The figure below shows the underside of a typical trilobite fossil, with cephalic, thoracic, and pygidial doublure, the rostral plate and associated sutures, and hypostome. The dark gray area represents the hollow interior of the dorsal shell. In this case, the hypostome is separated from the rostral plate, which is called the natant condition. In other species, the hypostome may be connected to the rostral plate (the conterminant condition), separated only by a suture (as in the ventral reconstruction of Olenoides serratus below), or even fused to the rostral plate. These hypostome types are important in trilobite classification. 

If you want more detailed definitions of the terms above, I have provided a glossary.

©1999-2006 by S. M. Gon III. Created using Macromedia Freehand

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In this depiction, the limbs and other ventral parts of the Burgess shale trilobite Olenoides serratus are shown. As mentioned above, only very rarely are these structures preserved and fossilized (other than the hypostome).  Long, many-segmented antennae emerge from lateral notches of the hypostome (which overlies the mouth), and many pairs of legs, varying very little except in size, proceed from the cephalon to the pygidium (three limb pairs under the cephalon, and one pair for each axial segment in the thorax and pygidium). This primitive lack of specialization is one of the features of trilobite limbs, shared with many other Paleozoic Arachnomorpha. 

The limbs are attached to a sequential set of axial sternites (ventral segments) bearing a thin, uncalcified exoskeleton. Each of the bases of the limbs possess jagged toothlike structures that are thought to have processed food passed between the legs forward to the hypostome and mouth. Such food-processing limb bases are referred to as gnathobases (gnathos = jaw). (see Trilobite Feeding Behavior.

Between the endopods (crawling limbs) and the body are pairs of finely branched feathery structures (typically interpreted as gills), borne on the exopods and here colored red, (see additional detail on trilobite limbs, below). In some trilobites, it is thought that movements of the exopods might have allowed the animal to swim (as similar movements provide swimming locomotion in modern marine and freshwater arthropods). Finally, at the rear of the trilobite, two antenna-like cerci (sense organs) are depicted, although Olenoides serratus is the only species among all trilobites that are known to have borne them.

Very rarely, conditions for preservation are so good that these delicate features are preserved. Only about twenty different species of trilobites have been found with preservation of antennae and limbs. To the right is an example of the olenid trilobite Triarthrus eatoni showing preserved antennae, legs, and gill filaments. The image can be found at Per Hansson's Trilobite Gallery, which I encourage you to visit!

Similarly, some of the Burgess Shale trilobites, notably Olenoides serratus, show antennae, limbs, and anal cerci, as in the double specimen below (image via the Smithsonian Institution). Images such as these were used to create the reconstruction of Olenoides above.

The limb details of Olenoides show that it was probably a predator or scavenger, bearing spines and a heavy gnathobase with which to tear at the tissues of its prey. 

The limb details of Triarthrus (below) are much more delicate, compared to Olenoides ((left), but there are many similarities in limb structure, notably the number of segments in the endopod (walking leg), and the delicate gill filaments of the exopod. These similarities were once considered an important shared character among trilobites and their relatives, but we now know that many of the Paleozoic arthropods have similar limb structure, and that limb similarity points to a shared primitive condition.

On the other hand, some trilobites have limbs that seem quite dissimilar to the pattern above. The limbs of Ceraurus bear an unusual paddle-like exopod with several distinct segments, while the endopod is remarkably simple and unadorned.

Perhaps the most unusual of known trilobite limbs are those borne by Agnostida. They are so unusual that some scientists cite Agnostine limbs as evidence that they must not be true trilobites!

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At the 2008 Trilobite Conference in Spain, this image of the New York trilobite Triarthrus eatoni won wide acclaim as the best preserved cephalic ventral features of a trilobite yet found. The arrangement of the cephalic limbs converging on the hypostome indicate how the gnathobases act as mouthparts processing and manipulating food in the vicinity of the mouth, which underlies the hypostome. Image courtesy of the collection of Dr. Ed Staver:

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