Suppose new molecular data showed that tunicates and vertebrates are sister taxa. What difference would this make to our assumptions about the form of the original chordate animal? What additional features might this animal have possessed?
What will be an ideal response?
This would mean that the original chordate was a mobile animal throughout life, rather than sessile as an adult. With this scenario, amphioxus serves as a model for what the basic chordate plan might have been like. In addition, the earliest chordate would probably have had the features that are shared by amphioxus and vertebrates (e.g., segmental muscles through the entire trunk, not just the tail), and these would have been ancestral characters for tunicates that were secondarily lost in the sessile adult forms.
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