More About Trilobites

As the picture above shows, the central lobe is called the axial lobe and the two side lobes are termed, the pleural lobes. The picture shows three other sections are present in the trilobite skeleton: these are the head (cephalon), body (thorax) and tail (pygidium). Like all other arthropods, trilobites are known to have periodically shed their exoskeleton during growth. This moulting is called ecdysis. Most trilobite fossils are in fact moults, as opposed to the remains of dead bodies. Entire trilobites are relatively uncommon, due to the fact that exoskeletons when molted tend to break apart.

It is known that trilobite appendages were biramous, which means they consisted of two branches, one above the other, with one pair on either side of the central axis per segment of the thorax. In each pair, the lower appendage was used as a walking limb, while the upper appendage supported a gill for respiration. Fossilised appendages are rare, apart from beds with exceptional preservation, like the Burgess Shale. However, studies of the trace fossils of trilobites gives evidence of the way trilobites moved, fed and rested. The trace fossils also show that trilobites had a diverse mode of life. From the fossil record it is believed that most trilobites lived on the sediment surface in shallow marine environments, feeding as scavengers on the sea floor.  Some trilobites were predators on the seafloor. Other trilobites are known to have lived pelagic or planktonic existences, in the deeper oceans.

As already mentioned the trilobite exoskeleton is divisible into three components, cephalon, thorax and pygidium. The skeleton was flexible, with the greatest flexibility being in the thorax, which was composed of a series of articulating segments, known as the pleura.
The cephalon is the most complex part of the exoskeleton and is where the sensory apparatus were housed. The cephalon has a basic shield, and in the middle of it, there is a central raised area, known as the glabella. The glabella contains many of the vital organs of the trilobite.  These were protected on the trilobites underside by a structure called the hypostome. Alongside the glabella, are the cheeks. Two definite suture lines are found on each cheek. The purpose of these sutures is to allow the cephalon to split during ecdysis.
The cephalon also 'houses' the eyes. Like insects today, trilobite eyes were compound, made up of many packed in cylinders of calcite. This type of eye is known as a holochroal eye.

Trilobite limiting factors