."

http://asia.nikkei.com/magazine/20170126/Tech-Science/Dinosaur-fossils-help-solve-a-big-mystery?page=1