 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 full-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 full-grown—which happens within their first year.