One of the hardest things to wrap our heads around when it comes to the dinosaur-to-bird transition is how massive, carnivorous ground-dwellers evolved into small, wispy birds fluttering from branch to branch. According to a new study published in Science this week, the dinosaur lineage that evolved into birds continuously shrank in body size over the last 50 million years and across at least 12 consecutive branches. 

"Birds out-shrank and out-evolved their dinosaurian ancestors, surviving where their larger, less evolvable relatives could not," says Michael Lee from the Australian Museum in Adelaide in a news release. The branch of dinosaurs that led up to modern birds -- called theropods, and included T. rex and Velociraptor -- was also the most evolutionarily innovative.

To trace evolving adaptations and changing body size over time, Lee and colleagues analyzed 1,549 anatomical traits from 120 species of theropods and early birds. They sampled traits across all branches of the theropod tree and across the entire dinosaur body in order to map out the transformation. 

Theropod body size, they found, decreased at least 12 times: from an average 163 kilograms (360 pounds) around 200 million years ago to the 0.8 kilograms (1.8 pounds) of the first bird, Archaeopteryx. 

The image below diagrams how theropods shrank continuously for 50 million years. From left to right: the ancestral neotheropod (220 million years ago), tetanuran (200 MYA), coelurosaur (175 MYA), paravian (165 MYA), and finally Archaeopteryx (150 MYA). 

Maintaining this long-term trend required sustained “miniaturization” -- a process that shortens an animal’s development, which brings about developmental changes. These changes, which started 50 million years before Archaeopteryx, allowed for the bird-specific traits: Snouts got shorter, teeth got smaller, and insulating feathers started to develop. 

According to these findings, prolonged miniaturization was one of two key drivers of the dino-to-bird transition. The second was the very fast evolution of new skeletal adaptations along theropod lines. Their adaptations -- like wings, wishbones, hollow skeletons, and three-fingered hands -- evolved four times faster than in other dinosaurs. No one ancestral bird trait drove the evolution of the rest; rather, the traits mutually influenced each other. 

In the image to the right, the feathered dinosaur Microraptor pounces on a nest of primitive birds (Sinornis). Both lived during the Cretaceous in what’s now northern China.

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

The theropods were the only group to continually push the envelope when it came to skeletal size, he tells Washington Post, although, maybe herbivores just couldn’t shrink, since their plant-based diet required a larger gut for digestion.

Here’s a video illustrating how birds arose from dinosaurs. Check out that hummingbird hovering next to that dino tooth!

 

 

Images: Brian Choo (top, bottom), Davide Bonnadonna (middle)
Video: Michael Lee