Enrolment is a unique feature among many families of trilobites but not all species could do this. Enrolment is the curling up of the body to form a "ball." It is thought to be a protective mechanism against predators, and sudden changes in environments. A recent study (Bruton and Haas, 1997) in Phacopsids suggests that enrolment is triggered by any event leading to changes in water and sediment changes.

There are six kinds of enrolments: incomplete, cylindrical, spheroidal, inverted spiral, spiral, uncoiled and basket and lid enrollment (Levi-setti, 1993).