Why were dinosaurs covered in a cloak such a long time before the early bird species Archaeopteryx first took flight? Researchers from the University of Bonn and the University of Göttingen attempt to answer that exact question in their article “Beyond the Rainbow” in the latest issue of the renowned journal Science. The research team hypothesises that these ancient lizards had a highly developed ability to discern colour. Their hypothesis: The evolution of feathers made dinosaurs more colourful, which in turn had a profoundly positive impact on communication, the selection of mates and on dinosaurs’ procreation.
The idea that birds and dinosaurs are close relatives dates back to the 19th century, when the father of evolutionary theory, Charles Darwin, was hard at work. However, it took more than 130 years for the first real proof to come to light with numerous discoveries of the remains of feathered dinosaurs, primarily in fossil sites in China. Thanks to these fossil discoveries, we now know that birds descend from a branch of medium-sized predatory dinosaurs, the so-called theropods. Tyrannosaurus rex and also velociraptors, made famous by the film Jurassic Park, are representative of these two-legged meat lovers. Just like later birds, these predatory dinosaurs had feathers – long before Archaeopteryx managed to spread its wings and gain flight. But why was this, particularly when dinosaurs lacked the ability to fly?
Dinosaur’s colour vision
“Up until now, the evolution of feathers was mainly considered to be an adaptation related to flight or to warm-bloodedness, seasoned with a few speculations about display capabilities” says the article’s first author, Marie-Claire Koschowitz of the Steinmann Institute of Geology, Mineralogy and Palaeontology at the University of Bonn. “I was never really convinced by any of these theories. There has to be some particularly important feature attached to feathers that makes them so unique and caused them to spread so rapidly amongst the ancestors of the birds we know today”, explains Koschowitz. She now implies that this feature is found in dinosaur’s colour vision. After analysing dinosaurs’ genetic relationships to reptiles and birds, the researcher determined that dinosaurs not only possessed the three colour receptors for red, green and blue that the human eyes possess, but that they, like their closest living relatives, crocodiles and birds, were possibly also able to see extremely short-wave and ultraviolet light through an additional receptor. “Based on the phylogenetic relationships and the presence of tetrachromacy in recent tetrapods it is most likely that the stem species-of all terrestrial vertebrates had photo receptors to detect blue, green, red and UV”, says Dr, Christian Fischer of the University of Göttingen.
This creates a much more colourful world for most animals than it is for humans and other mammals. In general, mammals have rather poor colour vision or even no colour vision at all because they tended to be nocturnal during the early stages of their evolution. To contrast, various studies on the social behaviour and choice of mates among reptiles and birds, which are active during the day, have demonstrated that information transmitted via colour exerts an enormous influence on those animal’s ability to communicate and procreate successfully.
Feathers allowed for more visible signals that fur
From dinosaur fossil finds we know that the precursors to feathers resembled hairs similar to mammal’s fur. Their primary function was to protect the smaller predatory dinosaurs – which would eventually give rise to birds – from losing too much body heat. The problem with these hair-like forerunners of feathers and with fur is that neither allow for much colour, but tend instead to come in basic patterns of brown and yellow tones as well as in black and white. Large flat feathers solved this issue by providing for the display of colour and heat insulation at the same time. Their broad surface area, created by interlocking strands of keratin, allows for the constant refraction of light, which consequently produced what is referred to as structural colouration. This refraction of light is necessary to produce colours like blue and green, the effect of metallic-like shimmering or even colours in the UV spectrum. “Feathers enable a much more noticeable optical signalling than fur would allow. Iridescent birds of paradise and hummingbirds are just two among a wealth of examples,” explains Koschowitz.
This work means we must see the evolution of feathers in a completely new way. They provided for a nearly infinite variety of colours and patterns while simultaneously providing heat insulation. Prof. Dr. Martin Sander of the University of Bonn’s Steinmann Institute summarises the implications of this development: “This allowed dinosaurs not only to show off their colourful feathery attire, but to be warm-blooded animals at the same time – something mammals never managed.”
Contributing Source: Universität Bonn
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