Marco: Giant kangaroosone of several extinct species of large mammals

Question

Marco: Giant kangaroos—one of several extinct species of large mammals (megafauna)—went extinct in Australia around 46,000 years ago. The deposits and wear patterns on the teeth of these animals from around the time of their extinction indicate that they fed mostly on saltbrush shrubs. Saltbrush thrives in arid climates, so it is not likely that the kangaroos’ food supply was adversely affected by the increasing aridity of the climate at the time. Thus, something else would have to account for their extinction, and the best candidate for that cause is predation by humans.

Fatima: That argument alone is not likely to satisfy many researchers in this field. Have you found any other evidence to bolster your conclusion?

Select  Marco  for the statement that, if true, most justifies Marco’s assertions, and select  Fatima  for the statement that, if true, most justifies Fatima’s skepticism about Marco’s assertions. Make only two selections, one in each column.

ID: 100371

LILICHKA · Accepted Answer

How is A true for Marco?
it actually undermines Marco's argument.
1. Marco says Kangaroos fed mainly on shrubs. 2. Shrubs feel fine when climate is arid. 3. Climate was getting more arid when GK went extinct.
Option A says THE OPPOSITE: the climate was getting LESS arid at the time kangaroos went extinct.
This totally ruins Marco's point. It might mean, shrubs became less available during that time, and the GK actually died out from hunger.

I'd like to hear your thoughts!

ChiranjeevSingh · Answer

Understanding the Passage   
  Marco:  Giant kangaroos - one of several extinct species of large mammals (megafauna) - went extinct around 46,000 years ago. 
 
 Giant Kangaroos - let’s call them GK - is an extinct species. 
 GK is a species of large mammals 
 Large mammals are called megafauna (mega means big or large; fauna means animals) 
 Several species of large mammals are extinct 
 GK went extinct around 46K years ago.

The deposits and wear patterns on the teeth of these animals from around the time of their extinction indicate that they fed mostly on saltbrush shrubs. 
 
 Analysis indicates that GK fed mostly on saltbrush shrubs

Saltbrush thrives in arid climates, so it is not likely that the kangaroos' food supply was adversely affected by the increasing aridity of the climate at that time. 
 
 Given that GK fed mostly on saltbrush shrubs and that these shrubs thrive in arid climates, we can expect that GK’s food supply was not affected by the increasing aridity (dryness) of the climate at the time. 
 Rather, we can expect the food supply to be abundant for GK at the time they went extinct since saltbrush thrive in arid climates.

Thus, something else would have to account for their extinction, and the best candidate for that cause is predation by humans. 
 
 “Thus” indicates the conclusion. 
 Marco concludes that something else would be the cause of GK’s extinction (Marco just eliminated the lack of food supply as a possible cause of GK’s extinction) 
 Marco further adds that predation by humans is the best candidate for being the cause of GK’s extinction 
 (As I read this conclusion, I see that the second part of the conclusion is a stretch. Marco has provided reasoning for eliminating one cause, but many causes can be possible. How can we conclude predation by humans as the best candidate for the cause of extinction?)

Fatima:  That argument alone is not likely to satisfy many researchers in this field. Have you found any other evidence to bolster your conclusion? 
 
 Fatima doesn’t provide any counterpoint. 
 She says that Marco’s argument is unlikely to satisfy many researchers in this field. (She says ‘many’ researchers, not ‘all’ researchers) 
 Then, she asks Marco whether he has more evidence to support his conclusion.  
    Understanding the Question Stem   
 Select Marco for the statement that, if true, most justifies Marco's assertions, and select Fatima, for the statement that, if true, most justifies Fatima's skepticism about Marco's assertions.

We’re looking for two options:
 
 One that supports Marco’s argument 
 One that supports Fatima’s doubt about Marco’s argument. This option should doubt Marco’s argument by supporting Fatima’s specific doubt. An option that weakens Marco’s argument in a way not aligned with Fatima’s doubt will not be correct.

Evaluating the Options

A. Giant kangaroos became extinct during a period that was less arid than previous periods they endured. 
 
 This option says that the period in which GK became extinct was LESS ARID than previous periods they endured (survived). 
 So, GK had survived periods with more arid climates than the one in which they went extinct. 
 This option seems to contradict or go against the information in the passage, which states that GK became extinct in a climate of increasing aridity. 
 However, since we don’t challenge the facts of the argument and we have to treat the options as true in strengthen or weaken questions, I’ll try to see whether this option can be true without contradicting the passage. 
 And I see that this option and the passage can be true simultaneously. 
 GK became extinct in a climate of ‘increasing’ aridity. So, the aridity was increasing at that time. Had the aridity become the most extreme ever? That’s not given. So, it is entirely possible that aridity was increasing at that time but was still lower than aridity of some of the previous periods. 
 So, the option is not contradicting the passage. 
 Given this understanding, what is the impact of this option on Marco’s argument? 
 We know that GK became extinct during a period of increasing aridity. We also know that GK’s food supply was likely not negatively affected. 
 However, the increasing aridity could affect GK negatively in some other way. GK’s health might have been affected. 
 So, while GK’s food supply was not negatively affected, other aspects linked to the increasing aridity of the climate could have led to GK’s extinction. 
 This option ELIMINATES OTHER CAUSES linked to climate aridity by saying that GK had endured more arid climates. GK’s enduring more arid climates indicates that no aspects linked to arid climates caused GK’s extinction. 
 Thus, this option increases the belief that predation by humans (an aspect not liked to arid climate) is likely the cause of GK’s extinction. 
 This option, thus, supports Marco’s argument.

B. Many researchers believe humans first arrived in Australia around 40,000 years ago. 
 
 This option provides info about what many researchers believe. 
 Many believe that humans first arrived in Australia around 40K years ago. 
 Given that GK went extinct around 46K years ago, this option seems to weaken the argument. 
 If humans arrived 40K years ago, they could not be the cause of extinction of GK that happened 46K years ago. 
 Fatima mentioned that Marco’s argument will not satisfy ‘many’ researchers. And this option presents the belief of ‘many’ researchers. 
 Thus, this option supports Fatima’s doubt about Marco’s argument.  
(A problem that I find with this option is that it talks about the time of arrival of humans in AUSTRALIA. Marco and Fatima DO NOT talk about Australia. I know that kangaroos are mainly found in Australia, but I don’t think that a GMAT question should assume this knowledge. So, this option seems a little off to me. However, no other option comes close.)

C. Approximately 60 different species in Australia died out in the wave of extinctions around 46,000 years ago. 
 
 This option says that MANY different species went extinct at the same time. 
 The important question for us is: what was the cause of this wave of extinctions? 
 Was human predation the cause? Or was something else the cause? 
 Since the option doesn’t indicate one way or other about the cause of these extinctions, the option has no impact on Marco’s argument.

D. Fossils of giant kangaroos also show evidence that those animals' diets routinely included plants other than saltbrush. 
 
 This option says that GK’s diets frequently included plants other than saltbrush. 
 The passage that GK’s diets mostly included saltbrush. 
 There’s no conflict between the passage and this option. GK’s diet was mostly saltbrush and regularly included other plants. 
 Now, the argument said that since saltbrush thrive in arid climates, GK’s food supply was NOT adversely affected. 
 However, if GK’s diet included other plants regularly, GK’s diet could have been adversely affected. 
 So, this option is a weakener. 
 However, this option is a VERY MILD one because we don’t know whether these other plants were adversely affected by arid climate and because these plants were, in any case, not a major part of GK’s diet 
 Thus, between B and D, B wins.

E. Several types of megafauna larger than the giant kangaroo went extinct around 46,000 years ago. 
 
 This option is similar to option C. 
 The reason this option has no impact is also the same as the reason why option C has no impact.

Abhishiek · Answer

Marco states that human predation might have caused the Giant kangaroos extinction. Fatima is skeptic about this.

A: None. This doesn't support Marco's statement. It might mean that kangaroo died due to less availability of saltbrush shrub. They haven't endured such an arid climate yet.

B : Fatima  
This directly supports her doubts. If humans reached Australia 40,000 years ago, they can't cause extinction 46,000 years ago.

C: This doesn't impact the argument.

D: Marco  
If Kangaroo's diet included something other than saltbrush shrub, it supports that something else (not unavailability of saltbrush shrub) would have to cause the extinction

E: This is mentioned in passage (one of several extinct species of large mammals ), and doesn't impact.

aliakberza · Answer

Hi  aakash214 . Thanks for posting this here.

I came across this question on GMAT Prep took and took a long time struggling with it before figuring it out. It is definitely one of the harder ones I have come across!

Firstly let's list down what each of them is saying:

GKs are one of many species of large mammals 
 Went extinct 46,000 years ago 
 Evidence from from teeth show they fed mostly on saltbrush 
 Saltbrush thrives in arid climates 
 We know climate was becoming more arid at the time  
 The aridity will help heir main food thrive 
 The aridity should in fact help the species thrive, because their main food source will thrive NOT make them extinct 
 Hence if it wasn't the climate it HAD to be something else and humans are the most likely.

This argument alone is not enough to convince researchers of  your conclusion  
 Is there any other evidence you have to support your claim?

So Marco lists down a bunch of facts about GKs, their habitats and the climate of the time they lived and concludes that increasing aridity of the time means that external factors such as humans were responsible for GK's extinction. Fatima is largely skeptical of the tall claims and asks for more evidence.

Questions asks:  Select Marco for the statement that, if true, most justifies Marco’s assertions, and select Fatima, for the statement that, if true, most justifies Fatima’s skepticism about Marco’s assertions. Make only two selections, one in each column.

So we are looking for facts that SUPPORT Marco's and Fatima's assertions (one option each), choices are:

Yes! Marco does note that the climate, at the time the GKs went extinct, was becoming increasingly arid and dry. It was not the main crux of the arguement but was part of the evidence used to support the conclusion for sure. Lets hold on to this for now as a candidate supporting Marco's assertion.
 
This is a perfect candidate for a fact that supports Fatima's skepticism, if humans arrived only 40 thousand years ago they could not possibly have been the cause of GK extinction 46 thousand years ago! Seems like a good match for Fatima! 
 
Does not help support either of the people's arguments.
 
This is actually irrelevant to what Marco has to say, simply because Marco says that their main diet saltbrush was in ample supply and only becoming more widely available due to the increasing aridity of the climate. If GKs did indeed eat something else as well, it really does not support Marco because he has already concluded based on available evidence that their main food source was in ample supply. 
 
Extinction of other larger megafauna are not relevant to either of their discussions.

You will see that once you rule out the improbable and unrelated choices only A and B match with Marco's and Fatima's assertions respectively.

Hope this helps. I can't say for sure but this seems like one of the harder IR Qs. I got it wrong too when I was taking the practice test myself.

nhatanh811 · Answer

Another reason why A will strengthen Marco's assessment is that it strengthens the argument that high aridity does not cause KG's extinction, since KG extinct in the period that is less arid than prior periods.

LILICHKA · Answer

I don't understand, why these are not the correct options in the exam answers. These are the only logical ones...

nhatanh811 · Answer

I am not one of the experts and tbh, I have a similar concern as you do so I will try my best to play devil's advocate on why A justifies Marco's statement

: 1. Notice the premise "the increasing aridity of the climate at the time" here, it is a premise so we should treat it as factual information. That means at that time, the shrub should increase even if the number of shrubs available is less than before (due to statement A shows less arid weather than previous periods). So GK's food supply will not be adversely affected 2. If we think of the Marco argument that High Aridity does not cause GK's extinction (since High Aridity = Plenty of food) then A makes sense because GK survived even more arid periods. Would love to hear from the experts AjiteshArun AndrewN GMATNinja on whether my rationale is sound!

AndrewN · Answer

I agree,  nhatanh811 . The notion behind (A) is that  less arid  cannot be equated to  not arid at all , and the primary food source of the giant kangaroos, saltbrush shrubs, could have endured well enough in less arid climates. Point 1 is to be taken as a true statement in assessing the argument Marco puts forth. Altogether, we can picture a V-shaped graph of aridity, perhaps, one in which each tip of the V represents maximum aridity, and the valley represents the least arid climate the giant kangaroos may have endured. If the timeline of the graph proceeded from left to right, then say that the kangaroos died in the middle of the right-hand side of that V. Both the premise from the passage and the answer choice given here could still hold true: the kangaroos had endured more arid periods in the past, but there was also an increasing aridity at the time of their extinction.

I think the process of elimination can help considerably here, since we know there can only be one choice for each person. (B) is easy to identify with Fatima. Meanwhile, (C) and (E) pursue a similar line of thought: other animals went extinct around the same time as the giant kangaroos; if there is nothing to separate the answers, then neither can be correct. Choice (D) does touch on the food supply, but it backs the idea that  other plants , not the one Marco has built his argument upon, may have allowed the kangaroos to survive. Thus, it cannot support the assertions Marco has made. This is GMAT™ linear logic at its best.

I hope that helps. Thank you for bringing me into the dialogue.

- Andrew

sidgvpta · Answer

While this is the OA, and I would be inclined to agree, I think this question is possibly in some way flawed- Nowhere in the original question is Australia mentioned, and (B) makes the assumption that Australia is where these giant kangaroos existed. The giant kangaroos could easily have existed in some other location, making this comment completely irrelevant.

Would love to hear more thoughts on this!

jasminelin · Answer

A is correct for Fatima because if true, it follows that the period was less arid so there was less food so the kangaroos died from lack of main food source (not humans). 
D is correct for Marco because if true, it means that kangaroos had other food to eat as well so food, whether or not the period was arid and whether or not the plants died, is not the issue.

why is this wrong? and how do we know the true answer?

AndrewN · Answer

I am sorry to say,  jasminelin , that both of your answers are incorrect. The  true answer  is the one provided by GMAC™, since this is an official question. I would suggest you read through the responses above and, if necessary, those to  the same question in the Beat the GMAT forum . If you still have questions or doubts, feel free to post them.

- Andrew

Tanchat · Answer

I read all comments but I don't understand why (A) is correct.

In the passage : The deposits and wear patterns on the teeth of these animals from around the time of their extinction indicate that they fed mostly on saltbrush shrubs.
So, mostly on saltbrush. So, it would have others that this Kangaroo can eat. Thus, it is D.

AndrewN · Answer

Hello,  Tanchat . If you have read all the comments but (D) still looks reasonable to you to support Marco's assertions, then I would recommend revisiting this thread in another week or so so that you can look at it with fresh eyes. I see several fine responses above, and there are others in the Beat the GMAT forum that I linked to in an earlier post. Moments of insight often occur when you are not actively pursuing them.

- Andrew

Abhishekgmat87 · Answer

Saltbrush thrives in arid climates, so it is not likely that the kangaroos’ food supply was adversely affected by the increasing aridity of the climate at the time. Thus, something else would have to account for their extinction, and the best candidate for that cause is predation by humans.

Saltbrush thrives in arid climates but it might not thrive in increased arid climates. Option A tells us that the aridity is not the reason for their extinction as they got extinct during less arid climate. It supports that something else is the reason for their extinction.

slueders · Answer

I must say... Horrid question

I see why A is the correct answer for Marco but the question does imply that Saltbrush "thrives" in arid climates with no evidence that it survives in less arid climates... Moreover, answer D states that there would be additional sources of food, therefore, supporting Marco's reasoning that there was an alternative cause for the Giant Kangaroo's extinction. This indirectly supports the argument that Giant Kangaroos did not die of starvation, therefore, increasing the chances that an alternative argument would be correct. This would boost Marco's argument. Sorry GMAC, you're wrong.

PReciSioN · Answer

Sajjad1994  - The original question in official guide does mention Australia in the stimulus, but it is not mentioned here which seems to be causing some confusion amongst people. Can we fix this please?

Sajjad1994 · Answer

Can you share a screenshot for the same?

Thank you!

PReciSioN · Answer

https://gmatclub.com/forum/download/file.php?id=81919 GMAT-Club-Forum-s577507t.png

KarishmaB · Answer

Marco - 
Giant kangaroos  went extinct in Australia around 46,000 years ago.
Their teeth, from around the time of their extinction, indicate that they fed mostly on saltbrush shrubs.
Saltbrush thrives in arid climates, so increasingly arid climate would have suited it.
Conclusion: Something else would have to account for their extinction, and the best candidate for that cause is predation by humans.

Fatima -
That argument alone is not likely to satisfy many researchers in this field. Have you found any other evidence to bolster your conclusion?

Basically Fatima is saying that I don't agree with your conlcusion.

Look at the options now and notice that options (C) and (E) are the same.

Approximately 60 different species in Australia died out in the wave of extinctions around 46,000 years ago.
Several types of megafauna larger than the giant kangaroo went extinct around 46,000 years ago.

They both are saying that several other species also went extinct at that time. Do we need to worry about whether there is some connect such as "Food source then may not have been the problem but something more large scale etc.?" No. Because both options say the same thing so neither can be the answer.

Giant kangaroos became extinct during a period that was less arid than previous periods they endured.

This strengthens Marco's conclusion that food scarcity was not the problem. That increasingly arid climate did not cause food scarcity. The kangaroos survived more arid periods in the past so why would this be any different. The cause must be something else. 
 MARCO - ANSWER

Many researchers believe humans first arrived in Australia around 40,000 years ago.

If this is true, then humans could not have been the reason for the extinction. 
This is against Marco's view and hence supports Fatima.
 FATIMA - ANSWER

Fossils of giant kangaroos also show evidence that those animals' diets routinely included plants other than saltbrush.

Irrelevant. Perhaps they did include other plants. But what we know is that "Their teeth, from around the time of their extinction, indicate that they fed mostly on saltbrush shrubs." Around the time of their extinction, they fed on saltbrush shrubs only and those were plenty. So then they shouldn't have died out. What they fed on at other times is irrelevant.

Marco: Giant kangaroosone of several extinct species of large mammals

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Marco	Fatima
		Giant kangaroos became extinct during a period that was less arid than previous periods they endured.
		Many researchers believe humans first arrived in Australia around 40,000 years ago.
		Approximately 60 different species in Australia died out in the wave of extinctions around 46,000 years ago.
		Fossils of giant kangaroos also show evidence that those animals' diets routinely included plants other than saltbrush.
		Several types of megafauna larger than the giant kangaroo went extinct around 46,000 years ago.

GMAT Two-part Analysis (TPA) Questions

Marco: Giant kangaroosone of several extinct species of large mammals

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