Though the theory that modern-day birds evolved from dinosaurs is widely accepted by scientists today, historically, this theory has lacked evidence. But researchers at Yale and the University of Chile have begun to use embryological data to as evidence of a bird-dinosaur relation.
In a study published this month in the Nature Ecology and Evolution journal, the researchers discovered evidence that two bones found in the skulls of dinosaurs — the postorbital and prefrontal bones — can also be seen in the embryos of birds.
“Nothing like [the postorbital bone in bird embryos] had ever been mentioned,” said Alexander Vargas, a professor at the University of Chile and lead author of the study. “It was really exciting to find it in an embryo — in exactly the same position where the bone would be in ancient dinosaurs.”
According to previous evidence, in mammals, bones that were supposedly lost were actually present in the embryo but fused to other bones early on, said Daniel Smith Paredes GRD ’22, a graduate student in Yale’s geology and geophysics department and first author of the study. The researchers, however, wanted to explore if this development is also seen in birds.
To support their hypothesis, the team looked at both fossils and embryos — a unique integration of the two data sources. The postorbital and prefrontal bones are not commonly seen in modern-day birds. Yet they are prevalent in the fossils of many species of dinosaurs, such as those of the Tyrannosaurus rex. Based on this fossil data, the researchers looked toward embryonic data to explain why modern birds, which evolved from dinosaurs, do not display this skull structure.
Using special staining technologies, the researchers found that both of these bones could be seen in the early development of bird embryos. These bones had gone undetected for decades because they fused to other bones in the skull very quickly after formation — in similar fashion to mammals.
But the fusion of these bones did not occur in a vacuum. This type of development most likely occurred for an evolutionary reason.
“This fossil record shows that the postorbital [bone] was lost about the time when an important expansion of brain size occurred in the evolution of birds,” explained Vargas, suggesting that this development was instrumental in allowing the skulls of birds to accommodate a larger brain.
The development of larger brains is especially interesting to Smith Paredes, as he sees the parallelisms between mammals and birds as indicative of a larger evolutionary trend. He explained that although mammals and birds are not closely related, they both have huge brains compared to other lineages and both demonstrate the fusion of bones in embryos.
A better understanding of the evolution of birds can help illuminate why these two groups appear to have evolved along similar patterns and why those specific patterns are important, Smith Paredes added.
Vargas has dedicated his research to determining exactly how this transition from dinosaurs to birds happened, as well as the evolutionary and genetic mechanisms used along the way. Although the dinosaur-bird hypothesis is widely accepted in the scientific community, some have been distrustful of the embryological data — which has often relied on old studies and outdated techniques. By providing stronger explanations for the evolution of birds, Vargas’ overall work has been able to fill in some of the last holes in this theory.
Looking toward future research, Vargas said he hopes to be able to visualize the formation of these bones even earlier using new labeling technologies. Additionally, he wants to expand his research to the development of other structures, such as the sternum and wishbone, which are “key to discussing the evolution of flight,” Vargas added.
The hypothesis that birds evolved as a modern dinosaur was first proposed in the mid-to-late 1800s following the discovery of fossils that displayed features common to both groups.
Karena Zhao | firstname.lastname@example.org