Egg coloration, so far thought to have originated in bird lineages, seems to have appeared much earlier, in dinosaurs, according to a new study. Scientists examined the well-preserved eggs of a bird-like dinosaur that lived in the Cretaceous period in the area of today’s China. The animal laid eggs that had a blue-green tint, the first evidence of pigment in dinosaur eggs.

The eggshells in question belonged to the oviraptorid Heyuannia huangi, a small-bodied, short-snouted group of dinosaurs with toothless beaks. Fossils of oviraptorids have been found in Mongolia and China.

Colour in eggs helps them to camouflage from predators as well as helping birds recognise their own eggs. The pigments that are responsible for eggshells’ colour are biliverdin, for blue-green shades, and protoporphyrin, for reddish-brown ones.

The researchers examined three oviraptorid eggs from the Late Cretaceous period, that had a very subtle shimmer of blue green. They found that there were both biliverding and protoprphyrin on the eggshells, but the amount of biliverdin was much higher, which means that the eggs in their original state were a solid colour. Fossilization caused the amount of biliverdin to drop, since it is more vulnerable to being dissolved by water. So, the eggs were probably of an intensively blue-greenish colour.

Further testing is required to determine whether the presence of the pigments is not associated with microbial activity, but since no pigment was found in the sediment around the eggs, researchers believe that it originated in the eggs.

The shape of the eggs helped researchers define the type of nest oviraptorids preferred, i.e. aboveground. This raised questions about whether nesting behaviour and egg colour can be associated. Potential linkage between blue-green egg colour and communal nesting remains an issue to be further investigated. Perhaps a connection between egg coloration and nesting behaviour can be established in the future.