A wonderfully preserved Jurassic relative of both birds and T. rex shows that rainbow-colored feathers were fashionable in ancient times, too
P Rajendran
It was long known that feathers sprouted on some dinosaurs of the group Theropoda – remember T. rex? – some members of which were also ancestral to modern birds. Now there is mounting evidence that some of these dino feathers came in iridescent color – rippling with apparently shifting rainbow hues with each movement.
Chinese researchers found a complete skeletal fossil of what they call Caihong juji, a theropod that, among its unique features, exhibits melanosomes – tiny structures responsible for color in cells in the iridescent feathers of modern birds. Fittingly, perhaps, Caihong means rainbow in Mandarin; ju means big; ji means crest (a reference to the therapod’s huge one). With its birdlike features, it bears only a passing resemblance to its more famous cousin, Tyrannosaurus rex.
Discovered by a farmer, the fossil was studied by scientists led by Dongyu Hu, a professor in the College of Paleontology at the Shenyang Normal University in China, and including a team of other Chinese, Belgian and American researchers.
With feathers like a bird though they are oddly asymmetrical in the tail, and a crested skull, the fossil shows its links with both old dinosaurs and birds.
Caihong belongs to the group Paraves, some other members of which do include modern birds. Like other theropods, Caihong got around on two feet, each boasting three toes; and had winged, feathered forelimbs. It is part of the Yanliao Biota, a collection of fossils that keeps on giving – plants, insects, and a rich variety of non-marine creatures – found in and around Gangou, Qinglong, in China’s Hebei Province, about 200 miles east of Beijing.
A fossil forms
The Caihong that is causing all this stir now died about 161 million years ago in the late Jurassic, and was preserved in hardened volcanic ash in lake deposits. The sediments pressed down and turned slowly into rock, while the organic material slowly broke down, leaving a well-defined fossil.
Among one of Caihong’s most interesting features were some microscopic structures that resembled melanosomes, cell structures that produce color in modern birds. In fact, the sausage-shaped structures found in Caihong are remarkably similar to those seen in modern hummingbirds with iridescent feathers.
Julia Clarke, a paleontologist at the University of Texas, Austin, who collaborated with the Chinese and Belgian groups, pointed out that known living birds create iridescence in different ways.
“Melanosomes has different shapes uniquely associated with different forms of melanin,” she said, pointing out to variations – from the shiny green and orangey colors seen in mallards, associated with long and skinny melanosomes, to the pancake-shaped ones in hummingbirds that produce brighter colors.
Melanosomes are found in specialized cells called melanocytes and in certain eye cells and produce, store and cart around the pigment called melanin around the cell. Around 1/50,000th of an inch in diameter, besides providing color and camouflage melanosomes also protect the skin from sunlight. Though they start as mere hollow bodies with no pigment they soon develop striations and get densely packed with melanin (though some melanosomes seen in iridescent feathers can be hollow).
There has been some uncertainty whether what researches have been claiming are melanosomes in fossils are really just bacteria immortalized in stone.
A few competing ideas
Michael Wuttke, of the Johannes Gutenberg University, Mainz, was the first to come up with the idea that these structures could be bacteria.
But even the earliest study on the presence of iridescence in fossils, by Jakob Vinther and colleagues, in 2008, found actual squid ink in fossils, granules of which were hard to distinguish from melanosomes seen in modern squid. They found that structures resembling melanosomes in fossils were limited to just those parts of similar living creatures that exhibited color – feathers, crests, etc. Also, sausage-shaped melanosomes in bird feathers often line up along the branches and sub-branches of the main feather – and the structures look remarkably similar in fossil feathers, too.
Clarke pointed out that in all the images in the scanning elecron microscope no structure showed a hint of mitotic division, a regular activity for bacteria. Also, unlike in bacteria, none of the structures observed in Caihong were hollow.
So, despite some lingering speculation, it appears likely that the structures seen in the Caihong fossil feathers are indeed melanosomes, and not bacterial doppelgängers.
While iridescence could always accidentally pop up in evolution, there is no reason for the trait to persist unless it supports or encourages the survival of the species.
A hue advantage
According to Clarke, iridescent colors could make dinosaurs – and some of their descendants, the birds – be more attractive to potential mates.
“Reptiles are generally diurnal. We were nocturnal. That’s why our eyes suck. The avian eye was put together sometime within Dinosauria. It’s a safe bet that these animals have good vision,” she said.
She pointed out that even a lizard brain has pretty good color vision.
Clarke cautions that because the first places branched feathers are found are on the tips of the arms and the tail, “it could be affecting locomotor performance and perhaps signaling strategies. It may not be an either/or (situation). It could be an interplay between sexual and … locomotor performance.”
“It is quite different from other paravians,” Clarke said. “This dinosaur shows ornament-type bony eyebrows.”
Going by variations seen in the melanosome-like structures, the team concluded that most of Caihong’s plumage was black, and that the head, chest and base of the tail could have been iridescent. But the team could not determine the actual play of colors in the iridescent areas, given that the structures would be positioned differently in a live animal. Also, there is no way of determining the effect of other pigments that no longer linger on in the fossil.
Whatever the reason, Caihong and other iridescent theropods have yet again changed human understanding of dino evolution. Let us hope that future editions of “Jurassic Park,” if the makers stick to the evidence, are far more colorful.
The original research appeared in Nature Communications
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