Washington | In a first, scientists have created experimental chickens with dinosaur-like feet by manipulating their genes, highlighting the evolutionary link between dinosaurs and birds. In dinosaurs – the ancestors of birds – fibula, one of the two long bones of the lower leg, is tube-shaped and reaches all the way down to the ankle.
However, in the evolution from dinosaurs to birds, it lost its lower end, and no longer connects to the ankle, being shorter than the other bone in the lower leg, the tibia. Scientists noted that bird embryos first develop a tubular, dinosaur-like fibula. Afterward, it becomes shorter than the tibia and acquires its adult, splinter-like shape. Brazilian researcher Joao Botelho, working at the lab of Alexander Vargas from the University of Chile studied the mechanisms that underlie this transformation.
In normal bone development, the shaft matures and ceases growth (cell division) long before the ends do. Botelho found that molecular mechanisms of maturation were active very early at the lower end, ceasing cell division and growth. Inhibiting a maturation gene called Indian Hedgehog resulted in chickens with a tubular fibula as long as the tibia and connected to the ankle, just like a dinosaur.
Researchers believe that early maturation at the lower end of the fibula occurs because of the influence of a nearby bone in the ankle, the calcaneum. Unlike other animals, the calcaneum in bird embryos presses against the lower end of the fibula. They are so close they have even been mistaken for a single element. Botelho proposes that at this stage, the lower end of the fibula receives signals more like those at the bone shaft.
In normal development, the calcaneum then becomes detached from the fibula. However, its distal end has already become committed to shaft-like development, and matures early. In the chickens with experimentally dinosaur-like lower legs, the calcaneum was attached to the fibula. Botelho also confirmed the calcaneum strongly expresses PthrP, a gene that allows growth at the ends of bones. Another interesting observation in the experimental chickens was that the other bone of the lower leg, the tibia, was significantly shorter, researchers said.
This suggests that a dinosaur-like fibula connected to the ankle stops the tibia from outgrowing the fibula, as it normally would. Working with Jingmai O’Connor from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in China, the research team realised this was consistent with an evolutionary pattern documented by the fossil record. The earliest forms to evolve reduced fibulas were toothed birds from the early cretaceous age, which lived alongside dinosaurs. The study was published in the journal Evolution.
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