Trilobite

 Trilobites are a group of arthropods with many body segments. The name "trilobite" comes from the fact that the body is divided into three sections, the pleural, axial, and pleural lobes, in the vertical direction. Trilobites are closely related to the horseshoe crab, which is a "living fossil".
 Trilobites appeared in the early Paleozoic Cambrian Era and flourished in the first half of the Paleozoic Era, playing a leading role in the shallow seas of that time. They are divided into many species, with more than 10,000 known. However, they gradually began to decline from the Silurian Period of the middle Paleozoic Era and became extinct by the end of the Paleozoic Era. The most popular view of the cause of the extinction of Paleozoic organisms, including trilobites, is that it was caused by volcanic activity in the Earth's interior.
 Many trilobite fossils have been found in Paleozoic strata, and they play an important role in determining the age of the strata. They vary in size and shape, and some fossils suggest that they adopted a defensive posture with their bodies curled up.