Reproductive Biology
When laying eggs, females of some bird species 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. Medullary bone has been found in fossils of the theropods Tyrannosaurus and Allosaurus and of 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, the presence of medullary bone in both groups suggests that dinosaurs in general produced medullary tissue. On the other hand crocodilians, which are dinosaurs' second closest extant relatives after birds, do not produce medullary bone. This tissue may have first appeared in ornithodires, the Triassic archosaur group from which dinosaurs are thought to have evolved.
Medullary bone has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity before they were fully-grown. Sexual maturity at sub-adult size is also found in reptiles and in medium- to large-sized mammals, but birds and small mammals reach sexual maturity only after they are fully-grown—which happens within their first year. Early sexual maturity is also associated with specific features of animals' life cycles: the young are born relatively well-developed rather than helpless; and the death-rate among adults is high.
Read more about this topic: Physiology Of Dinosaurs
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