Xiaoting Zheng, of Linyi University in China, and colleagues reported the development of a new theory of bird evolution and flight in the journal Science on March 14, 2013, that incorporates recent discoveries of feathered legs on all four legs of some species of dinosaurs that are considered bird ancestors.
The original four winged anatomy was replaced by two front wings and scales on the hind legs.
The researchers point to four major stages in the progression from four feathered limb as demonstrated in the lightly feathered hind limbs of the compsognathid Sinocalliopteryx that had feet and hind legs covered with very light small feathers to the basal deinonychosaurian Anchiornis and the basal avian Sapeornis that had large feathers on both legs and feet indicating a wing or wing like structure on the rear legs, proceeding to the enantiornithine Cathayornis with a reduced wing structure on the hind leg and culminating in ornithuromorphs (true birds) that had no foot feathers but small feathers on the upper back leg.
This discovery is the first physical proof that feathered dinosaurs evolved over time to flying dinosaurs with wings and eventually modern day birds.
The species of dinosaurs and birds considered span a time frame from 160 million years ago to the present and are related phylogentically.