They are speckled in blue, whitish or brown – there is a well-known variety of birds’ eggs. Was that the case with the eggs of the dinosaurs? A Analysis of fossil dinosaur eggs shows that a large group of primeval lizards already had egg shells with the same two pigments that also color today’s bird eggs. Their scrims already had a variety of colors similar to that of their modern descendants. According to the researchers, this proves that the dinosaurs camouflaged their eggs with colored shells – and that the birds inherited this ability from their ancestors.
Ornithologists and naturalists have been puzzling for centuries why the eggs of some bird species are conspicuously colorfully speckled, while others are rather plain and white or brown. It seems clear, however, that the coloring at least partially reflects the way of life and reproductive habits of the birds. Soil breeders such as the oystercatcher often have eggs in brownish camouflage colors so that their nest on the ground is more difficult to discover. By contrast, many birds of prey or species of bird breeding in tree hollows have plain white or strikingly greenish or bluish-colored eggs – possibly because their nests are difficult to access and well hidden anyway.
Pigments search for fossil dinosaur eggs
Because of this diversity and the close connection to the way of life, biologists have long assumed that the birds have developed the color of their eggs independently several times. Many dinosaurs also laid eggs, but for a long time there was no evidence that their shells were also colored. “The enormous variety of bird egg colors was attributed to the conquest of new ecological niches by the birds after the fall of the dinosaurs,” explain Jasmina Wiemann from Yale University and her colleagues. However, all bird eggs have one thing in common: their color is based on the different combination of only two pigments – the red-brown protoporphyrin IX and the blue-green biliverdin.
As part of their study, Wiemann and her colleagues specifically searched for traces of these pigments in fossil dinosaur eggs. To this end, they analyzed shell samples from 18 dinosaur species from different large groups of the dino kingdom – from herbivorous sauropods such as the titanium dinosaur to the predatory deinonychus running on the hind legs to early primeval birds. Using Raman spectroscopy, they were able to determine non-destructively whether their shells contained the two pigments protoporphyrin IX and biliverdin.
Colorful lizard eggs
It turned out that the eggs of the sauropods and many other early dinosaur groups are actually simply white and pigment-free. But the group of the Maniraptora, which is more closely related to the birds, apparently already had very similarly colored eggs to many of today’s birds: some were more brownish due to Protoporphyrin IX, others greenish due to Biliverdin, but most of the others both had pigments – they were speckled. The type of pigment deposition in the shell structure also matched that of today’s bird eggs. “This completely changed our ideas about how the egg colors developed,” says Wiemann. “For two centuries ornithologists have assumed that the shell color developed several times independently in modern birds. But we were able to show that the color of the skin has only an evolutionary origin in theropod dinosaurs. ”
Also surprising: the variety of shell colors for dinosaur eggs was apparently as large as for their modern descendants. “This indicates that the reproductive behavior of the non-avian dinosaurs was complex,” say the researchers. They suspect that the first colored eggs were developed by dinosaurs who laid their eggs in large nests on the floor. “Once the dinosaurs started using such open nests, they were exposed to the eyes of predators who hunted visually and also nest parasites,” explains Wiemann. “This favored the evolution of camouflage shell colors and also individually recognizable stains and patterns.”
The scientists have thus identified another legacy of the dinosaurs at Meise, Spatz and Co. “Colored eggs have long been a unique feature of birds. But apparently the egg colors developed much like the feathers and the wishbone long before they appeared in their dinosaur ancestors, ”says co-author Mark Norell from the American Museum of Natural History.
Source: Jasmina Wiemann (Yale University, New Haven) et al., Nature, doi: 10.1038 / s41586-018-0646-5