Six gill slits instead of five sounds like a rounding error. It’s actually a fossil hiding in plain sight. Nearly every shark alive today carries five pairs of gill slits. The bluntnose sixgill (Hexanchus griseus) keeps six — a plesiomorphic trait, meaning ancestral rather than newly derived. Its body plan traces back to the Triassic, placing the Hexanchidae among the oldest shark lineages still swimming. Where modern sharks streamlined down to five slits, the sixgill simply never did. That conservatism shows up across the whole animal: • A single dorsal fin set far back near the tail, rather than the two most living sharks carry • Comb-like (pectinate) lower teeth that interlock into a saw blade for shearing flesh from carcasses and large prey • A broad, heavy body reaching 5 m or more, built for cruising rather than sprinting It lives where that old design still pays off: the deep mesopelagic and bathyal zones, typically 200–1,100 m and recorded past 2,000 m, in water hovering...