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Imagine a bird's egg: it may be the brown cocoa of a hen in the open air. Or a creamy blue-green of robin. If it's a quail egg, it has ink stains. According to a new study published Wednesday in the journal Nature, these colors and variations have a common origin. The color of the eggshell, in all its splendor, is a characteristic trait of the animals of the past: the dinosaurs.
The birds, strange as it may sound, are living dinosaurs, the last of a lineage that has otherwise died out 66 million years ago. Prior to this work, however, many biologists had predicted that modern birds, not their ancestors, were developing colorful eggshells. Theories about eggs were "everywhere," said study author Mark Norell, a paleontologist at the American Museum of Natural History in New York. Some people hypothesized that colors evolved several times independently in the history of birds; others have suggested that this happened once, when the birds became the animals we know. The colored eggs, concludes this study, are much older.
"The discovery of a unique origin of the color of the eggshell in dinosaurs is a wonderful reminder that modern birds have inherited many traits from their dinosaur ancestors," said Mary Caswell Stoddard, biologist evolutionist from Princeton University who was not involved in this research. "We eat eggs for breakfast, but they have many clues about the evolutionary past."
Break a shell into its molecules and you will only find two types of pigment among the debris. A molecule called biliverdin is at the origin of green (a decades-old experience linked biliverdin to green spots that sometimes appear in bruises). Another molecule, protoporphyrin, provides rust and browns. These pigments blend together like watercolor paints to produce the full color palette of bird eggs.
A few years ago, Mark Hauber, an ornithologist from the University of Illinois at Urbana-Champaign, and his colleagues devised a protocol to examine biliverdin and protoporphyrin in eggshells , including shells of an extinct giant bird called moa. The authors of this study embraced this idea and "did a much better job than us", including a range of dinosaurs, said Hauber.
The author of the study, Jasmina Wiemann, currently a PhD student at Yale University, began her search for colored eggs with a dinosaur named Heyuannia huangi, an oviraptor with a parrot-like beak. Other scientists told her that she was "wasting her time," she said, as they thought the dinosaur eggs lacked pigment.
Using Raman spectroscopy, a non-destructive analytical tool, Wiemann researched the molecular signatures of protoporphyrin and biliverdin in the eggshell of the ovipositor. Heyuannia huangi, she discovered, has laid blue-green eggs.
This new research expands its previous study. Wiemann, Norell and Tzu-Ruei Yang from the University of Bonn in Germany examined eggs of 18 species. The non-extinct animals included an alligator and domesticated chicken. The crowd of dinosaurs was varied: oviraptors and other ancestors of birds, but also sauropods (long-necked dinosaurs) and hadrosaurs (duck-billed dinosaurs), which were not closely related to birds.
The authors of the study carefully selected their shells to find out what type of dinosaur was laying the eggs. Norell said, "We did not accept specimens for research that there was an embryo in the egg or if there was an adult closely associated with egg laying. Although the colors of the shell have faded over millions of years, eggs have become fossils, traces of pigment molecules remain.
Sauropods and hadrosaurs did not have colored eggs. They looked like alligator eggs – white. Bird parents of dinosaurs, however, had colored eggs, including spots and both types of pigments. (If you wonder why supermarket eggs are white, it's a human invention, the result of farmers who specifically raised chickens with genes for white eggs.
The dinosaurs in this study are from the Cretaceous, at the end of the dinosaur lineage. Norell said it was possible that "more primitive" animals, like a Tyrannosaurus, had laid colored eggs, but scientists had not yet found older shells to test.
These results confirm that Wiemann's discovery of blue oviraptor eggs was not a coincidence, Hauber said. "The extinct dinosaurs have laid their eggs like the rest of the birds," he said.
The egg patterns are a window on the nesting habits of dinosaurs. "A fairly easy prediction, but we are the first to have proof, is that the origin of colored eggs is associated with the origin of an open nest," Norell said. Crocodiles and turtles bury their white eggs, which means they do not have to be camouflaged. But many birds have colorful eggshells that camouflage their eggs in nests exposed to the elements and predators.
Hauber gave three other reasons, in addition to camouflage, for which birds have colored eggs: First, the pigments can act as an umbrella or sunscreen, protecting the embryos from too much heat. Second, some parent birds use punctual patterns to recognize their own eggs if they live in large colonies (or to prevent free-range birds, such as cuckoos, from sneaking into a brood). And third, the pigment deposits are like molecular mortars, reinforcing the structure of the hull. These are all possibilities for dinosaurs, too. "We are putting our puzzle thinking now!" Said Hauber.
So listen, pencil makers and paint mixers: Robin egg blue is perfect, but Oviraptor blue is the original.
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