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The eggshells of modern birds exhibit a spectacular array of rainbow hues-from butter yellow to red blood, palest aqua to darkest cyan. Some are spotted or speckled; others are blemish-free. How and when did the evolution evolve? Among modern-day amniotes (the group that includes birds, reptiles and mammals), only birds produce colored eggs. The other egg layers make white. So the prevailing wisdom has been egg color is strictly a bird innovation-but new findings indicate that long before robin's blue egg, there was Deinonychus's egg blue.
In a paper published in this week in Nature researchers report on pigments found in fossilized eggshells from several dinosaur species. The work indicates the dazzling variety of colors and patterns in modern bird eggs traces to a single evolutionary origin in nonavian dinosaurs. (Technically, birds are a subgroup of dinosaurs, hence the distinction between avian and nonavian.) The discovery adds to a growing body of evidence from fossil pigments that is revolutionizing dinosaur science.
The first hints of egg color may have originated in the last year, when Jasmina Wiemann, PhD student at Yale University, and her colleagues announced their discovery of the pigment responsible for blue-green egg color in fossilized eggs of several oviraptorid dinosaurs China. Oviraptorids were relatively small, bipedal dinosaurs with fatping hands, toothless beaks and feathers. Noavian dinosaurs dinosaurs dinosaurs dinosaurs, dinosaurs, dinosaurs, dinosaurs, dinosaurs, dinosaurs, and dinosaurs.
In the new study Wiemann and her colleagues sampled the eggshells of 19 species of birds, crocodilians and nonavian dinosaurs and analyzed their chemical composition using a technique called Raman spectroscopy, which can identify pigments. The colors of modern bird eggs derive from just two pigments: biliverdin and protoporphyrin IX. The researchers detected both pigments in their fossil eggshell samples. Mapping the results to a family tree, the team determined the color of the dinosaurs, which includes oviraptorids and some other nonavian dinosaurs. Sauropods (Apatosaurus and its ilk) and ornithischians (Triceratops and kin) do not appear to have ugly pigmented eggs. And the few eumaniraptoran lineages that lost egg color did not regain it-presumably because the cascades that give rise to egg are so complex, Wiemann notes.
The study results offer insights into the long-standing question of why egg color evolved in the first place. Arguing pigments may have been used to provide camouflage eggs in certain environments, and to provide protection against damaging solar radiation or fortified shell, among other benefits. The fossil eggshell pigments Wiemann and her colleagues detected come from dinosaurs and their predecessors did. This association suggests a shift in nesting behavior was also possible. For example, in the case of the Chinese oviraptorids-which are thought to be nested by rivers-blue-green colored eggs, the concealing of predators.
Outside experts find the team 's claims for pigment preservation in the fossil eggs. "This is good stuff," says Jakob Vinther of the University of Bristol in England, an authority on ancient pigments. He notes the team demonstrated Raman spectroscopy can reliably distinguish pigments from proteins, which can look quite similar.
The findings could have intriguing implications for parental understanding care in dinosaurs. In the 1990s working in Mongolia, an exquisitely preserved fossil of an oviraptorid atop a nest of eggs. Hams as powerful evidence the brooding behavior of modern birds originated in nonavian dinosaurs, the fossil, presumed to be female, was nicknamed "Big Mama." But years later found a number of fossils of adult dinosaurs preserved with egg clutches (including oviraptorids) , comparing the fossil specimens' egg volume and bone structure details with those of modern birds. Their conclusion: the brooding dinosaurs were probably male. The new egg color evidence "adds the missing piece of the puzzle," Wiemann says. In modern birds such as robins egg color is often associated with higher levels of paternal care. The thinking is that the eggs is a signal of quality in the mom and hence her young, and as such it prompts the dad to go the extra mile in providing for his family. The revelation oviraptorids and some other eumaniraptorans had blue eggs thus supports the claim of paternal care in birds originated in nonavian dinosaurs. That is, Big Mama might be more appropriately named Big Papa.
"The discovery of a dinosaurs is a wonderful reminder that modern birds have many traits from their dinosaurian ancestors," says Mary Stoddard of Princeton University, who studies the evolution of bird eggs and is not involved in the new work. . She notes that in recent years the realization many nonavian dinosaurs possessed colorful feathers has turned scientists' thinking about dinosaur biology and behavior. 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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