Sharks and Rays

By | September 9, 2016

Fossil teeth reveal that killer sharks have been cruising the seas for more than 400 million years—an astonishing length of time. Along with their flat-bodied relatives the rays, sharks belong to a truly ancient class of animals known as cartilaginous fish. These fish have no bones; instead, the skeleton is made of a rubbery material called cartilage.


Key features
■ Teeth are continuously shed and replaced
■ Skeleton made of cartilage
■ No ribs whatsoever
■ No air bladder for controlling buoyancy
■ Sharks must keep swimming or they will sink
■ Fins in pairs for steering, unlike earlier fish

The earliest known fossils of sharks and rays date back to the Late Silurian, almost 420 million years ago.

Hybodus (hy-BODE-us)


When: Late Permian to Late Cretaceous

Fossil location: Europe, N. America, Asia, Africa

Habitat: Oceans

Length: 6 ft (2 m)

Diet: Small marine animals

Hybodus looked as fierce as any modern shark and had the classic streamlined shape, but its teeth and fins were different from today’s sharks. It had two types of tooth: sharp ones at the front for seizing slippery prey such as fish, and flatter, more blunt teeth at the back of the mouth for crushing shells. In front of Hybodus’s dorsal fin (the fin on its back) was a long, bladelike spine. This may have helped the fin to cut through the water more easily or it may have been used for defense.

Heliobatis (he-lee-oh-BAT-iss)


When: 54–38 million years ago (Early to Middle Paleogene)

Fossil location: USA

Habitat: Freshwater streams and lakes

Length: 3 ft (1 m)

Diet: Crayfish, shrimp, and other invertebrates

Heliobatis may have been a relative of the stingray. Its tail contained up to three needlelike stingers that may have been able to inject venom. It lived at the bottom of lakes and possibly rivers, where it hunted for crayfish, small fish, and possibly snails. It was named Heliobatis (“Sun ray”) because of the way its fins fan out around it like rays of sunlight.

Notorynchus (no-toe-RIN-cuss)


Single Notorynchus tooth

When: 56 million years ago to now

Fossil location: Worldwide

Habitat: Cool, shallow marine waters

Length: 10 ft (3 m)

Diet: Sharks, rays, fish, seals, and dead animals

Also known as the seven gill shark, Notorynchus had seven gill slits (unlike the five seen in most sharks). Its strange teeth were each made up of many small points (cusps), creating a jagged, sawlike edge ideal for slicing flesh. Notorynchus is still found today and is common in cooler seas worldwide.

Squalicorax (SKWA-lih-CORE-ax)


Squalicorax’s fossilized tooth

When: 105–65 million years ago (Mid to Late Cretaceous)

Fossil location: Worldwide

Habitat: Oceans

Length: 15 ft (4.5 m)

Diet: Marine creatures

Fossilized shark teeth are common, since sharks shed thousands of teeth during their lives. Many Squalicorax teeth have been found, including one embedded in the foot of a hadrosaurid dinosaur. The rare find shows that Squalicorax sometimes scavenged from carcasses washed into the sea.

Stethacanthus (steth-a-CAN-thus)


When: Late Devonian to Early Carboniferous

Fossil location: N. America, Scotland

Habitat: Oceans

Length: 5 ft (1.5 m)

Diet: Marine animals

One of the oddest of all prehistoric fish, Stethacanthus had a dorsal fin shaped like an ironing board, with a cluster of toothlike scales (denticles) on top. It had more toothlike scales on its head, and its side fins had long, pointed rods called whips trailing behind them. These features may have been present only in males and were perhaps important in mating.
Stethacanthus usually lurked in shallow, coastal waters, where it nosed around for small fish and shellfish.

Helicoprion (HELL-ee-coe-PRY-on)


When: Early Permian

Fossil location: Worldwide

Habitat: Oceans

Length: 18 ft (5.5 m)

Diet: Marine animals

This bizarre shark was named Helicoprion (“spiral saw”) because the teeth of its lower jaw grew in a spiral, forming a disk as big as a dinner plate. Only Helicoprion’s teeth have been found. The disk is known to be from the lower jaw, but how the shark used it to feed is a mystery.

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