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。
阿尔伯塔大学-恐龙古生物学
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。
英语百科
Bipedalism
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped/ˈbaɪpɛd/, meaning "two feet" (from the Latin bis for "double" and pes for "foot"). Types of bipedal movement include walking, running, or hopping.