The presently accepted hypothesis of bird origins state that they evolved from small Triassic bipedal theropods . Within the past several years, a newly described fossil Sinosauropteryx prima , has provided information that initially would seem to affect the debate over the origin of birds. After examination, though, it was determined that this animal did not possess feathers. But, new fossil material described in 1998 by a multi-national team have demonstrated that feathers are not just for birds any more. These new specimens demonstrate that dinosaurs had feathers, too.
Birds are a dead-end lineage.
Earliest skeletons, Protoavis , are controversial. Bird skeletons, with teeth, known from the Cretaceous. The Royal Tyrrell Museum offers a Virtual Dinosaur for your very own. Read Chinsamy and Dodson's article on Inside a Dinosaur's Bone published in American Scientist. Recently, a specimen of Thescelosaurus was found with an intact fossilized heart.