Chapter 27, Problem 1TY

Summary Introduction

Introduction:

Jawfish includes bony and cartilaginous fish. Jaws are developed from third and fourth gill arches. The development of the jaw means the appearance of teeth. With the passage of time, jaws became important for feeding. Some 420 million years old, Entelognathus primordialis may be the first vertebrae with a modern jaw. Originally the jaws help to pump the water above the gills so that the fish can breathe more perfectly. Jaws help fishes to catch larger prey.

Answer to Problem 1TY

The correct answer is option (a) gnathostomes.

Explanation of Solution

Explanation/justification for the correct answer:

Option (a) gnathostomes. They are marine fishes. Jawed fishes belong to clade Gnathostomes. Gnathostomes possess 2 sets of paired fins (i.e. pelvic fin and pectoral fin). They are enclosed with bony plates. The gill filaments are ectodermal. An enormous placoderm was Dunkleosteous from the ‘Devonian period’, 380-360 million years ago. Gnathostomes belong to the clade Chondrichthyes and Osteichthyes.

Explanation for incorrect answer:

Option (b) lobe fins. Lobe-finned fishes or Sarcopterygii is a clade that contains coelacanths, tetrapods, lungfishes, etc. Sarcopterygii are sometimes also called Crossopterygii. Lobe fins are the most important characteristics of lobe-finned fishes. Their fins are very elastic. Therefore, this is an incorrect answer.

Option (b) tetrapods. The word "Tetrapoda" means "four feet" in Greek. Reptiles, birds, amphibians, and mammals are the chief groups of the Tetrapoda. They first appeared 370 million years back during the ‘Devonian period’. Tetrapods vary in size. One of the important characteristics of tetrapods is, they bear four legs or if they do not have four limbs, their ancestors are assumed to have four limbs. Therefore, this answer is not correct.

Option (d) amniotes.The characteristic feature of amniotes is an amniotic egg. Birds, reptiles, and prototherian mammal produce an amniotic egg. The outer shell of an egg is made of hard calcium to protect it from pathogens, predators, drying, and damage. Amniotic egg contains amnion. Amnion membrane is the special feature of amniotic animals.Therefore, this answer is not correct.

Option (e) cyclostomes.Cyclostomes are jawless fishes having a large sucking mouth. They are fresh-water and marine vertebrates. Due to the absence of jaw, they are termed as jawless vertebratesTherefore, this answer is not correct.

Conclusion

Gnathostomes are the jawed vertebrates. Gnathostomes include two clades i.e. Chondrichthyes and Osteichthyes. Hence, the correct answer is option (a).