The Age of Dinosaurs ended millions of years ago, but paleontologists are still attempting to get a handle on the immense diversity and diverse immensity of these creatures.

Take the report last month that Spinosaurus is now officially the biggest carnivorous dinosaur known to science. This two-legged beast actually strode onto the fossil scene in 1915, when a specimen was described by German paleontologist Ernst Stromer.

He figured this theropod (defined as a two-legged carnivorous dinosaur) was bigger than Tyrannosaurus rex, but the original Spinosaurus bones were destroyed by Allied bombs in 1944. So the T. rex reigned as the king-sized, carnivorous land beast for decades.

(Dinosaur illustrations are by Joe Tucciarone. Click here to see more of his dinosaur art.)

Then along came Giganotosaurus 11 years ago.

Now Cristiano Dal Sasso of the Civic Museum of Natural History in Milan, Italy, says Giganotosaurus has been dethroned based on estimates from a new Spinosaurus skull.

So just how do all these carnivores match up?

Tyrannosaurus rex was 40 to 50 feet long, weighed 6 tons, and had teeth up to 13 inches long. It lived 65 million years ago in North America. Giganotosaurus, by comparison, was 47 feet long and weighed 8 tons, but its serrated teeth were only 8 inches long. It roamed present-day Argentina 95 million years ago.

Spinosaurus had both of them beat. It was 55 feet long, weighed 8 tons and had long, crocodile-like jaws. It strode present-day North Africa and Argentina 100 million years ago.

For the ultimate in dinosaur length, though, a vegetarian diet prevailed. Herbivorous sauropods dwarfed carnivorous theropod dinosaurs, and most scientists think Argentinasaurus was the longest of all dinosaurs, at 120 feet, and one of the heaviest at 100 tons. It lived in South America near the end of the dinosaurs' reign 65 million years ago.

Paleontologists have only fossils to compare dinosaur sizes, and those fossils are often damaged or incomplete. The same bone from numerous specimens is often lacking, so scientists often must estimate the total size of an animal from a partial skeleton, as with the new Spinosaurus skull fragments.

Greg Erickson of Florida State University says mass, not length, is the best standard for comparing dinosaur size, because it gets around the problem of differently shaped animals. Mass is best estimated, he said, by measuring the circumference of the thighbone, which bears much of the animal's weight.

The new Spinosaurus size estimate is "compelling," Erickson told LiveScience. "Spinosaurus was probably considerably longer, and hence perhaps heavier, than T. rex and other large theropods."

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