Thursday, 28 May 2015

Reconstructing The Colour Of Dinosaur Eggs

Original colour is rarely preserved in the fossil record.
Madagascan ammonites are a notable exception
The fossil record is by no means limited in its palette. Remains are preserved in shades of black, ochre, white, grey, cream; some specimens contain vibrant greens, blues, purples and reds.

As colourful as fossils can be, they are not an accurate reflection of the creatures in life. Only the most recent sub-fossil material, such as mammoth hair from the Siberian permafrost retains its original colouration. Mineralised fossils on the other hand are stripped of this. The notable exception are certain ammonites where the pearlescent nacreous layer of the shell is preserved.

Sophisticated analysis techniques has allowed us to reconstruct the colour scheme of extinct organisms by examining microscopic, cellular structures which would have contained pigments. In rare circumstances, the original pigments themselves may be preserved. This form of chemical preservation has allowed us to say what colour dinosaur eggs were, or at least those belonging to certain species of oviraptor.

Dinosaurs were of course reptiles and so the best way of determining what colour dinosaurs eggs might have been is to examine those of modern reptiles. Some reptile eggs are marbled, speckled or even brightly coloured, but most are pale and leathery. This would suggest that dinosaur eggs were similarly pale. This new study, penned by researchers from Bonn University in Germany, has shown otherwise in the case of oviraptors. The most primitive modern bird species, the tinamou and the emu, which are most closely related to the earliest birds and in turn their dinosaur ancestors, lay colourful eggs.

Robin eggs are a vibrant blue
Animal behaviourist Mark Hauber from Hunter College in New York conducted chemical studies of birds eggs and found that the avocado-green from emus' eggs, the blue of robins' eggs, the brown of chickens' eggs and the pinks and purples from birds belonging to ancient living groups came from the way that two pigments in the shell, biliverdin and protoporphyrin, blend with each other and with the calcium carbonate that make up most of the shell.

Dinosaur egg fossils are typically shades of brown or black on account of minerals seeping into the shell during the fossilisation process. The research team, led by Martin Sander, examined oviraptor egg fossils which were abnormally pale and came from several ancient nesting sites across China. Sander's student, Jasmina Wiemann, found the oviraptor eggs contained both biliverdin and protoporphyrin. Most protoporphyrin came from the protein layer or cuticle still coating the fossil egg, as it does in modern bird eggs. Biliverdin came mostly from the calcium carbonate, also as in modern birds. Collectively the pigment evidence suggests oviraptors had blueish-green eggs.

Oviraptor eggs used in the study. These used to be blue-green
Colouration in eggs is typically to make them less visible to predators. While this may seem counter-intuitive, white eggs are actually easier to spot than coloured eggs. Colouration in oviraptor eggs fits with what we know about their nesting habits, namely laying eggs in open ground level nests. A blue green colour would help mask the eggs from predators.

There is a slight irony in this considering that the oviraptors were egg thieves. Additionally colouration would aid in parents recognising their own brood amongst several in communal nesting sites or in removing foreign eggs, as in the case of cuckoos and cowbirds today. The proteins themselves give the shell added structural strength.

Colouration in oviraptors does not mean that all dinosaur eggs were coloured. The giant sauropods are thought to have buried their eggs and would have no need for coloured shells. Colouration may have been found in the theropods, ancestral to the birds. The maternal advantage of colouration may have resulted in its evolution in other dinosaurs: Maiasaura for example whose name hints at its mothering tendencies and who laid eggs in large, communal nesting sites. From Jurassic Park to Walking with Dinosaurs, the eggs of these extinct reptiles are portrayed as pale and uninteresting. Now it's time to add a splash of colour.