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

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. 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. 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. 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. 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. 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. 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