Bird eggs have been admired since ancient times for their many hues and patterns, from the vivid blue of American robin eggs to the deep green-black of emu eggs. Now, a study shows these colors and markings are even older than we thought—they may have graced dinosaur eggs some 150 million years ago or more.
“[This] is a true discovery,” says Mark Hauber, an ornithologist who studies the evolution of bird eggs and brooding behaviors at the University of Illinois in Urbana and was not involved in the research. Until now, most experts thought colored eggs evolved more recently, in various groups of modern birds, and that the earliest birds laid pure white eggs like crocodiles, he says.
Research in the past few decades has shown many traits once thought to define modern birds, including feathers, wishbones, brooding behaviors, avian-style lungs, and hollow skeletons, evolved first in their dinosaur forebears. And last year, molecular paleobiologist Jasmina Wiemann of Yale University published the first evidence of dinosaur egg coloration, using chemical analysis to detect two pigments—blue-green biliverdin and red-brown protoporphyrin—in the eggs of a 70-million-year-old parrot-beaked oviraptorosaur called Heyuannia from China.
To find out whether colored dinosaur eggs shared an evolutionary origin with those of birds, Wiemann and her colleagues amassed well-preserved fragments of fossil eggshell from 15 Cretaceous era dinosaurs and extinct birds, as well as eggshell from living chickens, terns, emus, and alligators. Instead of using the chemical analysis they developed for the 2017 study, which required grinding up fossil specimens, they turned to an innovative technique for studying fossils: something called Raman microspectroscopy. The method requires no destruction of fossils, instead bouncing a laser off the surface of the eggshells to betray their molecular makeup, including whether they carried traces of the two pigments known to color modern eggshells.
The team found fossil eggs of many colors and speckling patterns. A relative of Velociraptor called Deinonychus laid eggs with blue-green color; the lightly built carnivorous troodontids had eggshells of blue-green, beige, or white; and the eggs of the Chinese oviraptorosaur Heyuannia previously tested were deep blue-green. Maps of protoporphyrin accumulation across the surface of many of the eggs, including Deinonychus and some troodontids revealed darker speckling patterns on top of the background color.
As the researchers write today in Nature, the fact that they found colored eggs in so many carnivorous theropod dinosaurs that are closely related to birds—and exactly the same method of eggshell pigmentation—means colored eggs evolved “deep within the dinosaur tree and long before the spectacular radiation of modern birds,” likely more than 150 million years ago.
Tinted shells probably camouflaged dinosaur eggs from predators, as do the shells of today’s birds, whereas distinctive speckling patterns may have helped parents distinguish their own eggs from those of cuckoolike dinosaurian nest parasites, Wiemann says. Birds with white eggs today, such as ostriches, parrots, and some domestic chickens, must have later lost the trait of coloring their eggs through evolution, she says.
Traditionally, dinosaurs were thought of as reptilian-style breeders that dumped their eggs and left. But because egg color in birds is associated with complex nesting behaviors, this—along with existing fossil evidence—signals such advanced parental care may have also taken place among dinosaurs. Egg coloration as camouflage likely evolved soon after dinosaurs switched from burying their eggs to building open nests, Weimann adds, because from that point on, they needed to hide them from predators.
No hint of pigment was detected in the eggs of several herbivorous long-necked sauropods or a duck-billed dinosaur Maiasaura, suggesting these species—which are on more distant branches of the dinosaur family tree—had white eggs that they buried in the ground like modern-day turtles.
David Varricchio, who studies dinosaur reproduction and brooding at Montana State University in Bozeman, says that because the authors used a novel technique, others will want to test its veracity. But, he says, the fact that we are even beginning to discuss the color of eggs more than 66 million years old is “pretty dang amazing.”
Wiemann plans to increase the dinosaur sample size to see whether she can pin down exactly where and when within the carnivorous theropod group of dinosaurs (which also includes birds) colored eggshells first evolved, and what color came first. Given the analytical methods from other scientific disciplines now available for work on fossils, “It’s an amazing time to be a paleontologist,” she says.