Jawless Fish

By | September 9, 2016

The first vertebrates were fish, but they were very different from today’s fish. The early fish couldn’t bite, since jaws were yet to evolve. Instead, they fed by sucking or scraping. With few or no fins, they swam by waggling their tails like tadpoles. They had no internal bones, but some had wide, bony shields covering their heads—protection from predators such as giant sea scorpions.

FAMILY FACT FILE

Key features
■ Mouths but no jaws
■ Many species lacked paired fins
■ Usually no stomach
■ Swam by beating a muscular tail

When
Some jawless fish fossils have been dated to the Cambrian Period, more than 500 million years ago. Many jawless fish died out at the end of the Devonian Period, almost 350 million years ago.

Drepanaspis (DREP-an-ASP-iss)

drepanaspis-fossil


When: 410 million years ago (Early Devonian)

Fossil location: Europe

Habitat: Ocean floor

Length: 14 in (35 cm)

With its flat, paddle-shaped head and narrow body, Drepanaspis was a strangely shaped fish. It hunted for food near the bottom of Devonian seas. Drepanaspis’s feeding methods are a mystery, since its jawless mouth faced upward instead of downward, a curious feature that would have made it difficult to scoop in food. Like many other jawless fish, Drepanaspis had bony armor to protect it from attack.

Birkenia (bir-KEEN-ee-a)

birkenia


When: 425 million years ago (Middle Silurian)

Fossil location: Europe

Habitat: Freshwater pools and streams

Length: 2 in (6 cm)

Although Birkenia did not have fins, it was still an active swimmer in the pools and streams in which it lived. It fed on the remains of dead plants and animals, probably sucking in scraps with its gaping mouth. Unlike many other jawless fish that had bony head-shields, Birkenia’s head was covered in small scales.

Zenaspis (zen-ASP-iss)

zenaspis


When: 410 million years ago (Early Devonian)

Fossil location: Europe

Habitat: Shallow seas and river mouths

Length: 10 in (25 cm)

Zenaspis had a horseshoe-shaped head protected by an armored shield, while the rest of its fairly flat body was protected by scales. Its eyes were placed close together on top of its head (a perfect position for spotting predators for a bottom-dwelling fish). Like lots of jawless fish, Zenaspis did not have teeth. Instead, its mouth, located on the underside of the body, was lined with bony plates. It probably fed on small creatures found on the seafloor or in river mouths.

Cephalaspis (SEFF-a-LASP-iss)

cephalaspis


When: 410 million years ago (Early Devonian)

Fossil location: Europe

Habitat: Freshwater pools and streams

Length: 9 in (22 cm)

This small fish lived at the bottom of pools or streams. Perhaps it moved its broad head-shield from side to side, stirring up mud as it searched for hidden worms and other creatures. It may also have fed on the waste of other animals that lived in the water. Pairs of scaly flaps balanced its body, and a fin on its back prevented it from rolling over.

Sacabambaspis (SAC-a-bam-BASP-iss)

sacabambaspis


When: 490 million years ago (Early Ordovician)

Fossil location: Bolivia

Habitat: Coastal waters

Length: 12 in (30 cm)

This fish had a broad head-shield and a body that narrowed to end in a small fin. With this shape, it probably swam very much like a tadpole, sucking in scraps of food through its ever-open mouth. Sacabambaspis had sense organs that helped it to feel movement in the water, allowing it to judge the distance to its prey—and avoid predators.

LIVING RELATIVE

lamprey

Toothed mouth of a lamprey

Two groups of jawless fish still exist today: hagfish and lampreys. Both are eel-shaped creatures with no bones, scales, or fins. Hagfish feed on worms or dead sea animals. Some lampreys are parasites – they use their circular, jawless mouths to latch on to fish so they can feed on their blood.

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