The astronomical event that is now thought to have occurred some 65 million years ago, resulting in the death of a large fraction of all living species and leaving a layer of clay containing an enhanced concentration of a rare metal, iridium, in the geological record in rocks throughout Earth was a(n):

Asteriod

Explanation:

The astronomical event called the K-T event (Cretaceous-Tertiary Mass Extinction event) that is now thought to have occurred some 65 million years ago, resulting in the death of a large fraction of all living species and leaving a layer of clay containing an enhanced concentration of a rare metal, iridium, in the geological record in rocks throughout Earth was an Asteroid.

This asteroid is thought to have been responsible for the death of the last of the dinosaurs and created a crater at the point of impact in the Yucatan region of Mexico. This crater is called the Chicxulub crater and has been age dated (by the 40Ar/39Ar method) at 65 million years.

It is said to be an 180-km diameter ring structure.

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