Which came f irst: the dinosaur or the egg?
Like modern birds, some dinosaurs had coloured eggs – and they may have been scribbled and speckled too.
BIRDS’ EGGS come in a beautiful diversity of colours, shapes and degrees of speckling.They range from the dark verdigris-green eggs of emus, which can be 13cm long, to the tiny eggs of comb-crested jacanas, which are just a fraction of that size, and beige, with lovely scribbly-gumlike lines daubed all over their surface (see AG 134).
A number of bird species are the only modern animals that lay coloured eggs – the eggshells of lizards and crocodiles are white, as are the eggs of many other species of bird. For this reason, ornithologists long thought that coloured eggs evolved among birds themselves. But a new analysis of dinosaur eggs from China has revealed some dinosaurs laid coloured eggs too, and that birds likely inherited this trait from their dinosaurian ancestors.
Researchers led by Jasmina Wiemann and Martin Sander at the University of Bonn in Germany have analysed eggs in a series of fossilised dinosaur nests found at three sites in the south and east of China.These eggs belonged to a species of oviraptorid dinosaur called Heyuannia huangi, which lived about 70–67 million years ago in the late Cretaceous Period and was an emu-like, fast-running species with a beak a little like a parrot.
Several years ago, the scientists noticed that these fossil eggs were often a pale blue-green colour, which stood out against the reddish surrounding rock.They noticed the similarity of the shade to the bluegreen eggs of many birds today and decided to chemically test the fossils for the pigments that colour these modern eggs.
They were proved correct. In the fossils they found biliverdin and protoporphyrin, which colour the eggs of birds such as emus, cassowaries and American robins today. “Non-avian dinosaurs and modern birds apparently use the same molecules to create eggshell coloration,” they wrote in a paper published in August in the journal PeerJ. They argued that this type of colouration may have first evolved in dinosaurs that laid their eggs in open nests.The colour may have helped camouflage them amid the vegetation of the habitats where the dinosaurs nested.
Living birds that lay pure-white eggs tend to be species that bury eggs in mounds, such as scrub turkeys and malleefowl, or those that sit on them continuously while brooding, and therefore have no need for camouflage. Oviraptorids such as Heyuannia laid their 20cm-long, capsule-shaped eggs around them in a circle in open nests of dirt on the ground, and may have benefited from them being less obvious to predators when fathers stepped away from the nests. As for many birds today, including cassowaries and emus, it was probably the males of dinosaurs such as oviraptors that brooded eggs and cared for the young.
Other dinosaurs, such as the long-necked giant sauropods, laid their round eggs in mounds that were buried under dirt and vegetation, so they may have been white in colour.
“We hypothesise egg coloration evolved after the switch from burying eggs to building an open and exposed nest,” wrote the authors of the study. “Selection for egg colour would only have occurred after the eggs themselves became visible.”The scientists point to Australia’s emus and cassowaries, which today lay their blue-green eggs in open nests on the ground where they blend in well with leaf litter and vegetation.
Colour is something that, until recently, experts thought we’d never understand about dinosaurs. But a series of studies since 2010 has made educated guesses about plumage colours of feathered dinosaurs (see AG 122).There is now every possibility more fossils will be found that begin telling us something also about the range of patterns and colours seen in dinosaur eggs.
This discovery about dinosaur eggs adds to the many features – such as beaks, feathers, paternal care and eggshell structure – that were once thought to be unique to birds, but now have been shown to be traits inherited from the dinosaurs.