What are trilobites?
Trilobites (pronounced TRY-lo-bites) are extinct crustacean-like animals that were one of the most numerous and successful marine creatures of the Palaeozoic Era, between 545 and 251 million years ago (mya).
An illustration of a fossil reconstruction of a Olenoides Trilobite, Cambiran Explosion.
Artist: Kate Nolan, Source: Museum Victoria.
Trilobites had a segmented body covered by an external shell, or exoskeleton. The exoskeleton covering the upper surface was thick and hard, and is normally the only part of the trilobite to be fossilized. The exoskeleton of both the head and the tail was composed of single plates in which the original segments had become joined together. However, in the middle part of the body, called the thorax, the segments were separate and hinged together, so that the thorax was flexible. This allowed the trilobite to roll up into a ball for protection from predators, like some slaters do today. Although the trilobite tail was a single rigid plate, the divisions between the original segments were usually marked by furrows and are still clearly visible, so it may be difficult to tell where the division is between the thorax and the tail.
Major features of the trilobite exoskeleton
Illustration adapted from: Woods, H. 1955. Palaeontology. Invertebrate. 8th edition. Cambridge University Press.
On the head there were a pair of multi-facetted, compound eyes that are the oldest visual organs known. Running the length of the trilobite’s body was a central convex lobe, called the axial lobe or axis, with two flatter lobes called pleural lobes on either side. These are the three lobes from which trilobites derive their name.
On the under surface of the trilobite were a number of legs (usually a large number, one pair for each segment), a pair of long antennae on the front of the head, and a similar pair of long sensory structures on the back of the tail. However, all of these structures are rarely fossilized as they were too fragile. The trilobite’s mouth was situated on the underside of the head.
Trilobites ranged in size from a few millimetres to about 90 cm long when fully grown. There was a great variety of different types, including forms that were very spiny, covered in knobs or swellings, or almost smooth. Some were highly convex in form and others very flattened. Some trilobites were probably scavengers feeding off the remains of dead animals, whereas others may have digested organic material from mud on the sea floor or filtered small food particles from sea water. Some may have been active predators.
Where are trilobites found?
Trilobites were most abundant during the Cambrian Period (545 to 490 mya), but after the middle Devonian (about 370 mya) they were in decline and they finally became extinct at the end of the Permian (251 mya). In Victoria, trilobites occur in Cambrian rocks in the Heathcote district and in Gippsland, and in early Ordovician rocks (about 489 my old) at Waratah Bay. However, they are most widespread in late Silurian to early Devonian (420-400 my old) siltstones and sandstones in the Heathcote, Kilmore, Lilydale and Kinglake districts.
Living relatives of trilobites
Trilobites are arthropods (literally, ‘with jointed limbs’) so they are related to such animals as spiders, scorpions, crabs and insects. Scientists do not agree on which of the modern arthropod groups are the closest living relatives of trilobites, partly because trilobite fossil remains do not preserve sufficient anatomical detail to allow complete comparison with living arthropods.
The trilobite Odontochile formosa; Early Devonian (c. 405 my), Kinglake district, Victoria.
Source: Museum Victoria
Fortey, R. 2000. Trilobite! Eyewitness to evolution. HarperCollins, London. [A generalized account.]
Whittington, H.B. 1992. Trilobites. Fossils illustrated, volume 2, Boydell Press, Woodbridge, Suffolk, UK. [A specialist, academic book containing comprehensive information on trilobites and many photographs.]