GMAT Critical Reasoning (CR) | Re: A large meteorite impact crater in a certain region

This is a confusing passage, so let's start with the conclusion: "the crystalline structure of rocks recovered at the[impact] site indicates that the impact that formed this crater was not the culprit" responsible for the mass extinction of plant and animal species that occurred at the end of the Mesozoic era.

What is the point of this passage?

The first sentence describes an explanation that had previously been accepted: "A
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