The event had the greatest impact on chordates greater complexity in body structure, appendages, etc. is option B: a brain with complex nervous system.
Within chordates, vertebrates have the most complex brains. The vertebrate brain has a tripartite organization since it is made up of a forebrain, a midbrain, and a hindbrain. The vertebrate brain has a tripartite organization since it is made up of a forebrain, a midbrain, and a hindbrain. Complexity in brain led to the complexity in body structure, appendages, etc.
The four distinguishing characteristics of animals in the phylum Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a tail. These characteristics all appear at some point during animal development. Only few chordates exhibit these traits throughout embryonic development. The notochord supports the skeleton, gives the phylum its name, and invertebrates develops into the vertebral column.
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