The group Trilobita existed from early in the Cambrian Period (520 million years ago) until the end of the Permian Period (250 million years ago). The name Trilobita is derived from the three (tri-) lobed structure of the exoskeleton, which has a raised central lobe (or axis) and a pair of side lobes, called pleurae. The trilobite body is also divided lengthwise into three regions or tagmata: a head or cephalon, a middle region (thorax) composed of several to many articulated segments, and a tail plate called a pygidium, which consists of fused segments.
Trilobites as arthropods
Despite a quarter billion years since their extinction, the zoological affinities of trilobites can be determined from structures preserved in fossils. Before the first trilobite with its legs fossilised was described in 1870, it was already well established that Trilobita belonged to the Arthropoda. Arthropods are the most species-rich animal phylum living today, and have been the most diverse group of multicellular animals since the Early Cambrian. Their main subgroups are the crustaceans (like prawns and crabs), the chelicerates (including spiders, scorpions, and mites), and the wholly terrestrial groups Insecta and Myriapoda (the latter including millipedes and centipedes).
Trilobita is the most species-rich entirely extinct arthropod group. In trilobites, the hard exoskeleton covering the dorsal body surface and its well marked segmentation (for example, the jointed segments of the thorax) are classic arthropod features. The trilobite exoskeleton was mineralised, constructed of calcite. The trilobite hypostome, a plate attached to the lower side of the head just in front of the mouth opening, corresponds to a similar structure (the labrum) in other arthropods. A pair of compound eyes is developed in most trilobites, with the arrangement of their units (ommatidia) being typical of Arthropoda (e.g., compound eyes in horseshoe crabs, crustaceans and insects). Trilobites periodically shed their exoskeleton to accommodate growth; trilobite fossils sometimes preserve so-called moult configurations that show various stages in the release of the old exoskeleton and escape of the then soft-bodied animal. Moulting is another diagnostic feature of the arthropods. In most trilobites moulting was accomplished by splitting the head shield along lines of weakness (called facial sutures) that run along the visual surface of the eye.
Legs of trilobites
The legs are preserved in fewer than 20 species of trilobites, but leg structure confirmed the group's position within Arthropoda. Trilobites had a pair of many-jointed antennae that projected in front of the head (but attached further back, against the hypostome), then three more pairs of head legs. The legs attached to the body beneath the middle lobe, the axis, and splayed outwards beneath the lateral lobes of the body, the pleurae.
All living arthropods also have a head composed of at least four leg-bearing segments. The three pairs of post-antennal head legs in trilobites are structurally very much like the limbs behind the head, one pair of which was attached at each segment of the thorax and the pygidium. The thorax consists of from two to 61 segments, usually being constant in a species (most trilobites have between six and 15 segments in their thorax).
All of the post-antennal legs have a similar two-branched (or biramous) structure. A seven-segmented inner branch is built like a typical walking leg in living arthropods, and an outer branch has a fringe of filaments that suggest it functioned as a gill. The inner and outer branch of each leg attach to a large median segment (called either a coxopodite or a basis) that bears a battery of strong spines along its inner margin. These spines would have functioned like the very similar spines along the inner margin of the legs in living horseshoe crabs, being used to tear food before it was transported to the mouth. The mouth in trilobites opened at the back of the hypostome, and must have been directed posteriorly (as in many crustaceans).
Cousins of horseshoe crabs?
That trilobites are arthropods is beyond doubt, but the exact position of Trilobita in the evolutionary tree of the arthropods is more controversial. Early workers took the geological antiquity of trilobites as evidence that they were the most primitive kind of arthropod, and may have included the ancestors of crustaceans and chelicerates.
A single pair of antennae is likely a primitive feature for all arthropods, and the similarity of leg structure along the trilobite body (e.g., without the specialised leg-derived mouthparts of crustaceans or insects) can also be interpreted as primitive. Most recent workers consider that, among living arthropods, the closest relatives of trilobites are the chelicerates. The similarity of these groups may not be obvious when we make comparison with the land-dwelling spiders, mites, or scorpions, but becomes more apparent when we examine the most primitive living chelicerates, the horseshoe crabs.
Trilobites, horseshoe crabs and sea scorpions have similar spine rows along the inner margin of their legs. The lamellae on the outer leg branch of trilobites are similar (and thought to have the same evolutionary origin) as the filaments of the book gills of horseshoe crabs and book lungs of arachnids.
The eyes of trilobites penetrate the dorsal surface of the head shield as in horseshoe crabs. Nonetheless, trilobites are not the direct ancestors of horseshoe crabs or other chelicerates. All trilobites share certain unique features (like the calcite mineralogy of the exoskselton and calcified eye) to indicate that they are a separate branch of Arthropoda.
For more detailed information on the structure of trilobites, including their soft-part anatomy, excellent sources are the revised Trilobita volume of the Treatise on Invertebrate Paleontology (1997, Geological Society of America and University of Kansas Press) and H. B. Whittington's book Trilobites (1992, Boydell).
A comprehensive website, A Guide to the Orders of Trilobites, is maintained by S. M. Gon III.
Australian Trilobites: A Species List and Bibliography. This list summarises the classification and species-level diversity of Trilobita in Australia.