3D analysis reveals a key similarity between dinosaur and bird embryos
It's all in the hips.
BIRDS ARE THE LAST DINOSAURS — but it took 150 years of analyzing fossils for scientists to come to that now-established conclusion. Yet a new study published last week in the journal Nature reveals living evidence for this fact, no fossils necessary. To see that birds are the inheritors of dinosaurs, you just need to look at birds’ bodies at the right moment in their development, the study finds. Before birds hatch, there is a brief period in the egg when their hip bones bear an uncanny resemblance to their reptilian ancestors.
HERE'S THE BACKGROUND — The first evidence that birds are dinosaurs came with the discovery of Archaeopteryx in Germany in 1861. This fossil represented a roughly 150-million-year-old missing link between reptiles and birds, possessing not only wings and feathers but also teeth, a long tail, and claws on its wing joints. Over time, scientists have unearthed more fossils of ancient, reptile-like birds, but also evidence that many raptor dinosaurs possessed feathers.
Still, modern birds possess bodies significantly different from most dinosaurs’ bodies. Myriad fossils shed light on the stepwise transformation leading up to modern bird physiology, but much remains uncertain about how this shift occurred and whether it is still ongoing in some fashion.
How the discovery was made — In the new study, paleontologist Bhart-Anjan Bhullar at Yale University in New Haven, Connecticut, and his colleagues focused on the avian hip bone, which is the core of the body of a bird. It runs the length of the avian frame, enabling a bird to stand, move, and carry the weight of its entire body.
In the past decade, Bhullar and his collaborators have investigated many key evolutionary transitions between dinosaurs, reptiles, and avian species, including the development of the dinosaurian inner ear, the bird beak, the mammalian jaw, and eyesight in vertebrates. They use computed tomographic (CT) scanning and microscopy to create 3D images of animal embryos to decipher these evolutionary steps.
The modern bird hip is decidedly different from its dinosaur ancestors. For example, in birds, the pubis — either of a pair of bones forming the two sides of the pelvis — faces backward, whereas, in many ancient dinosaurs and most animals with four limbs, the pubis is oriented forward. In addition, the end of the bird's pubis does not widen to form a “pubic boot” as it does in many ancient dinosaurs, and the ilium — the bone forming the upper part of each half of the pelvis — greatly extends both forward and backward in birds. All these changes are linked to how birds move their legs — the femur, or thighbone, stays largely horizontal and stationary while the lower part of the leg swings to propel the bird.
To learn more about the evolution of the bird hip, the researchers looked at pelvic development in alligators, domestic chickens, Japanese quail, Chilean tinamou, and parakeets. They compared 3D images of the hip bones, muscles, and nerves at various developmental stages in these animals with those of several dinosaurs, including Archaeopteryx.
The discovery — The scientists found many ancestral dinosaur features of the hip — the forward-facing pubis, a pubic boot, and a short ilium — do make a brief appearance during bird development in the egg.
"This discovery was made possible by new methods and techniques for imaging embryonic tissues at very early stages," study lead author Christopher Griffin, an evolutionary biologist at Yale University, tells Inverse.
"The ancestral dinosaurian features appear very early in the development of the skeleton before any bones are present and even before any cartilage is present. The dinosaur stages are most clear in the pre-cartilage developmental stages."
These features of the hip arrive at their typical avian forms after transitioning through a developmental sequence that, over the course of two days in the egg, mirrors what researchers have discovered from fossils about how birds acquired these features during evolution.
"As soon as we started imaging the pre-cartilage stages of bird embryos, we immediately recognized that they look like the hips of early dinosaurs," Griffin says. "The fact that the anatomy of long-extinct dinosaurs is still present in the embryos of birds was most surprising. These ancient features aren't locked away in fossils, like we always thought, but are temporarily present in living animals."
These changes are an example of "terminal addition," a biological mechanism in which ancestral features appear in an animal until late in its development. This was a surprise because many important features in the dinosaur-to-bird shift, such as the beak, are seen early in a bird's embryonic development.
"Probably the most important implication of this is that this retention of ancestral features in embryos may be more widespread among animal groups than we have originally thought," Griffin says. "This could open up avenues for investigating the origin of modern animals in a way that just wasn't seen as possible before."
The scientists initially made this discovery in 2017, "but the methods needed to be refined for a year or two before then to even get to the point where we had good data," Griffin recalls. "Over winter break that year, I left for a week to visit family and left all of our bird embryos in the lab, cleared, stained, and ready to be imaged under a microscope when I got back. Unfortunately, an issue with the thermostat in the lab caused the room to get extremely warm one day while everyone was out, and all the embryos rotted. It took a full year to get more embryos, clear, stain, and process all the images."
Griffin notes, "the ancestral dinosaurian features that we see in the bird embryo are not identical to those of adult dinosaurs. This isn't a carbon copy of a T. rex hip or a Velociraptor hip — those are large, made of bone, and have their own adult features that make them unique from all other species. What we see in the bird embryo is more likely to be similar to what a Tyrannosaurus or Velociraptor embryo would have looked like.”
“The difference being that in the extinct dinosaur,” he adds, “the embryo would stop changing at an earlier stage, stopping at the 'dinosaur stage.' The bird embryo keeps changing past the dinosaur stage until it looks like a bird."
WHAT'S NEXT? — "We still have no idea what the underlying genetic architecture of these features are, which opens up a lot of exciting research opportunities," Griffin says. Because birds are living with us today, scientists can use them as a live model to discover how dinosaurs developed and what genes may have played an integral role in their evolution.
"We thought this transition from dinosaurs to birds was locked away in stone, but since the transition is also seen in living animals with genes and development that can be studied directly, this means that we can potentially undertake the genetic changes it took to build a dinosaur."