General Anatomical Survey of Trilobites

Calling trilobite anatomy simple is not fair to these highly diverse creatures. Some of their fossils may make their anatomy appear to be highly simplified, but many of those fossils are lacking imprints and other evidence of the complex soft structures, fine details, and even fragile hard structures that also make up the bodies and appendages of trilobites. There are many great finds that do preserve these types of structures, so we do have information about them. These structures are variable across species, of course, and will be better described in individual circumstances. There are abundant excellently preserved core exoskeletons of these animals, and this generalized regional anatomy is our focus today. Discussing them in the simplest terms, however, we are able to break down trilobites into six general regions. 

 

We can break them down from one side to the other (4 - 6 below) and describe the right pleural, axial, and left pleural lobes. These three lobes are where the name trilobite originates from. Pleura is a word of medieval Latin and Greek origin that means "to the side of the body" and is often used in zoology to describe the sidewalls of arthropods. The pleural lobes are literally the "side lobes". The axial lobe, then, is the lobe that makes up the center of the body. 

We can also break down trilobite anatomy by discussing it from head to tail (1 - 3 below). The head region is known as the cephalon (1), the middle region is the thorax (2), and the tail region is the pygidium (3). The cephalon can be broken down into many different smaller regions based on facial sutures (natural fractures separating components of the head) that include preocular and postocular (before and after the eyes, respectively), the rostrum (the "nose" or front end part), and the hypostome ("mouth" part). The cephalon includes the mouth area of the trilobite, houses the compound eyes, and would have had attachments for antennae. The large lateral (side) flanges or fringes are also a part of the cephalon, regardless of how long they may have been; some trilobite species had lateral fringes that extended to being nearly the length of their body. 

 

The thorax is the middle region of the body and is made up of multiple articulated segments. The number was variable between species and could be as few as two and as many as 103. The thorax helped to protect vital organs of the trilobite, including the gills, and served as the attachment site for the limbs; the thorax  therefore protected the limbs as well. The articulation of these segments is such that they allow for "rolling up" as can be seen in modern pill bugs (I always called them "roly-pollies" when I was a kid). Fossils of this activity have been discovered and it is thought to have been a protective measure that trilobites employed against predators. The pygidium is the most posterior portion of the trilobite. It is formed by the fusing of a number of smaller segments, like those making up the thorax, and the most posterior portion of the animal, the telson.

 Sources to consider:

Bruton, D. L.; Nakrem, H. A. (2005), "Enrollment in a Middle Ordovician agnostoid trilobite", Acta Palaeontologica Polonica (3 ed.), 50: 441–448, retrieved June 22, 2009

Paterson, J.R.; Edgecombe, G.D. (2006). "The Early Cambrian trilobite Family Emuellidae Popock, 1970: Systematic position and revision of Australian Species". Journal of Paleontology. 85 (3): 496–513.

Whittington, H. B. (1997), "Morphology of the Exoskeleton", in Kaesler, R. L. (ed.), Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida, Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 1–85.