Washington: A new study has tried to explain as to how dinosaurs grew so large.
Large, plant-eating dinosaurs, including one group known as sauropods, were much more common than giant land-based mammals, which were also herbivores.
The largest sauropods frequently weighed more than 30 tons and included massive species such as Diplodocus and Apatosaurus.
One of the largest known land mammals, an extinct rhino-like herbivore, only grew to approximately one-third that size.
The new study, by Matthew Bonnan from The Richard Stockton College of New Jersey and his colleagues, suggests that one reason for this disparity may lie between their bones, Fox News reported.
Cartilage - the bendy, supportive material that cushions joints - shapes up differently in mammals and dinosaurs.
This difference may be one of the reasons why gigantic terrestrial mammals are so rare. Blue whales, the largest known mammals, would outweigh most sauropods by far.
The authors measured the width of the ends of thigh and arm bones in mammals, dinosaurs and their descendants - modern day reptiles and birds - to see how joints changed as the animals` size increased.
These widths, and distances between different points on the ends of bones, helped reconstruct the area of joints between the bones.
As mammals grow larger, their bones become more rounded at the ends.
However, bones in dinosaurs, reptiles and birds tend to grow wider, flatter ends as the animals grow in size. And the cartilage within these two differently-shaped joints looks different, too.
Bonnan likens cartilage to sheets of rubber stretched across the hard ends of bones to cushion them.
As mammalian bones grow rounder at the edges, cartilage stretches thin and tight across their surfaces.
The close-fitted, stretchy material transfers weight evenly across the bone surfaces. Dinosaur joints, however, appear to pack in more layers of cartilage as the animals size up.
The study is published in the journal PLOS ONE.