Scientists found many different types of fossils in a rock layer, which was dated 251-299 million years old using an index fossil. In layers directly above that point, scientists found a large decrease in the number of fossils, particularly of marine and insect fossils. When did the extinction event occur?

The extinction event occurred at the end of the Permian and the beginning of the Triassic, 251-250 million years ago.

Explanation:

The period that is described shortly, lasting from around 299 to 251 million years ago, was the Permian. This is a period in which life thrived on Earth, with big diversification among many different animal and plant groups. During this period, there has been an abundance of synapsids, anapsids, and diapsids, marine animals, large insects, etc., and numerous different types of plants dominating the landscape.

This all came to an end between 251-250 million years ago when the biggest mass extinction in the geological history of Earth happened, which is why it is referred to as the Great Dying. The estimates are that around 70% of the terrestrial species went extinct, and around 81% of the marine species.  What is very interesting and unexpected is that the biggest losses were in the species of insects, despite being animals that tend to be the toughest and most resilient. There are several theories about what caused this mass extinction, and it is most likely a combination of a supervolcano eruption that trigger volcanoes around the world to erupt as well, and probably a collision with a space body.