Eggs evolved color and speckles only once — during the age of dinosaurs

Author: Carolyn Gramling / Source: Science News

The colorful, speckled eggs of modern birds are an innovation inherited from their nonavian dinosaur ancestors.

A new analysis of the pigmentation in modern and fossilized eggshells suggests that eggs evolved to be colorful only once — in modern birds’ dinosaur ancestors, a team of vertebrate paleontologists report online October 31 in Nature. Color patterns found in the eggshells of theropod dinosaurs, a lineage that includes Tyrannosaurus rex and smaller winged dinosaurs such as Microraptor, were very similar to those of modern birds, says Jasmina Wiemann, of Yale University.

Scientists once thought only birds produced colorful eggshells, says coauthor Mark Norell of the American Museum of Natural History in New York City. But a growing body of literature suggests that many traits once thought to be unique to birds — feathers, flight, brain organization and adaptive metabolisms needed for flight — evolved long before modern birds did (SN: 4/14/18, p. 9; SN: 5/26/18, p. 8; SN Online: 10/19/18). “It’s kind of blurring the line between what’s a bird and what’s a [nonavian] dinosaur,” Norell says.

Bird eggs get their color from two pigments: a red-brown pigment called protoporphyrin, which creates speckles and is found only in an eggshell’s soft, outer cuticle layer; and a blue-green pigment called biliverdin, found in the deeper, crunchier part of the shell.

In 2015, Wiemann and her colleagues reported that fossilized eggshells of Heyuannia huangi, a short-beaked, crested dinosaur that lived about 70 million years ago, contained traces of both pigments (SN: 6/27/15, p. 14).

Eggstravaganza

Researchers found no traces of pigment in eggshells belonging to a long-necked titanosaurid dinosaur, or in the eggs of a modern American alligator, both of which have buried nests. But traces of blue-green or red-brown pigments — or both — were found in the eggshells of theropod dinosaurs such as Deinonychus antirrhopus as well as such modern…

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