恐龙在鸟类飞行进化史中的作用

A new study looking at the structure of feathers in bird-like dinosaurs² has shed light on one of nature's most remarkable³ inventions -- how flight might have evolved. Academics at the Universities of Bristol, Yale and Calgary have shown that prehistoric⁴ birds had a much more primitive⁵ version of the wings we see today, with rigid⁶ layers of feathers acting⁷ as simple airfoils（机翼） for gliding⁸.   Close examination of the earliest theropod（兽脚亚目的） dinosaurs suggests that feathers were initially⁹ developed for insulation¹⁰, arranged in multiple layers to preserve heat, before their shape evolved for display and camouflage¹¹（伪装）.   As evolution changed the configuration¹² of the feathers, their important role in the aerodynamics and mechanics of flight became more apparent. Natural selection over millions of years ultimately modified dinosaurs' forelimbs into highly-efficient, feathered wings that could rapidly change its span, shape and area -- a key innovation that allowed dinosaurs to rule the skies.   This basic wing configuration has remained more or less the same for the past 130 million years, with bird wings having a layer of long, asymmetrical¹³ flight feathers with short covert¹⁴ feathers on top. They are able to separate and rotate these flight feathers to gain height, change direction and even hover¹⁵.   This formation allows birds to move in such a way as to produce both lift and thrust simultaneously¹⁶ -- a capability¹⁷ that man, with the help of technology, is still trying to successfully imitate.   The research, published November 21 in Current Biology, looked at the dinosaur¹ Anchiornis huxleyi and the Jurassic bird Archaeopteryx lithographica. The latter is 155 million years old and widely considered to be the earliest known bird, presenting a combination of dinosaur and bird characteristics.   Their wings differed from modern day birds in being composed of multiple layers of long feathers, appearing to represent early experiments in the evolution of the wing. Although individual feathers were relatively¹⁸ weak due to slender feather shafts¹⁹, the layering of these wing feathers is likely to have produced a strong airfoil.   The inability to separate feathers suggests that taking off and flying at low speeds may have been limited, meaning that wings were primarily used in high-speed gliding or flapping flight.   Dr Jakob Vinther, from the University of Bristol's Schools of Biological and Earth Sciences, said: "We are starting to get an intricate picture of how feathers and birds evolved from within the dinosaurs. We now seem to see that feathers evolved initially for insulation. Later in evolution, more complex vaned or pinnate（羽状的） feathers evolved for display.   "These display feathers turned out to be excellent membranes#p#分页标题#e#²⁰ that could have been utilised for aerial locomotion²¹, which only very late in bird evolution became what we consider flapping flight. This new research is shedding light not just on how birds came to fly, but more specifically on how feathers came to be the way they are today -- one of the most amazing and highly specialised structures in nature."   Dr Nicholas Longrich of Yale University added: "By studying fossils carefully, we are now able to start piecing together how the wing evolved. Before, it seemed that we had more or less modern wings from the Jurassic onwards. Now it's clear that early birds were more primitive and represented transitional forms linking birds to dinosaurs. We can see the wing slowly becoming more advanced as we move from Anchiornis, to Archaeopteryx, to later birds."