The researchers said that the dinosaurs have experienced a long evolutionary cycle of 50 million years and become the birds we have today. Theropod dinosaurs, including Tyrannosaurus and Athlon, have experienced at least 12 over-biomorphological evolutions into modern birds.
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About 225 million years ago, some species fell from an average weight of 350 pounds to less than 2 pounds until about 163 million years ago when the first true bird appeared. Before that, they just kept reducing their height and weight for a full 50 million years. The researchers created a detailed phylogenetic tree by comparing fossils from the 120 different species of theropod dinosaurs and early bird fossils. They use complex mathematical models to reproduce changes in dinosaur body size during evolution.
Modern birds and theropods Carnivorous dinosaur species share many common features such as hollow bones, feathers and wishbone. Theropod dinosaurs are the only dinosaur species that are continuously shrinking. The shrinkage of evolution is consistent, the researchers said, and other dinosaur types have experienced ups and downs of body sizes exceeding millions of years. Not all theropods began to shrink. The researchers admitted that Tyrannosaurus rex kept the same size, indicating that it was a distant relative of the bird, not a direct family tree branch. 
