Logical Reasoning

PT17 · S2 · Q20 Until about 400 million years ago

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Until about 400 million years ago, fishes—the first true swimmers—were jawless.

Statements

Strong claim (first) Fish were the first true swimmers.

Until 400 million years ago they were jawless. They had to suck in surface plankton or suck in food particles from bottom mud.

Pivot (however) / Causal (allowed them to) Developing biting jaws changed their lives, allowed them to pursue prey, seize it in their jaws, manipulate it between their teeth.

Distinction (two lines of jawed fishes) One retained cartilage for its skeleton, the other adopted bone as its primary skeletal material.

Strong claim (the most abundant and diverse) Teleosts are the most abundant and diverse of all of today's vertebrate groups, and they evolved from the bone group.

Evaluate

There are way too many statements here to try to push ourselves to predict the correct answer.

We have a couple strong claims. We have a timeline. We have a two-pronged distinction.

There could be many derivable statements, so we might as well head to the answers.

The one prediction we might make based off the Pivot is that a correct answer will combine something before it with after it.

For example, we could probably infer that the jawless fishes had cartilage for skeletons. We could probably infer that teleosts never were forced to feed by sucking in surface plankton or bottom mud.

Goal

Let's find something, preferably safely worded, that we know we can derive from combining any of these facts.

20.

If all of the statements in the passage are true, which one of the following must also be true?

  1. Fish are the primary prey

    Too Strong: primary / all

    We actually don't know if fish are the prey of any jawed fishes, let alone the primary prey of all jawed fishes. The stimulus just tells us that "jawed fishes could actively pursue prey". We have no idea what type of prey.

    1% picked this

  2. Correct

    The jawless fishes did not

    Why this is right

    This can be derived from the first two sentences (making it highly obnoxious that we had to read and worry about the rest of that paragraph!). We were told that the jawless fishes' feeding methods were limited to sucking in surface plankton or sucking in food particles from bottom mud. So that means they weren't preying upon other fish, just suckin' up plankton from the roof or food particles from the floor. While those two claims are enough to justify this answer, this answer also reinforces the Pivot + Causal Difference-Maker. We were told that, "once they developed biting jaws, however, this allowed them to pursue prey". So this answer is a form of the classic Flip the Causal Difference-Maker inference, in which we say, "If jaws enabled them to pursue prey, then when they were jaw-less, they did not have the ability to pursue prey."

    Skill tested: Must be True · how this choice captures the argument's function is the move to repeat next time.

    68% picked this

  3. Teleosts do not feed upon

    Too Strong: do not

    We know that teleosts are not limited to feeding on the bottom mud. Their biting jaws allow them to pursue prey, but we were never told that they exclusively pursue prey. Someone's increased income might allow them to eat out at nicer restaurants; they used to be limited to McDonald's and other fast food, but their pay raise allows them to dine at fast causal or even sit-down restaurants. But that doesn't mean they never get a hankering for a Big Mac (which, by metaphor, in this case, is the particles found in bottom mud / sorry, McDonald's). Similarly, when times are tough and prey can't be found, or maybe just when they're feeling snacky, it's highly possible that teleosts still feed on particles found in bottom mud.

    6% picked this

  4. Jawless fishes did not have

    Opposite

    We could probably infer that jawless fishes didn't have bone as their skeletal material. We're told then after jaws developed, the jawed fishes broke into two main lines: one retained cartilage, the other adopted bone. So that implies that the status quo was cartilage skeletons. The bony fish were the ones who innovated a new evolutionary path after jaws had already evolved.

    18% picked this

  5. Jawless fishes became extinct approximately

    Too Strong: extinct

    We're told that around 400 million years ago, some fish started to have jaws. That doesn't mean that jawless fish were suddenly out of existence. Evolution takes a lot of time. It might take millions of years for jawed fishes to outcompete and out-replicated jawless fishes. But it's also possible that jawless fishes are still alive today. Nothing in the stimulus indicates they ever went extinct.

    7% picked this

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