Until the 1970s, the pattern of early marine animal evolution seemed t

Question

Until the 1970s, the pattern of early marine animal evolution seemed to be well established. Most present-day animal phyla had appeared during the “Cambrian explosion,” an extraordinary burgeoning of multicellular life in the warm seas of the Cambrian period, between 570 and 500 million years ago. It was assumed that, despite the very large number of species that appeared during the Cambrian explosion, nearly all fit into the same rather small number of phyla that exist today. Each phylum—a group of organisms with the same basic pattern of organization, such as the radial symmetry of jellyfish and other coelenterates or the segmented structure of worms and other annelids—was seen as evolutionarily stable. Innumerable individual species have arisen and died out, but development and extinction were assumed to take place within existing phyla; the elimination of entire phyla was thought to be extremely rare.

However, a diverse group of marine fossils, known collectively as the “Problematica,” presented difficulties for this interpretation. The Problematica show patterns of organization so bizarre that it is hard to fit any of them into present-day phyla. They include the banana-shaped Tullimonstrum and the spiked, spiny Hallucigenia, creatures whose very names reflect the classifier’s discomfort. The “Ediacaran fauna,” which respired, absorbed nutrients, and eliminated wastes directly through their external surfaces, are also included among the Problematica. Theirs was an approach taken by only a few modern multicelled creatures (such as tapeworms) that are otherwise totally unlike them.

Recently, several theorists have argued that the Problematica are not just hard to classify—they are evidence that the conventional view of the Cambrian explosion is wrong. They contend that the Cambrian explosion represented the simultaneous appearance of a much larger number of animal phyla than exists today. Each was a separate “experiment” in basic body design, and the Cambrian seas teemed with many different phyla, or basic body plans, each represented by only a few species. Today, the number of phyla has fallen drastically, but each surviving phylum contains a much larger number of species. The Problematica, then, were not unsuccessful variants within present-day phyla; each represented a distinct phylum in its own right.

Revisionists and conventional theorists agree that modern marine species are products of natural selection. But the revisionists contend that the selection process eliminated not only particular unfavorable traits, but entire body plans and approaches to survival. The Ediacaran fauna, for example, represented a particular structural solution to the basic problems of gas and fluid exchange with the environment. This approach to body engineering was discarded at the same time as the Ediacaran fauna themselves were wiped out; given the improbability of duplicating an entire body plan through chance mutation, it was unlikely that this particular approach would ever be tried again.    1 
 1. The author implies that revisionists would view efforts to classify the Problematica in present-day phyla

A. enthusiastically
B. optimistically
C. skeptically
D. with indifference
E. with outrage

2 
 2. The description in the latter half of the second paragraph of how the Ediacaran fauna carried out respiration, absorption, and excretion tends to support the view that they

A. were probably not members of any present-day phylum
B. had physiologic processes different from those of any other known organisms
C. could not absorb or excrete fluids
D. were members of the same phylum as Tullimonstrum
E. were closely related to tapeworms

3 
 3. The passage implies that present-day phyla contain

A. only a few species each
B. species more dissimilar than many phyla in the Cambrian period
C. many species showing basic structural similarities
D. species that undergo no evolutionary change
E. species that have existed continuously since the Cambrian period

4 
 4. The author mentions coelenterates and annelids in order to give examples of

A. phyla that died out because their body plans were not viable
B. the structural patterns characteristic of some modern phyla
C. phyla that are closely related to the Problematica
D. phyla that have evolved since the Cambrian period
E. groups of organisms that do not fit into conventional evolutionary models

5 
 5. The passage implies that conventional and revisionist theorists disagree about all of the following EXCEPT

A. the accuracy of the conventional view of early marine evolution
B. the probable number of marine animal phyla during the Cambrian period
C. the likelihood of entire phyla becoming extinct
D. the applicability of the theory of natural selection to the Cambrian period
E. whether or not the Problematica can be assigned to present-day phyla

6 
 6. According to the passage, the Problematica are difficult to classify because

I. some had unusual shapes
II. some of them functioned physiologically differently from modern organisms
III. they became extinct at the end of the Cambrian period

A. I only
B. II only
C. I and II only
D. I and III only
E. I, II, and III

Kaplan GRE Verbal Workbook 2007

Sajjad1994 · Answer

Official Explanation

4. The author mentions coelenterates and annelids in order to give examples of

Explanation

This is another logic question. These organisms are mentioned in the middle of  paragraph 1 , when the author defines phylum as  “a group of organisms with the same basic pattern of organization,”  such as the  “radial symmetry of jellyfish and other coelenterates.”  The author is giving familiar examples to illustrate the concept of a pattern of organization, and jellyfish and worms are present-day creatures.  Choice (B)  captures this idea. These phyla have not died out, so you can eliminate  choice (A) . As they are present-day phyla, they are not closely related to the Problematica  (choice (C) . The passage never mentions when these organisms evolved  (choice (D) . Since they are mentioned in paragraph 1, which deals with the conventional view, they do fit conventional models  (Choice (E) .

Answer: B

Sajjad1994 · Answer

Official Explanation

6. According to the passage, the Problematica are difficult to classify because

Explanation

This is a detail question, so go to  paragraph 2  in which the Problematica are discussed.

Statement I is true ; the Problematica had unusual  “patterns of organization” —that is, unusual shapes—which make them difficult to fit into modern phyla.

Statement II  is also true; the second half of  paragraph 3  notes that the unusual physiology of the Ediacaran fauna place it within the Problematica.

Statement III  is not mentioned in the passage; we don’t know when these organisms became extinct.

Answer: C

Sajjad1994 · Answer

Official Explanation

3. The passage implies that present-day phyla contain

Explanation

This is an inference question about present-day phyla, so look in the passage in which modern phyla are mentioned. A phylum (as stated in the middle of paragraph 1) is a group of organisms with  “the same basic pattern of organization,”  that is, possessing similar structures; today, there is a small number of phyla, each containing a large number of species (fourth sentence of paragraph 3). Putting these two points together gives us  choice (C) .

Choice (A)  is the situation that existed during the Cambrian, according to the revisionists.

Choice (B)  is also out; diversity within phyla is not mentioned.

Choices (D)  and  (E)  are contradicted by the discussion of natural selection in paragraph 4: All species undergo evolutionary change, and few, if any, have existed continuously for 500 million years.

Answer: C

Matty101 · Answer

Question 1

I do not have the official explanation. However, this is my take.

The correct answer is  C. skeptically . Revisionists view attempts to classify the Problematica within present-day phyla with skepticism because they believe these organisms are too distinct to fit within existing phyla. Instead, revisionists argue that each member of the Problematica likely represents a separate phylum entirely, reflecting a unique body plan that differs fundamentally from those of modern phyla. This view contrasts with the conventional approach, which assumes that nearly all Cambrian organisms can fit within present-day phyla.
 
 Key sentences that support this view include:

"Recently, several theorists have argued that the Problematica are not just hard to classify—they are evidence that the conventional view of the Cambrian explosion is wrong."  
  "The Problematica, then, were not unsuccessful variants within present-day phyla; each represented a distinct phylum in its own right."

chandy123 · Answer

Answers and Explanations:

Question 1:

The author implies that revisionists would view efforts to classify the Problematica in present-day phyla:

Correct Answer: C. skeptically

Explanation:

The revisionists argue that the Problematica represent entirely new phyla that do not fit into present-day categories. Thus, any attempt to classify them within current phyla would likely be viewed with skepticism.

Question 2:

The description in the latter half of the second paragraph of how the Ediacaran fauna carried out respiration, absorption, and excretion tends to support the view that they:

Correct Answer: A. were probably not members of any present-day phylum

Explanation:

The Ediacaran fauna used a structural solution (direct exchange through external surfaces) not found in most modern organisms. This unique approach supports the idea that they do not fit into any existing phyla.

Question 3:

The passage implies that present-day phyla contain:

Correct Answer: C. many species showing basic structural similarities

Explanation:

The passage explains that present-day phyla have many species sharing a common structural design, unlike the Cambrian period, when each phylum contained fewer species.

Question 4:

The author mentions coelenterates and annelids in order to give examples of:

Correct Answer: B. the structural patterns characteristic of some modern phyla

Explanation:

Coelenterates and annelids are examples of present-day phyla with distinct structural patterns, such as radial symmetry (coelenterates) and segmented bodies (annelids).

Question 5:

The passage implies that conventional and revisionist theorists disagree about all of the following EXCEPT:

Correct Answer: D. the applicability of the theory of natural selection to the Cambrian period

Explanation:

The passage explicitly states that both groups agree that modern species are products of natural selection, implying that natural selection applies to the Cambrian period as well.

Question 6:

According to the passage, the Problematica are difficult to classify because:

Correct Answer: C. I and II only

Explanation:

The Problematica are described as having unusual shapes (e.g., Hallucigenia) and physiological processes unlike those of modern organisms (e.g., Ediacaran fauna). However, the passage does not state that all Problematica became extinct at the end of the Cambrian period (III is incorrect).

Summary of Correct Answers:

1. C
2. A
3. C
4. B
5. D
6. C

guddo · Answer

1. The author implies that revisionists would view efforts to classify the Problematica in present-day phyla

The passage says conventional theorists tried to fit the Problematica into present-day phyla. Revisionists disagree. They argue that the Problematica were not odd variants within modern phyla, but separate extinct phyla in their own right.  So revisionists would doubt attempts to classify them within present-day phyla .

A. enthusiastically

This is incorrect. Revisionists would not welcome such efforts, because those efforts go against their view.

B. optimistically

This is incorrect. Revisionists do not think the Problematica can probably be placed successfully into present-day phyla.

C. skeptically

This is correct. Revisionists believe the Problematica represent distinct extinct phyla, not members of present-day phyla. So they would view classification attempts within modern phyla with  skepticism .

D. with indifference

This is too weak. The classification issue is central to the disagreement between revisionists and conventional theorists.

E. with outrage

This is too strong. The passage suggests disagreement, not anger.

Answer: (C)

Until the 1970s, the pattern of early marine animal evolution seemed t

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GMAT Reading Comprehension (RC) Questions

Until the 1970s, the pattern of early marine animal evolution seemed t

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615 to 715 in 15 Days (Ria's Strategies)

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