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Dinosaur reproduction
Dinosaur reproduction was relevant to archosaur physiology, with newborns hatching from eggs.
==Medullary bone==
A discovery of features in a ''Tyrannosaurus rex'' skeleton recently provided more evidence that dinosaurs and birds evolved from a common ancestor and, for the first time, allowed paleontologists to establish the sex of a dinosaur. When laying eggs, female birds grow a special type of bone between the hard outer bone and the marrow of their limbs. This ''medullary'' bone, which is rich in calcium, is used to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of the ''Tyrannosaurus rex'' specimen's hind limb suggested that ''T. rex'' used similar reproductive strategies, and revealed the specimen to be female.〔 Further research has found medullary bone in the theropod ''Allosaurus'' and the ornithopod ''Tenontosaurus''. Because the line of dinosaurs that includes ''Allosaurus'' and ''Tyrannosaurus'' diverged from the line that led to ''Tenontosaurus'' very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue. Medullary bone has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity rather quickly for such large animals.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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