Twoopposing scenarios, the “arboreal” hypothesis and the “cursorial” hypothesis, have traditionally been put forward concerning the originsof bird flight. The“arboreal” hypothesis holds that bird ancestors began to flyby climbing treesand gliding down from branches withthe help of incipient feathers: the height oftrees provides a good starting place for launching flight, especially through gliding. As feathers became larger over time,flapping flight evolvedand birds finally became fullyair-borne. This hypothesis makes intuitive sense, butcertain aspects are troubling. Archaeopteryx (the earliest known bird) and its maniraptoran dinosaur cousins have no obviously arborealadaptations, suchas feet fully adapted for perching.Perhaps some of them could climb trees, but noconvincing analysis has demonstrated how Archaeopteryx would have both climbed and flown with its forelimbs, and there were noplants taller than afew meters in the environments whereArchaeopteryx fossils have been found. Even ifthe animals could climb trees, this ability is not synonymous with gliding ability. (Many small animals, and even some goats andkangaroos, are capable ofclimbing trees but are not gliders.)Besides, Archaeopteryx shows no obvious featuresof gliders, such as a broad membrane connecting forelimbs and hindlimbs.
The“cursorial” (running) hypothesis holds that small dinosaurs ran alongthe groundand stretched out their arms forbalance as they leaped into the air after insectprey or, perhaps, to avoid predators. Even rudimentary feathers on forelimbs could have expanded the arm’s surface area toenhance lift slightly.Larger feathers could have increasedlift incrementally, until sustained flight wasgradually achieved. Of course, a leap into the air does not provide theaccelerationproduced by dropping out of a tree;an animal would have to run quite fast to takeoff. Still, some small terrestrial animals can achieve high speeds. The cursorial hypothesis is strengthened by the fact that theimmediate theropod dinosaurancestors of birds were terrestrial, and they had the traits needed for high lift off speeds: they were small, agile, lightlybuilt, long-legged, andgood runners. And because they werebipedal, their arms were free to evolve flappingflight, which cannot be said for other reptiles of their time.
2 oppsing hopsis: A and C ——point
A:somereasons for building the hopsis, but troubing. Reasons.
C: somereasons for building the hopsis, stengh by facts.
好了,下面说说注意事项吧。