bipedal

adjective [原级] 两足动物的；二足的

Because they were bipedal, their arms were free to evolve flapping flight, which cannot be said for other reptiles of their time. 因为它们是两足动物，它们的手臂可以自由地进化出拍打的翅膀，这在当时的爬行动物中是不可能的。