Which of the below is a feature shared between mammals and dinosaurs but not with amphibians or fish?

Question

BlackTom AI · Accepted Answer

Amniotes (a group that includes mammals and reptiles, the latter including dinosaurs) develop an amniotic egg or amniotic membrane during embryonic development. The amnion, chorion, yolk sac and allantois are extraembryonic membranes that protect the embryo, allow gas exchange, manage waste, and permit reproduction away from water. Amphibians and most fish are anamniotes: their eggs lack an amnion and must generally be laid in water or remain moist.

Step-by-step reasoning:
1. Identify the key distinction: presence of an amnion (amniotic egg/egg membranes) is a defining trait of amniotes (mammals, reptiles, birds) and absent in amphibians and fish.
2. Evaluate each option against the four groups (mammals, dinosaurs/reptiles, amphibians, fish).
   - Limbs: mammals and dinosaurs have limbs, but so do many amphibians (frogs, salamanders) — fish generally do not have true tetrapod limbs, so “limbs” is not uniquely shared only by mammals and dinosaurs while excluding amphibians.
   - Amniotic egg: present in mammals (embryos develop inside an amnion; monotremes lay shelled eggs; marsupials and placentals have internal amniotic sacs) and dinosaurs (reptilian amniotic eggs). Amphibians and fish lack an amnion — their eggs are anamniotic. This matches the requirement.
   - Teeth: teeth occur in mammals, many dinosaurs, many fish, and some amphibians; thus not exclusive to mammals and dinosaurs.
   - Nose: mammals and amphibians have external nostrils/nasal structures; fish have nares/olfactory organs (though not a true nasal respiratory system), and many reptiles/dinosaurs had nasal openings — so “nose” is not a clear distinguishing trait.
3. Conclusion: the amniotic egg (presence of an amnion and related membranes) is the shared feature of mammals and dinosaurs that is absent in amphibians and fish.

Therefore, the correct answer is: Amniotic egg. This matches the provided answer.

Which of the below is a feature shared between mammals and dinosaurs but not with amphibians or fish?单项选择题

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