There's no way that they could have possibly used their arms for walking, making them obligate bipeds.

他们不可能用他们的手臂走路，使他们成为专有的两足动物。

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。