Trilobites: Variations on a Theme

Over 300 million years, trilobites evolved a diverse and successful array of forms while maintaining a simple, common body plan. 

Three vertical lobes

BODY PLAN  Trilobites had three major sections from head to tail and three vertical lobes (trilobite means “three lobed”). Most species were able to roll into a ball to protect their antennae and soft underparts.

Cephalon

(head)

Eye

Enrollment

Segmented

thorax

Pygidium

(tail)

Stages of growth from

zero to six segments

GROWTH  Trilobites grew by molting, adding new segments and then splitting and discarding their old shells.

Stages of molting

VARIATIONS  Researchers have identified some 20,000 trilobite species. Representative specimens from 50 genera are shown below.

Agnostus

Lejopyge

Pagetia

Pleuroctenium

Yukonia

Bohemilla

Nileus

Novakella

Pricyclopyge

Robergia

Bathyuriscus

Harpillaenus

Kolihapeltis

Panderia

Zacanthoides

Acanthopyge

Ceratarges

Lobopyge

Terataspis

Uralichas

Boedaspis

Dicranurus

Kettneraspis

Miraspis

Selenopeltis

Bathycheilus

Ceraurus

Cheirurus

Deiphon

Encrinurus

Hollardops

Huntoniatonia

Odontocephalus

Phacops

Quadrops

Carolinites

Cheiropyge

Dimeropyge

Phaetonellus

Telephinus

Balnibarbi

Leptoplastus

Peltura

Pemphigaspis

Tricrepicephalus

Cambropallas

Emuella

Olenelloides

Olenellus

Peachella