The hypostome is a shield-shaped, calcified ventral structure of trilobites that is thought to be a mouthpart. It lies under the medial anterior cephalon. Its anterior edge is usually aligned strongly with the anterior edge of the overlying glabella (on the dorsal surface of the cephalon), and can either be separated and free of the anterior doublure (the natant condition), or attached to the doublure (conterminant). Most conterminant hypostomes are also strongly aligned with the anterior glabellar edge (as in the cheirurid specimen shown at left), but there are some exceptions, in which the conterminant hypostome does not match the anterior glabellar outline (the impendent condition).  Left: Leviceraurus mammiloides ventral preparation, showing conterminant hypostome. Image courtesy trilobites.com archives Hypostomal attachment and alignment: In the three examples below, the ventral cephalons of a natant, conterminant, and impendent species are shown. The hypostomes (H), dotted glabellar outlines (G), cephalic doublures (D), and cephalic cavities (dark grey) are shown.

Natant Hypostome free of anterior doublure, aligned with anterior edge of glabella	Conterminant Hypostome attached to anterior doublure,  aligned with anterior edge of glabella	Impendent Hypostome attached to anterior doublure,  but not aligned with the glabella
Ptychopariida:Ptychopariina:Ptychoparia	Corynexochida:Corynexochina:Fieldaspis	Corynexochida:Illaenina:Dysplanus

©2001 by S. M. Gon III.  images adapted from various sources

In the center of the hypostome is an ovoid, typically convex part called the median body. It is often divided into an anterior lobe and a posterior lobe. Flanking the median body is a border with various extensions, including anterior and posterior wings, sometimes bearing knob-like processes. The hypostome is hollow, and encloses the mouthparts, the anterior digestive track, and the bases of the antennae.

The lateral views of the hypostome shown above clearly show the anterior and posterior wings. The anterior wings of this species bear processes, while the posterior wings are simpler. Between the anterior and posterior wings are a pair of lateral notches with a circular outline. The antennae of the trilobite pass through these notches, then forward. The anterior wings are designed to rest firmly against internal structures of the glabella (see below).

©2001 by S. M. Gon III. Created using Macromedia Freehand 10

In the dorsal view of the cephalon of Ceraurinella typa, there are a pair of deep pits at the junction of the glabella furrow and the anterior-most pair of lateral glabellar furrows (see arrow above). These pits correspond to rigid, extensions called apodemes on the ventral surface of the exoskeleton (see paired light spots in the ventral view above). Those ventral apodemes correspond perfectly with the pair of knob-like processes on the anterior wings of the hypostome. When the hypostome is in place, it is very stable, attached directly to the anterior doublure via a suture, and also in contact with the two apodemes in the roof of the cephalic vault.

©2001 by S. M. Gon III. Created using Macromedia Freehand 10

A cut-away anterior view (above left) of  the cephalon and hypostome of the trilobite demonstrates the two points of contact (red circles) between the processes of the anterior wings of the hypostome and the apodemes of the cephalic vault. It also shows the outline of the anterior doublure (dotted black line) and the attachment suture along the anterior edge of the hypostome (red dotted line). A lateral x-ray view (above right) shows the same relationship, with the grey dotted line indicating the inner cephalic contours. It is very likely that the hypostome of Ceraurinella typa did not move, but was a hard, stable platform against which food was processed. Note the round lateral notch between the anterior wing and posterior wing in the hypostome lateral view. The diagonal lines leading downward and forward from the notch indicate where the antennae of the trilobite would have emerged at the front underside of the animal in life.

How are the hypostomal types related to trilobite classification?
Some trilobite orders are marked by having most of its members share a certain condition. For example, most of the order Ptychopariida have natant hypostomes, while most of the suborder Illaenina have impendent hypostomes. While there are exceptions to these patterns, the underlying shared characters provide part of the foundation for evolutionary patterns among the trilobites, important in higher level (e.g., order or suborder) classification. Fortey (1990. J. Paleontology. 33:529-76) reviewed the nature of hypostome attachment in trilobites and used combinations of ontogeny patterns and hypostomal attachment to augment the morphological basis for defining trilobite orders. Some additional general patterns are highlighted below:

Conservative design of natant hypostomes: The hypostomes of natant trilobites appears to have been conservative over the course of the evolution of trilobites. The overall form and shape of natant hypostomes does not vary much, bearing a simple ovoid shape without posterior extensions or ornamentation (see example natant hypostomes in the figure to right).  Fortey and Owens (Palaeontology 42(3):429-65) interpret the simple form of the natant hypostome as belonging to trilobites with generalized particle feeding habits, with little need to modify mouthparts to deal with specialized food items.

©2001 by S. M. Gon III. Created using Macromedia Freehand 10

Diversification of form among conterminant hypostomes: In contrast to natant hypostomes, the hypostomes of conterminant species exhibits a much wider array of diversification of size and shape (see examples to right). This variation has value in classification (for example, many members of the Order Asaphida bear forked hypostomes). The functional interpretations of hypostome shape also provide speculation on the feeding habits of the species.  In general, conterminant hypostomes are found on trilobites that are thought of as predators. Anchoring the hypostome against the structures of the anterior cephalon provides a strong and stable platform against which to tear apart prey. The specializations of hypostome form might reflect different kinds of prey, or different feeding behaviors.

©2001 by S. M. Gon III. Created using Macromedia Freehand 10

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

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Walking Trilobite animation ©2000 by S. M. Gon III