The ancestors of modern-day fish were vertebrate chordates. vertebrate chordates are believed to have evolved from invertebrate chordates, which were similar to modern-day tunicates.

what change must have occurred during the evolution from invertebrate chordates to vertebrate chordates?

a.
a backbone evolved to enclose the spinal cord, which develops from the dorsal nerve cord.
b.
an endoskeleton evolved to enclose the dorsal nerve cord, which develops from the notochord.
c.
an endoskeleton evolved to enclose the spinal cord, which develops from the notochord.
d.
a backbone evolved to enclose the notochord, which develops from the spinal cord.

The correct answer is option B, that is, an endoskeleton evolved to enclose the dorsal nerve cord, which develops from the notochord.

Explanation:

It has been discovered that the ancestors of the modern-day fish were vertebrate chordates. The vertebrate chordates are considered to have evolved from the invertebrate chordates that are identical to the present day tunicates. It has been found that in vertebrates the endoskeleton has evolved to encompass the dorsal nerve cord that has been developed from the notochord (a characteristic of invertebrates).

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