A group of living things
Sharks and rays
Chondrichthyes
Fish with light, tough skeletons, older than trees, that sense electricity.
Its members' last common ancestor lived about 410 million years ago.
You and sharks and rays last shared an ancestor about 465 million years ago (on 21 November, if life's history were one year).
Sharks and rays are cousins of the bony-fish group that includes people, but their skeletons are made of light but tough cartilage instead of bone. Shark-like scales go back about 450 million years, to before trees existed.
Most sharks are torpedo-shaped, with gill slits on the sides of the head. Most sharks have rows of sharp teeth. Most rays are flattened discs that swim with wide wings, with eyes on top and mouth underneath. Shark skin feels rough like sandpaper from tiny tooth-like scales.
Sharks and rays can sense hidden prey with tiny pores around the head that pick up weak electric signals. They live in all oceans, from shallow coral reefs to the deep sea.
Sizes run from the dwarf lanternshark, which grows to only about 20 centimetres long and is small enough to hold in a hand, to large whale sharks, which can grow to about as long as a bus.
A nose that points the way
In tests, some sharks smell in stereo. They turn toward the nostril that smells food first, even when the gap is only about a tenth of a second.
Mermaid's purses
Some sharks and rays lay eggs in tough cases with tendrils that grip seaweed. Empty cases wash up on temperate beaches for beachcombers to find.
Spotty bellies
Each manta ray has its own pattern of black spots on its white belly. Scientists use photos of the pattern to tell individuals apart.