Despite their reputation as living dinosaurs, birds have come a long way since the days of T. rex and friends.
Gone is the toothy rostrum, replaced by the more familiar beak. Their skeletons have adapted to flight, as have their forelimbs. Even their pelvis is twisted into a shape their oldest ancestors would barely recognize.
These changes were not spontaneous or absolute developments. For a brief period early in their development, in fact, birds of all kinds develop a pelvis that would not have seemed out of place as part of a fossilized dinosaur skeleton.
As part of an ongoing investigation into how the common ancestors of reptiles, birds, and dinosaurs developed their characteristics over the eons, researchers across the United States have mapped the fetal development of crocodilian skeletons. and birds.
They found that the pelvis is an example of what is called a terminal addition – a feature that begins remarkably well preserved, resembling a structure associated with their ancestors, before changing into a newer form later in the development of ‘an individual.
“It was unexpected to find that these early stages of bird development looked so much like the hips of an early dinosaur,” says Christopher Griffin, an evolutionary biologist at Yale University with a particular interest in history. vertebrates.
“In just two days, the developing embryo changes in a way that mirrors how it changed during evolution, from looking like an early dinosaur to looking like a modern bird. .”
Buried deep inside the abdomen, the pelvises of reptiles, birds, and dinosaurs are not the first thing that stands out for the quintessential difference between these three branches of the vertebrate tree.
Given the distinction of their methods of locomotion—from the lizard sprawled on a rock, to the stately gait of a hunting theropod, to the Mick Jagger strut of a chicken—it’s not hard to imagine the types of changes required in their bones and muscles.
Birds’ femurs are more horizontal than those of any dinosaur, for example, allowing their lower legs to carry their movement.
To accommodate changes in their movement style, their ilium (our hip bones) extends further towards their chest and buttocks, carrying the mass of their body; their pubic bone slopes backward rather than forward, with the pubic bones left wide open.
Along with all of these skeletal changes, the ligaments, muscles and tendons also had to undergo major overhauls without compromising the animal’s movements.
Watching how a growing organism builds its body from scratch can provide insight into how evolution might tinker with an ancestor’s plans to innovate.
Griffin and his team therefore used fluorescent antibodies to show skeletal development in the embryos of several species of birds, including domestic chickens and parakeets, as well as alligators.
Using CT scans to build 3D models of growing bones, muscles, nerves and connective tissues, the researchers mapped changes in morphology from early development through hatching.
The bird-like adaptations of the pelvic skeletal and muscular system, they found, appeared much later in development than expected, emerging from features that all seemed more at home in a dinosaur’s body.
“Every bird, early in its life, has this dinosaur form,” says Yale vertebrate paleontologist Bhart-Anjan S. Bhullar.
“Then, at the last minute, it’s like he remembered it was a bird and needed a birdbath.”
Finding an example of terminal addition representative of an entire class of organisms is a great discovery, which until now had only been debated as possible.
Since the muscles and bones of the hips all had to evolve together to do their job effectively, it’s perhaps no surprise to find that they develop this way.
The researchers believe there could easily be other instances of terminal addition worth looking for in large groups of organisms, and may even be common across major evolutionary transitions.
For dinosaur enthusiasts, it is fascinating to know that jurassic park is never as far as a fertilized chicken egg.
This research was published in Nature.