They were able to estimate the body weights of the living animals by measuring the bones' thickness. The ages of the animals came from counting annual growth rings in the bones, a common feature in fish, reptiles and dinosaurs. The researchers then drew growth curves for the six dinosaur species they studied, plotting body mass against age. They found that some of the dinosaurs, especially the largest ones, grew as fast as almost any animal that ever lived. Generally, researchers find that within animal groups, growth rates are fairly uniform.
Birds grow fastest, followed by mammals, marsupials, reptiles and then fish.
But Erickson and his colleagues were surprised to find that in dinosaurs, small species grew more slowly. The smallest dinosaur studied, the 3-pound Shuvuuia deserti, grew only twice as fast as a reptile of comparable size. It suggests that something different is going on, but what that difference is is a tough call,'' Farlow said. Please read them.