Living birds have bodies substantially modified from the ancestral reptilian condition. The bird pelvis (hip bone) in particular experienced major changes during the transition from early archosaurs (dinosaurs plus crocodiles) to living birds. This stepwise transformation is well documented by an excellent fossil record; however, the developmental alterations that underly it are less well understood. In new research, scientists at Yale University used embryological imaging techniques to examine bird pelvic tissues in 3D, allowing direct comparison with the fossil record.
“Every single bird, in its early life, possesses this dinosaurian form,” said senior author Dr. Bhart-Anjan Bhullar, a researcher in the Department of Earth and Planetary Sciences and the Yale Peabody Museum of Natural History at Yale University.
“Then, at the last minute, it’s like it remembers it’s a bird and needs a bird’s pelvis.”
In their study, Dr. Bhullar and colleagues looked at pelvic development in alligators, domestic chickens, Japanese quail, Chilean tinamou, and parakeets, and compared their developmental stages with those of dinosaurs, including Archaeopteryx.
They labeled embryonic pelvis with antibodies to look for proteins that are expressed in developing cartilage, connective tissue, skeletal muscles, and nerves.
The researchers then created 3D images for the pelvis, muscles, and nerves with confocal microscopes and CT scanning.
They found that the bird pelvis is 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 transition, such as the bird’s beak, are seen early in a bird’s embryonic development,” said first author Dr. Christopher Griffin, a postdoctoral researcher in the Department of Earth and Planetary Sciences and the Yale Peabody Museum of Natural History at Yale University, and in the Department of Geosciences at Virginia Tech.
“It was unexpected to find these initial stages of bird development look so much like the pelvis of an early dinosaur.”
“During just two days, the developing embryo changes in a way that reflects how they changed in evolution, transitioning from looking like an early dinosaur to looking like a modern bird.”
The pelvis is the core of a bird’s body. It runs the length of the avian frame, engulfing the torso, while also enabling a bird to stand, move, and carry the weight of its entire body.
“The bird body is incredibly modified in virtually every way to create an optimized flying machine,” Dr. Bhullar said.
“Its body structures are tightly constrained by the necessities of aeronautic design.”
The scientists also looked at avian muscles and nerves related to the development of the pelvis.
The development of those systems were not synchronous with bone development, implying that each system was somewhat uncoupled from the others.
“Our data provide evidence that the avian pelvis, whose early development has been little studied, evolved through terminal addition — a mechanism whereby new apomorphic states are added to the end of a developmental sequence, resulting in expression of ancestral character states earlier in that sequence,” the authors said.
“The phenotypic integration we detected suggests a previously unrecognized mechanism for terminal addition and hints that retention of ancestral states in development is common during evolutionary transitions.”
The study was published in the journal Nature.
C.T. Griffin et al. The developing bird pelvis passes through ancestral dinosaurian conditions. Nature, published online July 27, 2022; doi: 10.1038/s41586-022-04982-w
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