- Thu Mar 22, 2018 11:16 am
#44793
Hi Marie,
This is a Must Be True question. The stimulus is providing a brief description of how fish evolved from all of them being jawless to some of them having jaws.
Until around 400 million years ago all fish were jawless.
Jawless fish sucked food into their mouthes, since they were physically unable to grab and gnaw on their food. For example they could suck in plankton from the surface of water or suck in particles from the bottom of the water source they were in. This makes B the correct answer since they were ‘suckers’ they were not able to prey on other fish. These early jawless fish had cartilage for their skeletal system. (This fact comes from the sentence that says, ‘one retained cartilage for its skeletons’). This makes answer choice D factually incorrect.
When some fish evolved to have jaws they no longer had to rely upon sucking in food, instead they were able to pursue prey, grab the prey using their jaws and use their teeth to chew their prey. The stimulus does not go so far as to say that fish are the primary prey of all jawed fish. So A is an exaggerated, or unknown answer. Additionally, the stimulus never says that all jawless fish have become extinct, the evolution of a species does not necessarily mean that the ‘original’ species disappears. This makes E an incorrect answer.
There were 2 main lines of jawed fish: 1) The fish that retained the cartilage skeletal materials of their jawless predecessors (sharks, rays). 2) The fish that evolved to have bone as their primary skeletal materials. This second evolutionary line includes the teleosts which is defined as ‘the most abundant and diverse of all today’s vertebrate groups.’ Though it is true that teleosts have jaws and as such may have the ability to pursue, grab and chew prey, the stimulus never says that that is their only method of eating. This leaves C as an unsupported assertion, and thus an incorrect answer.
Hope that helps!
-Malila
