The largest animals ever to have walked the face of the earth just got a little smaller, according to a paper published today in the Zoological Society of London's Journal of Zoology.

Why aren't they as big as previously thought?   The researchers say that the original statistical model used to calculate dinosaur mass is flawed, which led to them suggesting dinosaurs have been oversized.  Widely cited estimates for the mass of Apatosaurus louisae, one of the largest of the dinosaurs, may be double that of its actual mass instead of the commonly cited 33-38 tons it may be as light as 18 tons. 

Of all dinosaur estimates, mass has always been the least likely to be taken literally.  Estimates of mass have always varied between specimens and mass varies by the cube of distance - errors in size make mass even less accurate at large sizes.

Brachiosaurus in Berlin, for example, is something of a benchmark in paleontology but it is actually a 'chimera' rather than a complete fossil, comprised of up to 5 animals, so dimensions are educated guesses even in the best large fossils.   A mistake in the actual number of vertebrae, at nearly a foot each, can throw off estimates dramtically.

"Paleontologists have for 25 years used a published statistical model to estimate body weight of giant dinosaurs and other extraordinarily large animals in extinct lineages. By re-examining data in the original reference sample, we show that the statistical model is seriously flawed and that the giant dinosaurs probably were only about half as heavy as is generally believed" says Gary Packard from Colorado State University.

Apatosaurus louisae at the Carnegie Museum of Natural History.

The new predictions have implications for numerous theories about the biology of dinosaurs, ranging from their energy metabolism to their food requirements and to their modes of locomotion.