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The hues and patterns of modern bird eggs date back to their dinosaur ancestors
The modern bird eggshells present a dramatic range of rainbow hues, ranging from butter yellow to blood red, from the lightest aqua to the darkest cyan. Some are stained or speckled; others are without imperfections. How and when did the incredible diversity of colors and patterns of eggs evolve? Of the modern-day amniotes (the group that includes birds, reptiles and mammals), only birds produce colored eggs. The other layers of eggs form white ones. The wisdom that has prevailed has therefore been that the color of the eggs is a bird's innovation, but new discoveries indicate that long before the blue of Robin's egg does Deinonychus's egg blue.
In an article published this week in Nature Researchers have described pigments found in fossilized eggshells from several species of dinosaurs. The work indicates that the dazzling variety of colors and patterns in modern bird eggs dates back to a single, evolutionary origin in non-avian dinosaurs. (Technically, birds are a subset of dinosaurs, hence the distinction between avian and non-avian.) The discovery adds to a growing body of evidence of fossil pigments that is revolutionizing dinosaur science.
Jasmina Wiemann, a Ph.D. student at Yale University, and her colleagues announced the discovery of the pigment responsible for the color of the blue-green egg in the fossilized eggshells of several Chinese oviraptorid dinosaurs. Oviraptorides were relatively small bipedal dinosaurs with gripping hands, beaks without teeth and feathers. The discovery established that at least one group of non-avian dinosaurs had colored eggs, asking whether the birds had inherited the color of their ancestors' eggs from non-avian dinosaurs or if the coloring had evolved independently in birds and non-avian dinosaurs.
In this new study, Wiemann and his colleagues sampled the egg shells of 19 species of birds, crocodilians and non-avian dinosaurs and analyzed their chemical composition for the purpose. a technique called Raman spectroscopy, able to identify pigments. The colors of modern bird eggs come only from two pigments: biliverdin and protoporphyrin IX. The researchers detected both pigments in their samples of fossil eggshells. By analyzing the results in a family tree, the team determined that the color of the egg had appeared only once – in the Eumaniraptora group of dinosaurs, which includes oviraptorids and other non-avian dinosaurs, as well as all modern birds. Sauropods (Apatosaurus and his ilk) and ornithischians (Triceratops and kin) do not appear to have laid pigmented eggs. And the few lines of eumaniraptorane that have lost the color of the egg have not found it – probably because the gene cascades giving rise to the color of the egg are so complex, note Wiemann.
The results of the study provide insight into why the color of the egg has changed. Scientists have already proposed a number of hypotheses to explain the phenomenon, stating that the pigments could have helped hide eggs in certain environments, provide protection against damaging solar radiation or fortify the shell, among other benefits. The fossil egg-shell pigments that Wiemann and his colleagues have detected come from dinosaurs known to have deposited their eggs in air nests instead of burying them as their predecessors. This association suggests that a change in nesting behavior was a key factor in the appearance of egg color, although other factors also contributed to this development. For example, in the case of Chinese oviraptorids, believed to be nestled near rivers, the blue-green color of the eggs may have helped them to blend in with the nest material and surrounding vegetation, hiding them concealed predators.
Outside experts find convincing the claims of the team for preserving pigments in fossil eggs. "It's a good thing," says Jakob Vinther of Bristol University in England, an authority on ancient pigments. He notes that the team has demonstrated that Raman spectroscopy can reliably distinguish pigments from proteins, which may seem very similar.
The findings could have interesting implications for understanding parental care in dinosaurs. In the 1990s, paleontologists working in Mongolia recovered a fossil of an extremely preserved oviraptorid, placed on top of an egg nest. Considered a convincing proof of the monumental behavior of modern birds of origin of non-avian dinosaurs, the fossil, supposed to be a woman, was nicknamed "the big mother". But a few years later, researchers examined a number of adult dinosaur fossils preserved with eggs (including oviraptorides). , comparing the volume of eggs and the details of the bone structure of fossil specimens with those of modern birds. Their conclusion: the thirsty dinosaurs were probably men. The new proof of the color of the egg "adds the missing piece of the puzzle," says Wiemann. In modern birds such as blackbirds, the color of the blue egg is often associated with higher levels of paternal care. The idea is that the color of the eggs is a sign of quality in the mother and therefore in the young and that it therefore encourages the father to make an extra effort to support his family. The revelation oviraptorids and some other eumaniraptorans had blue eggs therefore supports the affirmation of paternal care in birds native to non-avian dinosaurs. That is, Big Mama could be called Big Papa.
"The discovery of a unique origin of eggshell color in dinosaurs is a wonderful reminder that modern birds have inherited many traits from their dinosaur ancestors," says Mary Stoddard of Princeton University, who studies the evolution of the eggs of birds and has not participated in recent work. . She notes that in recent years, the awareness by many non-avian dinosaurs of colorful feathers has transformed scientists' thinking about the biology and behavior of dinosaurs. This new study, she adds, "is likely to inspire a whole new area of research on dinosaur nesting and incubation behavior."
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