Coloured eggshells: Raman traces dinosaur egg pigmentation

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  • Published: Nov 1, 2018
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Coloured eggshells: Raman traces dinosaur egg pigmentation

Colour coded

Egg colour reconstruction. Eggshell-pigment surface maps. Wiemann et al/Nature

Raman spectroscopy has been used to demonstrated that adding pigment to eggs evolved only once and in the dinosaurs and the process is the same as used by the many thousands of bird species we see around the world today.

Writing in the journal Nature, Jasmina Wiemann of the Department of Geology & Geophysics, at Yale University, in New Haven, Connecticut, USA, Tzu-Ruei Yang of the Steinmann Institute for Geology, Mineralogy, and Paleontology, at the University of Bonn, Bonn, in Germany, and Mark A. Norell of the Division of Vertebrate Paleontology, at the American Museum of Natural History, in New York, USA, explain their findings in detail.

Pigment problem

The team explains how there are many thousands of species of extant amniotes, which includes the birds, reptiles, and egg-laying mammals. They all lay eggs, by definition. However, birds are the only members of that umbrella grouping that lay coloured eggs. Recently, Wiemann and her colleagues reported how they had identified the same red-brown and blue-green pigments (the tetrapyrroles biliverdin and protoporphyrin IX) that some birds use to colour and camouflage their eggs in certain fossil oviraptor dinosaur eggshells. One question that immediately arose from that finding, published in 2017, was whether or not the birds had inherited these egg pigments from their theropod dinosaur ancestors or whether egg pigmentation was a lesson in convergent evolution and a biochemical trick that had evolved separately and for a second time in the birds. Of course, the simplest and seemingly obvious answer is that no it had not evolved again; the birds had simply carried the skill with them from their dinosaur forebears. Now, the team has found the evidence to support this latter hypothesis using high-resolution Raman microspectroscopy. The team points out that Raman can distinguish between true egg-colour pigments and pigment-like compounds known as protein fossilization products (PFPs).

Wiemann and her colleagues analysed a set of fossil eggshells, which included representatives of all major dinosaur groups, to look for evidence of pigmentation. They found traces of pigments preserved in the eggshells of all Maniraptora. This was a group of small, bipedal, and often feathered dinosaurs. They used the Raman data to map out and reveal spotted and speckled patterns. The team suggests that the pigments were deposited in the eggshell in much the same way as the colourants are embedded in the eggshell of a modern bird.

Egg strategy

In stark contrast to coloured bird and dinosaur eggs, the eggs of the ornithischian and sauropod dinosaurs, much more distant relatives of birds, which included the likes of Triceratops and Diplodocus, had pale, essentially colourless eggshells. Pigment-free from the beginning rather than having lost their colouration as the eggshells fossilized. Taken together, these two findings suggest that pigmented eggs evolved only once, in the bird-like theropod dinosaurs. The same pigments that perhaps helped Tyrannosaurus rex hide her eggs among the prehistoric vegetation have been carried for to this day and help our feathered friends in much the same way and to allow for recognition of the bird's own eggs in communal nesting sites. The implications of this latter point being that dinosaurs too may have had more complicated laying strategies than previously supposed.

Related Links

Nature 2018, online: "Dinosaur egg colour had a single evolutionary origin"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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