How does the pelvic structure of Saurischia differ from Ornithischians according to hip bone arrangement?
Answer
The pubis bone retained an arrangement ancestral to modern reptiles.
The distinction between Saurischia ('lizard-hipped') and Ornithischia ('bird-hipped') centers entirely on the arrangement of the pelvic girdle bones. Saurischians retained a hip structure that is more primitive or ancestral when compared to the hip bones found in modern reptiles. Conversely, Ornithischians evolved a pelvic structure where the pubis bone is directed forward, representing a specialization away from that ancestral reptilian pattern that the Saurischians maintained, even as they evolved into predatory theropods.
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