The arthropods are an invertebrate phylum characterized by segmentation, the specialization of various regions of the body, an exoskeleton which is jointed at the limbs, and blood cavities (among other features).
Although the exoskeleton makes an efficient support and armour, it inhibits the growth of the soft parts inside and must be periodically shed (molting) to allow this growth.
Each arthropod passes through a series of growth stages that give rise to a discarded exoskeleton which may form part of the fossil record. It is not surprising, therefore, that trilobites have a good fossil record. Yet fossils are rarely complete. The segments of the exoskeleton were held together by organic material which decomposed after death, usually allowing them to drift apart before burial.
The trilobites are usually considered to be a separate class within the phylum Arthropoda, characterized by many primitive features.
Their name refers to three longitudinal lobes, defined by two grooves that divide the body into an axial (central) and two pleural lobes (to the left and right of axial lobe). The cephalon (head) shows the most interesting features of the trilobite: a central raised part called the glabella, a line called suture along which the exoskeleton splits during molting, and the eyes on this suture.
Nearly all trilobite fossils are of the upper surface of the animal only. The undersurface must have been only lightly calcified and thin. The edges of the exoskeleton bend over to form a frame surrounding the undersurface. This frame, quite wide in such genera as Isotelus, is called the doublure. At the front of the undersurface of the cephalon a small plate called the hypostome is located just in front of the mouth.
Draw a complete specimen of one of the following genera: Ogygia, Ogygopsis, Olenoides(1). Mark as many of the identifiable features as you can: glabella, cephalon, pygidium, thorax, axial lobe, pleural lobes, facial suture, segments of the glabella (free and fixed cheeks), etc.
The cephalon of trilobites seemed to have been formed primitively by the fusion of several segments and the specialization of certain pair of appendages.
Examine plaster restorations of Triarthrus from the Middle Ordovician Utica shale. How many segments make up the cephalon (2a)? Does your answer make sense when you compare it to the number of glabellar furrows (2b)? What type and how many pairs of appendages are present on the cephalon (2c)?
Look for its hypostome on the underside. Is this an example of a natant hypostome (left, above) or conterminant (more specialized) hypostome (right, above) ?
The appendages of trilobites are almost never preserved. When they are preserved they consist of delicate two-branched appendages that are similar on each segment but of different lengths.
Examine the specimens of Isotelus . Find a sketch the specimen displaying an undersurface with the hypostome and doublure (3a).
One specimen preserves appendages: on which part of the body are they present (3b)?
How can you tell the head from the tail on specimen O339.2 (3c)?
What type of observations would help determine whether the smaller specimen of Isotelus is an adult or a juvenile form of the larger specimens (3d)?