Answer:
Something changed that caused them to be unsuccessful in their environment
Step-by-step explanation:
The trilobites are a very old organism that existed in the distant past. The trilobites were one of the dominant animals during their existence, prospering for quite a long time, and being spread over a very large area. But these animals are no longer on the planet, and the reason for that is that something dramatic happened that caused significant change in their environment. Because of the dramatic change of their environment, the trilobites were not able to adapt themselves to the rapidly created new living conditions, leading to their extinction because they were not well suited for those new conditions.