Final answer:
The major class within the subphylum Trilobita is Class Trilobita. Trilobites were diverse and widespread during the Paleozoic era. Their fossils are used to determine relative ages of rocks.
Step-by-step explanation:
The major class within the subphylum Trilobita is Class Trilobita. Trilobites are extinct arthropods that lived during the Paleozoic era. They were among the most well-known fossils and had three major body sections: cephalon, thorax, and pygidium. Trilobites were diverse and widespread, and their fossils are used to provide relative ages of rocks.