Trilobites

Sinespinaspis markhami from the Lower Silurian of the Forbes district, New South Wales. Source: Museum Victoria.

These marine animals were the earliest known arthropods (invertebrate animals with a hard external skeleton and jointed limbs), and so were distant relatives of modern crustaceans (e.g. crabs).

Trilobites ranged in size from 0.5 mm to nearly 1 m in length, although most species were between 3 and 10 cm long. The trilobite body was divided into a head, consisting of at least five fused segments, a thorax which was also segmented and enabled the animal to roll itself into a ball, and a tail. The length of the body was also divided into three by a pair of furrows, and it is this characteristic which accounts for their name ('three lobes'). Trilobites had gills on their many pairs of limbs, and compound eyes like insects. The lenses in each eye were often larger and more separate than in insects, and pointed in different directions to give a wide-angled field of vision. Their eyes are the most ancient visual system known, and have sometimes been preserved in amazing detail.

Odontochile formosa from the Lower Devonian of the Kinglake district, Victoria. Source: Museum Victoria.

Trilobites were once very plentiful-several thousand different species are known-but all are now extinct. They first arose in the Cambrian Period (545 to 490 million years ago), and died out at the end of the Permian Period (about 251 million years ago), probably as a result of a lowering of the level of the sea.

Trilobites are useful in determining the age of rocks, particularly those deposited during the Cambrian and Ordovician periods.

Trilobites can be found in Victoria in the Kinglake, Lilydale and Kilmore districts.

Gravicalymene angustior, from the Lower Devonian of Lilydale, Victoria. Source: Museum Victoria.

Acanthopyge australis, from the Lower Devonian of Killara, Victoria. Source: Museum Victoria.