What critical method confirms the placement of modern fish classes versus *Xenacanthus* classifications?

DNA sequencing

A key methodological difference exists when establishing the certainty of classification ranks between extant and extinct species. For living animals, confirmation of high-level placements, such as within Kingdom Animalia, Phylum Chordata, and Class Chondrichthyes, is achieved with high certainty through modern DNA sequencing techniques. Conversely, for *Xenacanthus*, which cannot be sequenced, these placements must rely entirely on the principle of anatomical homology—comparing shared structures like cartilage composition, fin placement, and the unique cranial spine—with other fossil groups. This reliance on morphology makes the exact relationships between extinct shark orders inherently more speculative than those confirmed genetically for modern relatives.