Why would evidence of sense organs in the head of a fossil animal (such as Haikouella) suggest a close relationship with vertebrates that is, which critical vertebrate feature would have to be present?
What will be an ideal response?
Vertebrate sense organs are formed from neural-crest tissue and epidermal placodes, both of which are unique vertebrate features. The presence of such structures in an adult chordate suggests the presence of these vertebrate embryological tissues, and hence a closer relationship to vertebrates than amphioxus or tunicates (which lack these tissues).
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