Chapter 31, Problem 9STP

Summary Introduction

To write:

The evolution of the jaw and explain how it was an important advancement for fishes.

Introduction:

Evolution is the process of changes occurring in an organism which increases its chances of survival and reproduction.

Explanation of Solution

The term, fish, includes jawless fish (Agnatha) and jawed vertebrates (Gnathostoma).

Agnatha:

• The most primitive jawless fishes are different from cephalochordates which have a bone.
• The living agnathans are either parasitic in nature (lampreys) or scavengers (hagfish).
• Their circular mouths lack jaws and are used as both respiratory structures and straining devices.
• As they evolved, the agnathans developed more streamlined bodies with dorsiventrally flattened heads.

Gnasthostomes:

• The evolution of jaws was very beneficial for the organisms. It allowed the gills to develop and focus only on the process of respiration.
• Embryological evidence shows that modification of slender bones supporting the gill arches in the throat region evolved into jaws.
• The first jawed fishes were the Acanthodians.

Conclusion

The evolution of jaws was very important for the fishes as it increased the variety of food that could be ingested by fishes, thereby increasing their chances of survival. It helped in the modification and specialization of gills for the process of respiration.