Correct answer: Both changing environments and competition for survival.
Trilobites are fossil group of extinct marine arthropods that are known to form the class trilobita. Trilobites are the creatures that are known to live 526 million years ago in the earth's seas. Now they become the important cultural icons.
The trilobite is the marine animals that first known to appear in the Cambrian period when they dominated the seas. There is a change in the exoskeleton of trilobite to save them self