Which trilobite characteristic did not change through time?

The answer is a presence of three lobes.

The name trilobite means three lobes. Their body is divided into three lobes - a central axial and two pleural lobes. This characteristic did not change through time and it can be found between all fossils. But, some changes are noticed regarding the eyes size, shape, and position, suture position, as well as a number of body segments and spine.