The Incredible Shrinking Theropods

Dinosaur line that became birds shrank incredibly fast, study finds

How did birds emerge from a lineage of large dinosaurs whose clawed feet were planted firmly on the ground? Size really matters, according to a team of scientists that traced the incredibly fast shrinkage along 50 million years of ancient avian evolution.
The findings, published in the journal Science, show how the continuous miniaturization of this dinosaur lineage allowed for a whole host of physical changes that made powered flight possible.
Paleontologists have long known that birds evolved from dinosaurs known as theropods, a group that included the formidable Tyrannosaurus rex. But there’s been considerable argument over whether the branch of theropods leading to birds really was quickly shrinking until the earliest birds emerged roughly 160 million years ago.
Link