Jaws improved feeding efficiency

During the Devonian period, many new kinds of fishes evolved in the seas, estuaries, and fresh waters. Although most of these fishes were jawless, in one lineage, some of the skeletal arches that supported the gills evolved into jaws (Figure 34.11). A fish with jaws can grasp and subdue large prey. Further development of jaws and teeth enabled some fishes to chew both soft and hard body parts of prey. Chewing aided chemical digestion and improved the fishes' ability to extract nutrients from their prey.

The dominant early jawed fishes were the heavily armored placoderms (class Placodermi). Some of these fishes evolved elaborate fins and relatively sleek body forms that improved their ability to maneuver in open water. A few became huge (10 m long) and, together with squids (cephalo-pod mollusks), were probably the major predators in the Devonian oceans. Despite their early abundance, however, most placoderms had disappeared by the end of the Devonian period; none survived to the end of the Paleozoic era.

Peritoneum

(a) Eptatretus stouti

(a) Eptatretus stouti

Pacific Hagfish

34.10 Modern Jawless Fishes (a) The Pacific hagfish. (b) Two sea lampreys using their large,jawless mouths to suck blood and flesh from a trout.The sea lamprey can live in either fresh or saltwater.

(b) Petromyzon marinus

34.10 Modern Jawless Fishes (a) The Pacific hagfish. (b) Two sea lampreys using their large,jawless mouths to suck blood and flesh from a trout.The sea lamprey can live in either fresh or saltwater.

(b) Petromyzon marinus

Petromyzon Marinus

Jawless fishes (agnathans)

Extinct and living forms Skull (cartilage)

Jawless fishes (agnathans)

Extinct and living forms Skull (cartilage)

Placoderm Diagram
Early jawed fishes (placoderms) Extinct

Some anterior gill arches became modified to formjaws.

Modern jawed fishes (cartilaginous and bony fishes)

Jawless Skulls With Blood

Some anterior gill arches became modified to formjaws.

Living forms

Living forms

Additional gill arches help support heavier, more efficientjaws.

Additional gill arches help support heavier, more efficientjaws.

34.11 Jaws from Gill Arches This series of diagrams illustrates one probable scenario for the evolution ofjaws from the anterior gill arches of fishes.

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