It's basically the bone structures of dinosaurs, particularly theropods that are similar to modern day birds.
First Ever Fossil Of Sleeping Dinosaur Found In China
Today we know that theropod dinosaurs and birds share more than 100 anatomical features, including a wishbone, swiveling wrists, and three forward-pointing toes.
Many are of the opinion that birds are not descendants of dinosaurs and did not evolve from them but rather modern birds are descendants of birds. They are considered by many scientists to be a branch of the dinosaur family that survived, probably because they could fly away from predators. There have been reported fossils of birds found that are 290 million years old.
Try pouring a big bunch of sand or something on a sleeping bird, and just SEE if it stays in that postition to die!