Question

Which of the following could be considered the most recent common ancestor of living tetrapods?

(A) a sturdy-finned, shallow-water lobe-fin whose appendages had skeletal supports similar to those of terrestrial vertebrates

(B) an armored, jawed placoderm with two pairs of appendages

(C) an early ray-finned fish that developed bony skeletal supports in its paired fins

(D) a salamander that had legs supported by a bony skeleton but moved with the side-to-side bending typical of fishes

Short answer

(A) The statement "a sturdy-finned, shallow-water lobe-fin whose appendages had skeletal supports similar to those of terrestrial vertebrates " is true.

(B) The statement "an armored, jawed placoderm with two pairs of appendages " is false.

(C) The statement "an early ray-finned fish that developed bony skeletal supports in its paired fins" is false.

(D) The statement "a salamander that had legs supported by a bony skeleton but moved with the side-to-side bending typical of fishes" is false.

Step-by-step solution

Tetrapods

The term tetrapods is used for describing vertebrate animals that have four limbs. These limbs occur in pairs, and so, the tetrapod animals possess two pairs of limbs. The classes Amphibia, Mammalia, Reptilia, and Aves, along with ancestral organisms that evolved into these classes, are regarded as tetrapods.

Explanation of option '(A)'

The evolution of chordates their divergence to various classes and sub-classes occurred slowly. The skeleton is either comprised of bony tissues or cartilage-like tissues. Among the bony members, the lobe-finned fishes (evolved during late Devonian) are considered to be the closest relative of the extant tetrapods.

Thus, the sturdy-finned, shallow-water lobe-fin fishes are the most recent common ancestor for all the extant tetrapods.

Therefore, the given statement is true.

Explanation of option '(B)'

The armored jawed fishes, such as the extinct placoderm species, are ancestors to all the bony fish groups (the ray-fin fishes and lobe-fin fishes). Still, their existence in the evolutionary lineage is distant from the living tetrapods. This indicates that they are not the recent ancestors of living tetrapods.

Thus, armored jawed fishes are old ancestors of living tetrapods.

Therefore, the given statement is false.

Explanation of option '(C)'

The ray-finned fishes are highly diverse and have several extant lineages. The evolution of this category of bony fishes occurred before the lobe-finned fishes (at around 444 million years ago). So these are not the most recent ancestors of extant tetrapods.

Thus, ray-finned fishes are not the recent ancestors of living tetrapod organisms.

Therefore, the given statement is false.

Explanation of option '(D)'

Salamander is an example of an extant amphibian organism (one of the tetrapods), which evolved from lobe-finned ancestral fishes. So these cannot be considered the most recent ancestors of all extant tetrapod species.

Thus, the salamander is one of the living tetrapods and not an ancestral species.

Therefore, the given statement is false.