Final answer:
The sister taxon of the Chordata is the Echinodermata, which shares a closer evolutionary relationship with Chordata than other invertebrate phyla due to their common deuterostome development and endoskeletal features.
Step-by-step explanation:
The sister taxon of the Chordata is the Echinodermata. The phylum Chordata includes animals with a notochord, a dorsal hollow nerve cord, pharyngeal slits, a post-anus tail, and an endostyle/thyroid. The Chordata and Echinodermata are both part of the Deuterostomia group of organisms, which is distinct from Protostomia, and these two phyla share closer evolutionary ancestry with each other than with any other phyla. Echinoderms and chordates share important developmental processes, as they both develop their mouths secondarily in the embryonic stage, hence the name 'Deuterostomia' (meaning 'second mouth'). Moreover, among the most primitive deuterostomes, the Echinodermata also produce an endoskeleton composed of ossicles, which is a feature common to this group.