 On the other hand, he concluded that gastroliths found with fossils of advanced theropod dinosaurs such as "Sinornithomimus" and "Caudipteryx" resemble those of birds, and that the use of gastroliths for grinding food may have appeared early in the group of dinosaurs from which these dinosaurs and birds both evolved.When laying eggs, female birds grow a special type of bone in their limbs between the hard outer bone and the marrow.This medullary bone, which is rich in calcium, is used to make eggshells, and the birds that produced it absorb it when they have finished laying eggs.