Answer:
Trilobites
Step-by-step explanation:
The Phanerozoic Eon is characterized by the presence of organisms that possessed hard shells as an adaptation to the environmental conditions. Among these organisms, the most common hard-shelled animals were the trilobites.
Trilobites were present during most of the Paleozoic Era, which belongs to the Phanerozoic Eon. These animals were marine organisms with remarkable and unique phenotypes.
The body of a trilobite was composed of three segments: the cephalon, the thorax, and the pygidium (tail). In addition, they possessed an exoskeleton composed of chitin, which was the reason behind their hard shells. They shed their exoskeletons to be able to grow.
Trilobites are currently extinct.