The detailed feather pattern and color of Microraptor - a pigeon-sized, four-winged dinosaur that lived about 120 million years ago - once boasted a glossy iridescent sheen.
According to scientists, the Microraptor's tail was narrow and adorned with a pair of streamer feathers, which suggests the importance of display in the early evolution of feathers.
By comparing the patterns of pigment-containing organelles from a Microraptor fossil to those in modern birds, researchers have determined the dinosaur's plumage was actually iridescent with a glossy sheen like the feathers of a modern crow.
An ongoing reconstruction of Microraptor is expected to help scientists approach the question of how dinosaurs began the transition to flight.
"Specifying the color and iridescence of feathers in avian dinosaurs was not possible 20 years ago," explained H. Richard Lane, program director in the National Science Foundation's (NSF) Division of Earth Sciences, which funded the research. "This development, in combination with the arrangement of tail feathers, is leading to a deeper understanding of the early development of avian plumage signaling."
Since it was discovered as the first four-winged dinosaur in 2003, Microraptor has been the focus of questions about the evolution of feathers and flight. Indeed, scientists have proposed aerodynamic functions for various feathery features such as its tail, forewing shape and hind limbs.
Once thought to be a broad, teardrop-shaped surface, or with a shape more like that of a paper airplane meant to help generate lift, Microraptor's tail fan is actually much narrower with two elongate feathers off its tip. Researchers hypothesize the tail feathering may have been ornamental, and likely evolved for courtship and other social interactions, rather than as an adaptation for flight.
"Most aspects of early dinosaur feathering continue to be interpreted as fundamentally aerodynamic, optimized for some aspect of aerial locomotion," said Julia Clarke, a paleontologist at The University of Texas at Austin (UT-Austin).
"Some of these structures were clearly ancestral characteristics that arose for other functions and stuck around, while others may be linked to display behaviors or signaling of mate quality."
Feather features were shaped by early locomotor styles, Clarke believes. "But, as any birder will tell you, feather colors and shapes may also be tied with complex behavioral repertoires and, if anything, may be costly in terms of aerodynamics."
As Matt Shawkey, a biologist at the University of Akron notes, modern birds use feathers for many different functions including flight, thermoregulation and mating.
"Iridescence is widespread in modern birds, and is frequently used in displays... The evidence that Microraptor was largely iridescent suggests that feathers were important for display even relatively early in their evolution."
The feather color displayed by many modern birds is partially produced by arrays of pigment-bearing organelles called melanosomes, about a hundred of which can fit across a human hair. Generally found in a round or cigar-like shape, a melanosome's structure is constant for a given color. Iridescence arises when narrow melanosomes are organized in stacked layers.
After a breakthrough by Jakob Vinther of UT-Austin in 2009, paleontologists started analyzing the shape of melanosomes in well-preserved fossilized feather imprints. By comparing these patterns to those in living birds, scientists can infer the color of dinosaurs that lived many millions of years ago. To be sure, paleontologists concluded Microraptor was iridescent when Shawkey discovered that melanosomes in the most common iridescent feathers were uniquely narrow.
Interestingly enough, additional information on the feather color of a variety of dinosaurs has recently been determined. For example, the first color map of an extinct dinosaur showed black-and-white spangles, red coloration and grey body color in a species called Anchiornis. Based on the new data from Microraptor and other findings, a complex color repertoire that includes iridescence is likely ancestral to a group of dinosaurs called Paraves that originated at least 140 million years ago. It includes dinosaurs like Velociraptor as well as Archaeopteryx, Anchiornis and living birds.
"This study gives us an unprecedented glimpse of what this animal [Microraptor] looked like when it was alive," added Mark Norell, a paleontologist at the American Museum of Natural History (AMNH).