Coloured eggs evolved only once, with the eggshell pigmentation method of modern birds having been laid down by their dinosaur predecessors, reports a paper published online this week in Nature.

Of the living amniotes (birds, reptiles, and egg-laying mammals), birds are the only members that produce coloured eggs. The same red-brown and blue-green pigments used to colour bird eggs have recently been identified in certain fossil dinosaur eggshells. However, whether birds inherited egg colouration from their dinosaur forebears or evolved eggshell colouring separately was not known.

To address this, Jasmina Wiemann and colleagues use Raman spectroscopy to analyse a set of fossil eggshells, which included representatives of all major dinosaur groups, to look for evidence of pigmentation. They find traces of pigments preserved in the eggshells of all Maniraptora - the group of small, bipedal and often feathered dinosaurs that includes birds - which they map out to reveal spotted and speckled patterns. The authors determine that the pigments would also have been deposited in the same way as the colourants in modern bird eggs.

By contrast, ornithischian, and sauropod dinosaurs - the more distant relatives of birds, which famously include Triceratops and Diplodocus - had eggshells that were pigment-free, confirming that eggs of these dinosaurs were always plain, rather than having lost their colouration as the eggshells fossilized. Together, these findings suggest that pigmented eggs evolved only once, in the bird-like therapod dinosaurs, and these pigments are carried through until the present day.