A new study is out detailing the evolution of massive dinosaurs into today’s birds. Scientists compared fossils of 120 different species and 1,500 skeletal features in order to construct a family tree for a class of dinosaur known as theropods. Theropods were two-legged meat-eaters.

Go outside right now and you’ll see what theropods have become. Gone are the ferocious features of meat-eating dinosaurs. Today, they are robins, finches and other small birds.

“They just kept on shrinking and shrinking and shrinking for about 50 million years,” said study author Michael S. Y. Lee of the University of Adelaide in Australia.

“Shape-shifters” is how Lee describes them.

The image below takes inspiration from the ape-to-man evolution drawing. Check out how the massive meat-eating dinosaur transforms into the earliest bird over millions of years.

dinosaur bird to evolution

While theropods kept shrinking to become today’s modern bird, its fellow feathered dinosaurs stayed bulky. This evolutionary edge was key to theropods survival according to researchers.

“Ultimately, this evolutionary flexibility helped birds survive the deadly meteorite impact which killed off all their dinosaurian cousins,” said Lee.

Theropods ended up shrinking drastically at least 12 times. The average mass started at 163 kilograms (around 360 pounds) to the 0.8 kilogram (1.7 pounds) Archaeopteryx, which is considered the earliest bird.

Why did the meat-eating theropods shrink, but not the herbivores? It may have something to do with a plant-based diet requiring a sizable stomach for digestion.

Shrinking theropods would have held several advantages over traditional dinosaurs.

“Being smaller and lighter in the land of giants, with rapidly evolving anatomical adaptations, provided these bird ancestors with new ecological opportunities, such as the ability to climb trees, glide and fly,” said Lee.

The study was published in the journal Science.

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