No one knows for sure what caused the dinosaurs to become extinct.

(Grammar Questions I Couldn't Answer)

Recently I was teaching the present perfect in unit 6 of Expert IELTS 6.  

There was a supplement workbook that contained written exercises in which the students had to distinguish between the present perfect and the past simple.  The teaching assistant assigned it as homework, and was marking the answers.

One of the exercise was:

No one knows for sure what _________________ the dinosaurs to become extinct.  ( Expert IELTS 6 Workbook, page 59, exercise 4, number 5.)

Students had to choose between the past simple and present perfect.  A number of students chose the present perfect ("has caused"), but the answer key of course said "caused".

The teaching assistant, who was Vietnamese, approached me, and asked if perhaps the answer key might be mistaken.

"No," I said.  "The answer is definitely 'caused'."

"Are you sure?" she asked.  "We use the past simple for definite times in the past, and the present perfect for the unspecified times in the past.  So shouldn't the present perfect be acceptable here?"

"No, the present perfect is definitely wrong," I said.  "I mean, yes, you are right.  That is the rule that we teach for the present perfect.  But  the present perfect is definitely wrong here.  I can't tell you why, but it just sounds wrong."   (Sometimes the fact that native speakers make grammaticality judgements based solely on native speaker intuition is difficult to explain to non-native speakers.)

I promised the  TA I would research the answer, and then get back to her.

I asked around the staffroom, and got 2 different answers from colleagues:

"pres.perfect connects past and present time situations/statuses. The extinction is simply an event (sudden climate change) / immutable fact, therefore past simple is more suitable."

and 

"I think the 'exact time' is contained in 'what' - the something that happened in the past. I guess you could say 'what has happened to Bob', though..."

and

Is it also that they are making a choice between past and recent past? If it was the rhinos in Vietnam, we might say that there are many reasons that have caused them to become extinct? But dinosaurs are long gone?

I showed all these responses back to the teaching assistant.  "Well, there you go," I said to her.  "We've got three answers.  The trouble is, they're all different answers.  I don't know if that helps you at all, or just makes things more confusing."

She indicated it just made things more confusing.

"If I had to choose," I said, "I think the first explanation--the one about how the extinction is just an event in the past--is probably the right answer."

"The thing is," she said, "we use the present perfect when there's a past action with a present result, right?"

"Ah, I see what you mean," I said.  "You mean that because there aren't any dinosaurs alive now, then that's the present result."

She nodded.

"The thing is," I said, "in the actual sentence, that present result isn't being emphasized.  Arguably all past actions have present results.  But we only use the present perfect when the speaker is thinking about that present result."

Admittedly, I was just making stuff up as I spoke at this point.  But that was the best explanation I could think of on the spot.

What do you guys think?  Does that sound  reasonable, or am I off base here?