At some point, the obligate quadrupeds like the sauropods would have had facultative bipedal ancestors.

在某个时候，像蜥脚类动物这样的专性四足动物会有兼性双足动物的祖先。

If we think about some of the largest dinosaurs, the sauropods, they were definitely obligate quadrupeds.

如果我们想想一些最大的恐龙，蜥脚类动物，它们绝对是四足动物。

So, an animal that walks primarily on two legs is an obligate biped and an animal that walks mainly on four legs is an obligate quadruped.

因此，主要用两条腿行走的动物是专性两足动物，而主要用四足行走的动物是专性四足动物。