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

An endoskeleton evolved to enclose the dorsal nerve cord, which develops from the notochord.

Explanation:

The ancestors of modern-day fish are vertebrate chordates that are believed to be the descendants of invertebrate chordates. The phylogenetic understandings on evolutionary developments have revealed that these invertebrates are closer relatives of the present-day tunicates.

Nevertheless, these evolutionary adaptations lead to the development of endoskeleton invertebrates. The primary function of endoskeleton development was to protect the dorsal nerve cord. The dorsal nerve cord is the only unique feature in all chordates and it develops from the notochord.

not too sure maybe olecranon process

then that particular organisms become extinct.