The coelacanth—a prehistoric fish species believed extinct for 66 million years until a live specimen was discovered in 1938, along...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

• The coelacanth—
• a prehistoric fish species
  • believed extinct for 66 million years
  • until a live specimen was discovered in 1938,
• along with numerous fossils and ancient DNA samples [?]
• continues to fascinate evolutionary biologists today.

• [?] = what punctuation should follow "ancient DNA samples"
  • A: —
  • B: (nothing)
  • C: ,
  • D: ;

Understanding the Meaning

Let's start reading from the beginning:

'The coelacanth—'

• Right away we see an opening em dash after the subject
• This signals that some descriptive or explanatory information is coming

Then we get:

• 'a prehistoric fish species believed extinct for 66 million years until a live specimen was discovered in 1938, along with numerous fossils and ancient DNA samples'
• This whole phrase is telling us about the coelacanth—
  • what it is (a prehistoric fish)
  • what happened to it (believed extinct for 66 million years)
  • what changed that (live specimen discovered in 1938, plus fossils and DNA samples found)

Now we've reached the blank. Let's look at our choices—they're asking us what punctuation should come after "ancient DNA samples."

To see what works here, let's read the rest of the sentence!

• 'continues to fascinate evolutionary biologists today.'
• Ah! This is a verb with the rest of the main clause.

Now let's understand the complete structure. What do we notice?

• We have: The coelacanth [opening em dash] descriptive information [blank] continues to fascinate...
• The main sentence, without the interruption, would be:
  • "The coelacanth continues to fascinate evolutionary biologists today."
• Everything between the opening em dash and our blank is descriptive information that interrupts the main sentence—sitting between the subject (coelacanth) and its verb (continues).

Here's the key relationship:

• When you open an em dash to insert descriptive information in the middle of a sentence, you need to CLOSE it with another em dash
• Think of em dashes like parentheses—they work in pairs
• The closing em dash signals "okay, interruption over, back to the main sentence"

So we need Choice A: the closing em dash (—) to match the opening one and return us to the main sentence.

GRAMMAR CONCEPT APPLIED

Using Paired Em Dashes for Interrupting Information

When you use an em dash to insert descriptive or explanatory information in the middle of a sentence (called a parenthetical element in grammar terms), you need a second em dash to close that interruption and return to the main sentence. The em dashes work as a pair, like bookends or parentheses.

Pattern:

• Main sentence interrupted: The discovery—which surprised everyone—changed the field
  • Subject: The discovery
  • Interrupting information: which surprised everyone
  • Closing dash brings us back to: changed the field
• Without the interruption: The discovery changed the field
  • This is the core sentence that would work on its own

In our question:

• Core sentence: The coelacanth continues to fascinate evolutionary biologists today
• Interruption added: a prehistoric fish species believed extinct for 66 million years until a live specimen was discovered in 1938, along with numerous fossils and ancient DNA samples
• Complete structure: The coelacanth—[interruption]—continues to fascinate...

The opening em dash signals "here comes extra information" and the closing em dash signals "interruption over, back to the main point." You need both to maintain clear sentence structure.