Which shark is often called a 'living fossil'?

The frilled shark is often called a “living fossil” because its eel-like body and several primitive anatomical traits have changed relatively little over a long evolutionary history.

Its scientific name is Chlamydoselachus anguineus, and it lives mainly in deep water along continental shelves and slopes. The shark has six pairs of gill slits with frilly margins, a dark, eel-like body, and jaws lined with hundreds of recurved teeth. It can swallow soft-bodied prey such as squid whole.

The label does not mean the species is literally unchanged or that it is a direct survivor of the earliest sharks. “Living fossil” is an informal description for organisms that retain ancient-looking features. Modern genetic and anatomical research places the frilled shark among living shark lineages, while its closest relationships and classification have been debated.

The frilled shark was formally described in 1884 from a specimen associated with Japan. A second living species, the southern African frilled shark, was recognized in 2009, so the name can refer to a broader group as well as the familiar species.