Trilobites were some of the first multicellular animals to live in the world's oceans. Their fossil record extends back million years to the early part of the Cambrian Period and indicates that trilobites evolved rapidly in the shallow seas of this time. In fact, trilobite fossils are so common in Cambrian rocks that the Cambrian Period is sometimes called the age of trilobites.
Although trilobite diversity decreased about million years ago, following the major extinction event at the end of the Ordovican Period and again during a series of extinctions in the Middle to Late Devonian Period, trilobites have one of the most extensive fossil records of any group of animals.
Thousands of species are known from rocks around the world, leading some to call trilobites the beetles of the Paleozoic. Trilobite fossils are fairly rare in the Pennsylvanian and Permian rocks that crop out in eastern Kansas, a testament to the decline in their diversity during the later Paleozoic. The very last trilobites appear to have died out completely near the end of the Permian Period, just before another huge extinction that marked the end of the Paleozoic Era. Like the insects, trilobites are classified as arthropods, animals with jointed legs.
Their closest living relative is the horseshoe crab Limulus. Trilobites had a hard external skeleton, called a carapace, which is convex and approximately oval-shaped. The carapace is divided lengthwise into three lobes hence, the name trilobite : a prominent central axis and two flatter lobes. The carapace consists of a head, or cephalon, which is typically crescent shaped; a middle region called the thorax, made up of a number of overlapping segments anywhere from 2 to 61 ; and a tail, or pygidium, in which the segments were fused.
Pricyclopyge , a trilobite with large eyes and all-round vision which probably swam rather than crawling on the sea floor. Trinucleus , a trilobite with no eyes, which lived in mud or at great depths. Paradoxides, one of the giants of the trilobite world. This specimen, though incomplete, is 12 cm long. This tiny agnostid trilobite is 5 mm long. Some have questioned whether agnostids are really trilobites at all. Trilobites appeared in the Cambrian Period and became extinct at the end of the Permian Period.
In Britain, trilobites occur in rocks of Cambrian, Ordovician and Silurian age, for example in Wales and the Welsh Borderland, in Devonian rocks of south-west England and in Carboniferous rocks, for example in Lancashire. Geologists use trilobites in a variety of ways to help them understand how the Earth has developed. One use is in the relative dating and stratigraphical correlation of sedimentary rock successions, especially in rocks of Cambrian and early Ordovician age.
Trilobites are particularly important for correlating Cambrian rocks. Paradoxides , for example, occurs in rocks in England, Wales, Newfoundland, Sweden, Spain and Siberia, and shows that these rocks are all of the same, middle Cambrian age.
Other stratigraphically useful trilobites include species of Merlinia in the early Ordovician and Calymene in the Silurian. Another use of trilobites in geology is in reconstructing past geographies palaeogeography and past environments palaeoenvironments.
The early Ordovician trilobite Petigurus is found in north-west Scotland but nowhere else in Britain, although it can be found in North America. Early Ordovician trilobites from England and Wales e. This is part of the evidence showing that much of Scotland was close to North America about million years ago and was separated from southern Britain by an ocean that has been named Iapetus.
Merlin the magician even has a trilobite from this area named after him — Merlinia. Ectillaenus bergaminus Whittard. Some of the species below have been found, but are quite rare: Bolasidella housensis , Alokistocare harrisi , and Olenoides nevadensis. Other types of fossils can be found, including brachiopods, sponges, worm tracks, and phyllocarids.
The quarry is part of the House Range, dated mid-Cambrian. Trilobite Information. A typical trilobite is about 2 inches long, but some are less than half an inch in length and giants of the group measure fully 2 feet. Like the living crustaceans crabs, lobsters, crayfish, etc.
In some species a single trilobite produced 27 shells or more. In fact, it is very likely that most trilobite fossils are the discarded shells. Although trilobite fragments are rather abundant in some rocks, complete specimens are rare. It was only under the most exceptional conditions, such as burial by sediment before or immediately after death, that complete trilobites were preserved relatively unchanged. Because of their unusual and interesting appearance, trilobites are among the fossils most sought after for collection and study.
Avid collectors continually comb the countryside searching for new trilobite localities. The best collecting is at outcrops of shale , limestone , and dolomite in quarries, roadcuts, and natural exposures.
The Paleozoic rocks of Illinois have long been known for their abundant and well-preserved trilobite fossils.
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