Wednesday, 31 October 2018

Dinosaur Eggs Confirm Bird Evolution

A selection of bird eggs and a fossil theropod egg (on the right).
Image credit: Jasmina Wiemann/Yale University
Dinosaurs put all colored bird eggs in one basket, evolutionarily speaking | YaleNews

More evidence today, if any more were needed, of the dinosarian origin of modern birds. This time it is evidence that the pigments birds use to colour their eggs evolved in dinosaurs who needed to camouflage their eggs in open or partially open nests on the ground.

The study also changes our thinking on how and when bird egg colour evolved.

It had been believed that birds evolved egg colours multiple times according to environmental pressures, however, all bird use the same two pigments, red and blue, in various combinations and patterns to achieve the different markings. This study challenges that view by showing that dinosaurs also had pigmented shells.

We infer that egg color co-evolved with open nesting habits in dinosaurs. Once dinosaurs started to build open nests, exposure of the eggs to visually hunting predators and even nesting parasites favored the evolution of camouflaging egg colors, and individually recognizable patterns of spots and speckles.

Jasmina Wiemann, lead author
Palaeontologist, Yale
This was shown by a team of researchers from Yale, the American Museum of Natural History, and the University of Bonn, led by Yale paleontologist Jasmina Wiemann. Their findings are published today in Nature.

Weimann and her colleagues arrived at this conclusion after examining 18 fossils eggshells from around the world, using non-destructive laser microspectroscopy to test for the presence of the two eggshell pigments. They found them in small carnivorous Eumaniraptoran dinosaurs, a group which include Velociraptor - probable ancestors of birds.

Birds are the only living amniotes with coloured eggs1,2,3,4, which have long been considered to be an avian innovation1,3. A recent study has demonstrated the presence of both red-brown protoporphyrin IX and blue-green biliverdin5—the pigments responsible for all the variation in avian egg colour—in fossilized eggshell of a nonavian dinosaur6. This raises the fundamental question of whether modern birds inherited egg colour from their nonavian dinosaur ancestors, or whether egg colour evolved independently multiple times. Here we present a phylogenetic assessment of egg colour in nonavian dinosaurs. We applied high-resolution Raman microspectroscopy to eggshells that represent all of the major clades of dinosaurs, and found that egg colour pigments were preserved in all eumaniraptorans: egg colour had a single evolutionary origin in nonavian theropod dinosaurs. The absence of colour in ornithischian and sauropod eggs represents a true signal rather than a taphonomic artefact. Pigment surface maps revealed that nonavian eumaniraptoran eggs were spotted and speckled, and colour pattern diversity in these eggs approaches that in extant birds, which indicates that reproductive behaviours in nonavian dinosaurs were far more complex than previously known3. Depth profiles demonstrated identical mechanisms of pigment deposition in nonavian and avian dinosaur eggs. Birds were not the first amniotes to produce coloured eggs: as with many other characteristics7,8 this is an attribute that evolved deep within the dinosaur tree and long before the spectacular radiation of modern birds.

Colored eggs have been considered a unique bird characteristic for over a century. Like feathers and wishbones, we now know that egg color evolved in their dinosaur predecessors long before birds appeared.

Mark Norell, co-author
Macaulay Curator of Paleontology
American Museum of Natural History
Little bits of evidence such as this don't in themselves confirm a common origin of modern birds in a group of dinosaurs, but the evidence is now so compelling that this is merely filling in the gaps, and improving our understanding of the origins of relatively minor details such as the two pigments common to all bird eggshells.

Again, there is also the entirely incidental and unintentional confirmation of evolution as the only rational explanation for what we see today. No biologists these days sets out to prove the theory of evolution; they do it quite incidentally because the theory correctly describes the reality of how things came to be as they are.

Just as no geologist would pick up a rock fallen from a cliff and rush off to write a paper on how this confirms the theory of gravity so no biologist writes a paper on how a discovery such as this one confirms the theory of evolution. They simply report the facts and the facts confirm evolution because evolution is factual.

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