According to the lecture, what feature of sauropods were lacking in "prosauropods"?

Question

BlackTom AI · Accepted Answer

Obligate quadrupedal posture was absent in prosauropods because they retained a facultative bipedal anatomy, unlike true sauropods which evolved a permanent four-legged stance.

Relevant knowledge points and concepts:
- Prosauropods are stem-sauropodomorph dinosaurs that preceded true sauropods; sauropods are the large, long-necked, quadrupedal herbivores of the Jurassic–Cretaceous.
- Key anatomical indicators of locomotor stance include relative limb proportions (hindlimb vs forelimb length), limb robustness, pelvic and shoulder girdle morphology, and center-of-mass placement.
- Other diagnostic features in both groups include neck length, skull size, and manual (thumb) claws.

Step-by-step reasoning:
1. Limb proportions: Prosauropods typically have longer hindlimbs relative to forelimbs and a more gracile, mobile forelimb. This suggests they could adopt a bipedal stance or rear up to reach vegetation. In contrast, sauropods evolved forelimbs and hindlimbs of more similar columnar proportions suited to supporting massive bodies on all fours.
2. Center of mass: In prosauropods the center of mass remained posterior enough to allow bipedal locomotion; sauropods shifted center of mass forward and evolved columnar limbs and a rigid, obligately quadrupedal posture.
3. Manual anatomy: Prosauropods commonly possess a large thumb claw (e.g., Plateosaurus), which is reduced or lost in many sauropods.
4. Neck and head: Prosauropods do have relatively elongated necks and small heads for their bodies, though less extreme than later sauropods.

Why the correct answer is correct:
- “Obligate quadrupedal posture” is the feature sauropods consistently had that prosauropods lacked; prosauropods were not obligate quadrupeds but facultative bipeds or plantigrade bipeds at times.

Why the other options are incorrect:
- Elongated neck: prosauropods already show neck elongation (though less extreme), so it is not lacking.
- Small head for the body: prosauropods have relatively small skulls, so this feature is present.
- Thumb claw: prosauropods typically possess a large thumb claw, so they do not lack this.

Conclusion: The correct answer is “Obligate quadrupedal posture,” which matches the provided answer.

According to the lecture, what feature of sauropods were lacking in "prosauropods"?单项选择题

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