| Palæos: |  |
Trilobites |
| INVERTEBRATES | Trilobite Characteristics |
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Trilobites are the most diverse of the extinct arthropod groups, known from perhaps 5000 genera. The classification of trilobites within the Arthropoda has generated much controversy, much of which is still not completely resolved (see above). Whatever their higher position among Arthropoda, there are a number of characters that distinguish trilobites from within the arachnomorph clade, the most significant noted below:
| eye
ridges: These are consistently present in
primitive trilobites, connecting the front of the palpebral lobe with the axial furrow (a feature lost in many post-Cambrian trilobites) |
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pygidium: The posterior tagma of greater than one segment is a conspicuous feature of all trilobites. Pygidia are typically very small in primitive forms (e.g., Olenellina)
Together with the organization of
the body into three anterior-posterior divisions (cephalon, thorax, and
pygidium), and the three longitudinal lobes (axial lobe and two flanking
pleural lobes), the body features on this page serve to readily distinguish
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compound
eyes:
While other compound eyes are
found in Cambrian arthropods, only those of trilobites have corneal surfaces composed of prismatic calcite lenses (with the crystallographic axis normal to the lens surface). circumocular sutures:
In Cambrian holochroal
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| rostral plate:
a ventral anterior plate separated
from the rest of the cephalic doublure by sutures is very well developed in primitive trilobites (e.g., Redlichiida), narrower in other trilobite orders, and secondarily lost in some advanced forms (e.g., Asaphida and Phacopida) 
all images ©2000 by S. M. Gon III unless otherwise noted |
hypostomal wings:
The
trilobite
hypostome may be homologous to the labrum in Crustacea, but all trilobite hypostoma bear anterior wings which fit in pits in the anterior axial glabellar furrows (or homo- logous locations). calcified cuticle: Trilobites
bear a
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