There is little known about the ancestor of trilobites because trilobites were among the first shelly fossils found in early Cambrian. Trilobites were possibly derived from the same ancestor of annelids.

Eldredge (1977) and Fortey and Owens (1990) summarized several evolution patterns, including the increasing specialization of complex eyes, improved enrolment, increased size and proportion of pygidium. Spines had developed in some species.

However, eyes in some separate taxa disappeared. This suggests that organism is not necessarily evolved into more complex form. It is a misconception that evolution always make "better" organisms.

A recent study suggests that the morphological diversity increases even the total number of the family decreases over time (Foote,1997).

Development of spines in some taxa. Image source: www.trilobites.com