The great beasts of the Jurrassic and Cretaceous may be long gone, their influence lives on in ways large and small. The latest example is the discovery that the coloring of birds' eggs is something they inherited from the Eumaniraptora, a clade of dinosaurs that include modern birds and their closest relatives, including the (much exaggerated) Velociraptors.
The colors of bird eggs are characteristic of their chosen nesting environment, and though they only use two pigments, red and blue, these shades can be used for camouflage, and to detect intruders in the nest.
Useful as this trait is, it's not one other egg layers have adopted, which makes its origins intriguing.
Yale University PhD student Jasmina Wiemann studied samples from 18 dinosaur eggshell fossils, looking for the two pigments used by modern birds to produce their eggs’ diversity of colors, speckles, and spots.
In Nature Wiemann reports these same pigments were found in all but one eumaniraptoran eggshell she studied across many species and two continents, but not in the shells of other classes of dinosaurs such as sauropods. Looking closer, eumaniraptoran eggs were spotted and speckled in a manner similar to modern bird eggs.
"This completely changes our understanding of how egg colors evolved," Wiemann said in a statement. "For two centuries, ornithologists assumed that egg color appeared in modern birds' eggs multiple times, independently."
Wiemann and her co-authors think color appeared about the same time some dinosaurs started using open nests, rather than hiding their eggs in sand like modern reptiles. This left the eggs vulnerable to predators, giving an advantage to those with camouflage.
Co-author Mark Norell of the American Museum of Natural History noted that egg color can be added to feathers and wishbones as features of modern birds that appeared first among non-avian dinosaurs.
The work implies this eumaniraptors had more complex reproductive behavior than previously recognized, quite possibly including the brood parasitism practiced today by cuckoos. Birds that do this lay their eggs in the nests of others, and leave it to the hosts to go to all the effort of raising the young. This can lead to a sort of arms race, where hosts develop methods to recognize, and expel, the parasite’s offspring, while the parasite finds ways to outwit them.
Biliverdin gives birds' eggs their blue coloring and permeates deep into the eggshell. Protoporphyrin IX produces the brown and red specks and spots on the surface. Some of the eumaniraptoran eggs Wiemann examined showed traces of both, with patterns similar to modern birds. Other species had only one pigment or the other.