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March 08, 2012

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Bird-like dino had glossy plumage for the ladies, research suggests

March 8, 2012
Courtesy of the University of Texas at Austin 
and World Science staff

Sci­en­tists have un­cov­ered what they say is the de­tailed feath­er pat­tern and col­or of Mi­cro­rap­tor, a pigeon-sized, four-winged di­no­saur that lived about 120 mil­lion years ago.

An an­ces­tral form of mod­ern birds, the an­i­mal boasted a glossy ir­i­des­cent sheen, like a crow, and a thin tail adorned with a pair of “stream­er” feath­ers, re­search­ers re­port in the March 9 is­sue of the jour­nal Sci­ence.

An artist's illustration of Micro­raptor plum­age. (Cour­tesy Ja­son Broug­ham/U. of Tex­as)
The in­ves­ti­ga­tors com­pared the pat­terns of pigment-containing or­ganelles, or cel­lu­lar com­part­ments, from a Mi­cro­rap­tor fos­sil with those in mod­ern birds. The find­ings mark the ear­li­est rec­ord of ir­i­des­cent col­or in feath­ers, they said. A new re­con­struc­tion of the di­no­saur is al­so ex­pected to help sci­en­tists ap­proach the con­tro­ver­sy of how di­no­saurs be­gan the tran­si­tion to flight.

Since it was discov­ered as the first four-winged di­no­saur in 2003, Mi­cro­rap­tor has fu­eled ques­tions about the ev­o­lu­tion of feath­ers and flight. Some sci­en­tists have pro­posed aer­o­dy­nam­ic func­tions for var­i­ous feath­ery fea­tures such as its tail, fore­wing shape and hind limbs, go­ing so far as to place Mi­cro­rap­tor mod­els in wind tun­nels and launch them from cat­a­pults. 

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 long feath­ers off of its tip, re­search­ers said. They be­lieve the tail feath­ering may have been or­na­men­tal rath­er than prac­ti­cal, and probably evolved for court­ship and oth­er so­cial in­ter­ac­tions.

“Most as­pects of early di­no­saur feath­ering con­tin­ue to be in­ter­preted as fun­da­men­tally aer­o­dy­namic, op­ti­mized for some as­pect of” fly­ing, said Jul­ia Clarke, one of the pa­per’s co-authors and a pa­le­on­tol­ogist at The Uni­vers­ity of Tex­as at 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 surely shaped by early lo­co­mo­tor [fly­ing] styles. 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.”

“Mod­ern birds use their feath­ers for many dif­fer­ent things, rang­ing from flight to [temp­erature] reg­u­la­t­ion to mate-attracting dis­plays,” said Matt Shaw­key, a co-author and bi­ol­o­gist at the Uni­vers­ity of Ak­ron. “Iri­des­cence is wide­spread in mod­ern birds and is fre­quently used in dis­plays. Our ev­i­dence that Mi­cro­rap­tor was largely ir­i­des­cent thus sug­gests that feath­ers were im­por­tant for dis­play even rel­a­tively early in their ev­o­lu­tion.”

The sci­en­tists de­duced Mi­cro­rap­tor was ir­i­des­cent when Shaw­key discov­ered that in the most com­mon ir­i­des­cent feath­ers, ar­rays of pigment-bearing or­ganelles called me­lan­o­somes were un­iquely nar­row.

In­forma­t­ion on feath­er col­or of a va­ri­e­ty of di­no­saurs has re­cently come to light, since the first col­or map of an ex­tinct di­no­saur showed black and white span­gles, red col­ora­t­ion and gray body col­or in a spe­cies called An­chior­nis in 2010. Based on the new da­ta from Mi­cro­rap­tor and these oth­er finds, the sci­en­tists con­tend a com­plex pal­ette of col­ors in­clud­ing ir­i­des­cence is likely an­ces­tral to a line­age of di­no­saurs called Par­aves that orig­i­nat­ed at least 140 mil­lion years ago and in­cludes di­no­saurs such as Ve­loci­rap­tor as well as Ar­chae­op­ter­yx, An­chior­nis and liv­ing birds.

“This study gives us an un­prec­e­dent­ed glimpse at what this an­i­mal looked like when it was alive,” said Mark Norell, co-author and chair of the Amer­i­can Mu­se­um of Nat­u­ral His­to­ry’s Di­vi­sion of Pa­le­on­tol­ogy. The re­search­ers worked with col­la­bo­ra­tors at the Bei­jing Mu­se­um of Nat­u­ral His­to­ry.

The feath­er col­or dis­played by many mod­ern birds is pro­duced par­tially by ar­rays of 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 always about the same for a giv­en col­or. Af­ter a break­through by Vin­ther in 2009, pa­le­on­tol­ogists have started an­a­lyz­ing the shape of me­lan­o­somes in well-pre­served fos­silized feath­er im­prints. By com­par­ing these pat­terns with those in liv­ing birds, sci­en­tists can in­fer the col­or of di­no­saurs that lived many mil­lions of years ago. Ir­i­des­cence arises when the nar­row me­lan­o­somes are or­gan­ized in stacked lay­ers.

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Scientists have uncovered what they say is the detailed feather pattern and color of Microraptor, a pigeon-sized, four-winged dinosaur that lived about 120 million years ago. An ancestral form of modern birds, the animal boasted a glossy iridescent sheen, like a crow, and a narrow tail adorned with a pair of “streamer” feathers, researchers report in the March 9 issue of the journal Science. The investigators compared the patterns of pigment-containing organelles, or cellular compartments, from a Microraptor fossil with those in modern birds. The findings mark the earliest record of iridescent color in feathers, they said. A new reconstruction of the dinosaur is also expected to help scientists approach the controversy of how dinosaurs began the transition to flight. Since it was discovered as the first four-winged dinosaur in 2003, Microraptor has fueled questions about the evolution of feathers and flight. Some scientists have proposed aerodynamic functions for various feathery features such as its tail, forewing shape and hind limbs, going so far as to place Microraptor models in wind tunnels and launch them from catapults. 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 long feathers off of its tip, researchers said. They believe the tail feathering may have been ornamental rather than practical, and probably evolved for courtship and other social interactions. “Most aspects of early dinosaur feathering continue to be interpreted as fundamentally aerodynamic, optimized for some aspect of” flying, said Julia Clarke, one of the paper’s co-authors and a paleontologist at The University of Texas at 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 surely shaped by early locomotor [flying] styles. 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.” “Modern birds use their feathers for many different things, ranging from flight to thermoregulation to mate-attracting displays,” said Matt Shawkey, a co-author and biologist at the University of Akron. “Iridescence is widespread in modern birds and is frequently used in displays. Our evidence that Microraptor was largely iridescent thus suggests that feathers were important for display even relatively early in their evolution.” The scientists deduced Microraptor was iridescent when Shawkey discovered that in the most common iridescent feathers, arrays of pigment-bearing organelles called melanosomes were uniquely narrow. Information on feather color of a variety of dinosaurs has recently come to light, since the first color map of an extinct dinosaur showed black and white spangles, red coloration and gray body color in a species called Anchiornis in 2010. Based on the new data from Microraptor and these other finds, the scientists contend a complex palette of colors including iridescence is likely ancestral to a lineage of dinosaurs called Paraves that originated at least 140 million years ago and includes dinosaurs such as Velociraptor as well as Archaeopteryx, Anchiornis and living birds. “This study gives us an unprecedented glimpse at what this animal looked like when it was alive,” said Mark Norell, co-author and chair of the American Museum of Natural History’s Division of Paleontology. The researchers studied feathering, melanosome shape and density from a Microraptor fossil working with collaborators at the Beijing Museum of Natural History. The feather color displayed by many modern birds is produced partially by arrays of 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. After a breakthrough by Vinther in 2009, paleontologists have started analyzing the shape of melanosomes in well-preserved fossilized feather imprints. By comparing these patterns with those in living birds, scientists can infer the color of dinosaurs that lived many millions of years ago. Iridescence arises when the narrow melanosomes are organized in stacked layers.