ScienceShot: When Did Feathered Dinosaurs Become Birds?
Detailed analyses of Chinese fossils may have isolated the point on the reptile family tree where feathered dinosaurs began evolving into warm-blooded birds. In a new study, researchers looked at tiny pigment bodies known as melanosomes in the skin, hair, and feathers of 181 modern species (including mammals, birds, and several types of reptiles) and in the fossils of 13 species that lived in what is now China between 120 million and 160 million years ago (including lizards, turtles, dinosaurs, and winged reptiles known as pterosaurs). In most groups of reptiles, melanosomes typically had a slightly ovoid shape (as shown in upper right, a close view of melanosomes from a fossil of the dinosaur Beipiaosaurus [upper left]; black blob depicts average melanosome shape, and scale bar represents 1 micrometer). Dinosaurs that sported simple filaments known as “dinofuzz” also had ovoid melanosomes, the team found. But in a group called maniraptoran dinosaurs (which sported modern-style feathers like today’s birds), melanosomes often had a wide variety of shapes, including the vastly elongated structures seen at lower right (from the fossils of a yet-to-be-described bird [lower left]), the researchers report today in Nature. Implications of the finding aren’t yet clear, the researchers say. However, because a wide variety of melanosome shapes is seen today only in warm-blooded creatures such as birds and mammals, the burst of melanosome diversity among early maniraptoran dinosaurs may chronicle the point at which several critical biological characteristics in the lineage fundamentally changed, including the shift toward warm-bloodedness. That boost in metabolism, plus associated changes in reproduction and feeding habits, may have set the stage for avian success.