Birds lay eggs, reptiles lay eggs, even some mammals lay eggs. But among all the amniotes, only birds lay eggs with colors – some quite eye-popping. Everyone else lays white eggs.
We have known for years that some dinosaurs had colorful eggs too. The birdlike Heyuannia huangi, a stunted-winged beaked dinosaur found in China, laid blue-green eggs that look startlingly like those of the latter-day emu.
So, scientists had already figured that color in eggshells originated in dinosaurs. Now a team writing in Nature explain how new findings demonstrate that pigmented eggshells evolved exactly once – among maniraptors, the type of dinosaur that would evolve into birds.
A broad study of dinosaur eggshells from a range of types show that the eggs of non-maniraptor dinosaurs were unpigmented. In other words, white.
What needed demonstrating was that birds inherited the ability to produce red, green, brown and blue eggshell pigments from their dinosaur ancestors, rather than evolving it separately by convergent evolution.
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Convergent evolution is when totally different beings separately evolve similar solutions. For instance, birds have wings and cockroaches have wings, and both can fly with their wings, but they do not have the same evolutionary origin.
How can colors of extinct animals – feathers, skin or eggshells – be deduced anyway? Key to this theory was the eureka moment in which scientists realized what they were seeing under the microscope wasn’t fossilized bacteria but fossilized melanosomes.
Melanosomes are subcellular organelles that contain melanin. Different colors of melanin give different colors to hair, feathers, skin and eyes. Crucially, different types of melanin are found in organelles of different shapes.
For instance, melanosomes with the color red are shaped like little meatballs, while black melanosomes look like cocktail sausages, explains a 2015 paper on 50-million-year-old fossil bats, which turn out to have been reddish-brown.
Regarding feathered dinosaurs, a turning point was the study of a spectacularly preserved banded 108-million-year-old feather found in Brazil. Microscopic examination found that its black stripes contained sausage-like melanosomes, while the white stripes had no melanosomic structures.
This latest ovi-research group, headed by Jasmina Wiemann of Yale, used advanced spectroscopic analysis to study dinosaur eggshells from all major groups, seeking evidence of pigmentation. Pictures of garishly striped velociraptors are still more imagination than reliable fact, but this team definitely did show that bird eggs and maniraptor eggs both have red-brown protoporphyrin IX and blue-green biliverdin.
Maniraptors are the group of small, bipedal dinosaurs, some of which generated the birds we love to watch and eat. Not only did the maniraptors have colored eggs: the patterns on their shells are akin to the speckling and spotting patterns we can see in today’s bird eggs.
The scientists also demonstrated that ornithischian and sauropod eggs were colorless (i.e., white), and demonstrated that this was a true signal rather than an artifact of their fossilization. Famous examples of ornithischians are the triceratops and stegosaurus. An example of sauropods is the titanic long-necked brontosaurus and their ilk.
It was not only the ancestors of birds among the maniraptors that had colored eggs. So did some non-avian maniraptors, and their color pattern diversity is like that of today’s birds.
In other words, colored eggs evolved “deep within the dinosaur tree,” as the team put it. It happened long before birds evolved. Separate research also suggested that egg color evolved with open nesting habits in dinosaurs predating the split to birds.
Among the dinosaurs found to have colored eggs were the Heyuannia, whose eggs – founds in the thousands – were blue-green; troodontids, small dinosaurs that looked like mean birds and had serrated teeth in their beaks; the dromaeosaurid Deinonychus antirrhopus, which might have been amusing if not for those talons that look like they could eviscerate a crocodile; the wee enantiornithine, very birdy though with a toothed beak too, and many more.
One wonders why the maniraptors would evolve colored eggs. We do not know but can factor in the observation that bird-egg color has been demonstrated to adapt to changes in incubation strategy or climate, or even in mating behavior.
This all leads to speculation that egg color evolved among animals that didn’t bury or somehow conceal their eggs, but laid them and left them exposed. White eggs stand out and are more likely to be eaten, but speckled eggs can be camouflaged.