What characteristic confirms Xiphactinus's placement within Phylum Chordata?
Answer
It possessed, at some point in its life, a notochord.
Placement within Phylum Chordata is a fundamental step in biological classification, confirming basic structural organization common to vertebrates. For *Xiphactinus*, this membership asserts that the organism possessed specific chordate features during at least some phase of its life cycle. The core characteristics required for Chordata inclusion are the presence of a notochord, a dorsal nerve cord, and pharyngeal slits. The possession of the notochord—a flexible rod providing skeletal support—is the key feature mentioned that places it firmly within this massive phylum alongside all other vertebrates.
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