Trilobite

A creature whose fossils make up about three quarters of the Cambrian record. There have been thousands of variants discovered. Many can be found in the Burgess Shale in the Canadian Rockies. They shed their carapaces (like a modern crab) from time to time. By the end of the Paleozoic era they were extinct.

Oddly, see Webby's Trinucleus for more.

The trilobite is not only the state fossil of Wisconsin (Calymene celebra), it is also the state fossil of Ohio (Isotelus) and Pennsylvania (Phacops rana).

These marine arthropods first appeared 570 million years ago and survived for more than 300 million years (longer than dinosaurs and humans). Trilobites were one of the first living creatures to secrete hard shells which allowed them to be fossilized. They are found in Paleozoic sedimentary rocks around the world.

The word trilobite means "three-lobed" in Greek. This described the body of the trilobite which was divided into three sections: the head, the segmented thorax, and the tail. These three sections each had a pair of legs and gill-like appendages which were probably used for swimming or breathing.

The end-Permian extinction (251 million years ago) eliminated over 90% of Earth's species, triggered by massive Siberian volcanic eruptions that released CO2 and acidified the oceans. While commonly called the Great Dying, it particularly impacted trilobites, whose diversity was already declining. Prior to this extinction, only the Proetida order remained, confined to shallow-water habitats amid dropping sea levels.

Of all the fossil arthropods, Trilobites have been given the most attention. This is mainly because they are abundant in Paleozoic rocks, useful for correlation of location and time and attractive in appearance.

Complete sections of trilobite fossils are rare, because like all arthropods they were composed of rigid segments of exoskeleton joined by flexible organic connections that decayed after the death of the animal.

Although complete exoskeletons are uncommon, the seperated parts of the carapace are common fossils in Cambrian, Ordovician and Silurian rocks, and are proof of the abundance of these animals in the early Paleozoic seas. The carapaces of trilobites were highly calcified and are preserved as calcite: they are assumed to have contained some organic component, probably chitinous, during life.

Except in extraordinary conditions, the only part of the trilobite that is preserved is the exoskeleton that covered the back or dorsal side. The ventral side and the appendages (much like that of a crab: spiny walking legs) were not calcified and are known only from a few specimens in which this soft tissue was preserved as a thin film, or replaced by pyrite.

Trilobites are also the first organism known to have eyes. These were compound, made of many individual lenses, much like a modern insect.

Tri"lo*bite (tr&imac;"l&osl;*b&imac;t), n. [Cf. F. trilobite. See Trilobate.] Paleon.

Any one of numerous species of extinct arthropods belonging to the order Trilobita. Trilobites were very common in the Silurian and Devonian periods, but became extinct at the close of the Paleozoic. So named from the three lobes usually seen on each segment.

 

© Webster 1913.