What is the feature that is common among terrestrial vertebrates but not in more basal vertebrates?

Question

BlackTom AI · Accepted Answer

The amniotic egg is the key innovation that allowed vertebrates to complete their life cycle away from water: it contains extraembryonic membranes (amnion, chorion, allantois, and yolk sac) that cushion the embryo, prevent desiccation, permit gas exchange, and handle waste — enabling reproduction on land.

1) Relevant concepts
- Amniotes: the clade of terrestrial vertebrates that includes reptiles (including birds) and mammals; defined by having an amniotic egg (or its modified internal form).
- Amniotic egg membranes:
  - Amnion: encloses the fluid-filled sac that protects the embryo.
  - Chorion: facilitates gas exchange.
  - Allantois: stores metabolic wastes and also aids gas exchange.
  - Yolk sac: provides nutrients.
- Basal vertebrates: earlier branching groups (jawless fishes, many fishes, amphibians) that either lack these membranes or require aquatic environments for reproduction.

2) Step-by-step reasoning
Step 1: Evaluate each option for presence in terrestrial vertebrates and in more basal vertebrates.
Step 2: Vertebrae — by definition all vertebrates possess vertebrae (basal vertebrates included), so this is not unique to terrestrial groups.
Step 3: Jaw — jaws evolved in early gnathostomes (jawed fishes) well before terrestrial amniotes; many basal vertebrates (jawed fishes) already have jaws, so this is not unique.
Step 4: Limb — true tetrapod limbs appear with the first land-adapted tetrapods; however, limbs are not the defining feature of the amniote radiation and some basal vertebrates (fishes) lack limbs while amphibians (which are more basal than amniotes) do have limbs but lack the amniotic egg.
Step 5: Amniotic egg — appears at the base of the amniote clade and is present in reptiles, birds, and mammals (modified internally in mammals), but absent in more basal vertebrates such as fishes and amphibians that rely on water for reproduction.

3) Intermediate logical steps
- Compare presence/absence across groups: vertebrae (present everywhere) → eliminate; jaw (present in many aquatic vertebrates) → eliminate; limb (appears at tetrapods but not an innovation distinguishing terrestrial amniotes from more basal tetrapods) → less precise → eliminate; amniotic egg (distinct synapomorphy of amniotes) → select.

4) Why the correct answer is correct
The amniotic egg is the synapomorphic trait that distinguishes amniotes (the terrestrial vertebrate clade of reptiles, birds, mammals) from more basal vertebrates (fishes and amphibians) because it enables reproduction independent of standing water.

5) Why other options are incorrect (brief)
- Limb: not unique to terrestrial amniotes (some basal tetrapods have limbs; limbs evolved earlier at the base of tetrapods).
- Vertebrae: present in all vertebrates, including basal groups.
- Jaw: evolved earlier in jawed fishes, so not specific to terrestrial vertebrates.

Conclusion: The correct feature is Amniotic egg, which matches the provided answer.

What is the feature that is common among terrestrial vertebrates but not in more basal vertebrates?单项选择题

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