First sharks and cartilaginous fish

Chondrichthyans, the cartilaginous fish, include sharks, rays and chimaeras. They have skeletons of cartilage that is often hardened with mineral, skin covered with tiny tooth-like scales, and teeth that are shed and replaced. The oldest teeth and scales, from Qianodus in Guizhou, China, described in 2022 by Plamen Andreev and colleagues, date to about 439 million years ago. The earliest articulated skeleton with spines, Doliodus problematicus from New Brunswick, Canada, is roughly 410 million years old.¹²

Sharks evolved from the first jawed fish, the jaws event of the Silurian, and diversified in the Devonian. Cladoselache, a sleek 2-metre shark from the Cleveland Shale of Ohio about 370 million years ago, had no scales over much of its body and strong tail fins. By the Carboniferous there were many odd groups, such as Stethacanthus with an anvil-shaped fin. Cartilage rarely fossilises, so most of what we know comes from teeth, spines and scales. The group survived all five mass extinctions, and modern sharks and rays, with their keen senses of smell and electric field detection, are among the most ancient lineages of jawed vertebrates alive.²