In many parts of the world, there is a rock layer rich in the metal iridium. Iridium is very rare on Earth, but it is relatively common in meteorites and other debris that comes from outer space. All occurrences of this iridium layer have an age of about 65 million years. Most of the fossils in rock layers below the iridium layer, including dinosaur fossils, do not appear in layers above the iridium layer.

Question

Based on this evidence from the rock and fossil record,
A.
an asteroid impact likely caused a mass extinction 65 million years ago.
B.
meteorites and other space debris did not impact the Earth 65 million years ago.
C.
an asteroid impact caused dinosaurs to become more common 65 million years ago.
D.
meteorites and other space debris no longer contain the metal iridium.

Answer 1

Based on the evidence provided from the rock and fossil record, the most logical conclusion is:

A. an asteroid impact likely caused a mass extinction 65 million years ago.

This conclusion is supported by the presence of the iridium layer, which is associated with extraterrestrial sources, and the observed extinction of many species, including dinosaurs, immediately following that layer in the geological record.