Major Trilobite Features
This page last revised 20 July 2008 by S. M. Gon III

The major trilobite body features that are typically preserved are summarized and illustrated below. These hard pieces of exoskeleton are preserved readily because of their mineral content, high in calcite (CaCO3). Most of these terms for major body features figure prominently in trilobite descriptions. I provide more detailed charts of trilobite dorsal and ventral morphological terms you can access among other choices below. A glossary gives further definitions of terms, and there is a page dealing with special terms that apply only to particular trilobites.

 ©1999 by S.M. Gon III

The trilobite body is divided into three tagmata (major sections), a cephalon  with eyes, mouthparts and antennae, a thorax of multiple articulated segments (that in some species allowed enrollment), and a pygidium, or tail section of fused segments.
3 lobes 
The name "trilobite" (meaning "three-lobed") is not based on the tagmata (cephalon, thorax, and pygidium), but on the three longitudinal lobes: a central axial lobe, and two pleural lobes that flank the axis.
cephalic terms
When describing differences between different taxa of trilobites, the presence, size, and shape of the cephalic features above are often mentioned. To the right are cephalic (cranidial) features described when the librigenae are missing.
The cheeks (genae) are the pleural lobes on each side of the axial feature, the glabella. When trilobites molt or die, the librigenae (the so-called "free cheeks") often separate along the facial sutures, leaving only the cranidium --that is, the glabella and fixigenae ("fixed cheeks").
genal spines

Terms for genal spines: while the typical  spine placement is at the genal angle (the lateral posterior corner of the cephalon), where it is called simply a genal spine, other spine locations may be anterior (pro) or adaxial (meta) of the genal angle, and are further defined by their placement on either the fixigena (shwon in yellow here) or the librigena (in purple). Note that a prolibrigenal spine might occur close to the genal angle, or be placed far forward along the anterior margin of the cephalon. A profixigenal spine is usually on the anterior margin. In some trilobites, a series of small spines might be present along the genal margin (e.g., see  specimens in the family Odontopleuridae)
Emuelloid features

In some trilobites, the thorax is divided into an anterior prothorax followed by an opisthothorax, often with many segments. In the example above, the emuelloid trilobite Balcoracania has a six-segmented prothorax (the sixth segment is macropleural), followed by a many-segmented opisthothorax. The pygidium is a minute, segmented disc at the end of the animal.

A superb specimen of Balcoracania dailyi from the collection of David Simpson, South Australia. Please refer to his website of Australian Lower Cambrian Trilobites for other specimens of Balcoracania and its Cambrian contemporaries.

all figures above ©1999-2008 by S.M. Gon III  created with Macromedia Freehand

stroll with me among the topics below...
Walking Trilobite animation ©2000 by S. M. Gon III