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
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: