An extinct primitive marine arthropod, characterized by two grooves along its body producing a tri-lobed appearance; diverse and widespread from the Cambrian to the Permian periods; ranged from minute to 1 m/3 ft long; mostly living on sea bottom; sometimes planktonic. (Phylum: Arthropoda. Class: Trilobita, c.4000 species.)
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iTrilobite Fossil range: Cambrian - Permian |
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Asaphiscus wheeleri, a trilobite from Cambrian-age shale in Utah |
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| Agnostida Redlichiida Corynexochida Lichida Phacopida Proetida Asaphida Harpetida Ptychopariida Nektaspida (doubtful order) |
Trilobites are extinct arthropods in the class Trilobita. The last of the trilobites disappeared in the mass extinction at the end of the Permian 250 million years ago (m.y.a.).
Trilobites are very well-known, and possibly the second-most famous fossil group after the dinosaurs.
Physical description
The bodies of trilobites are divided into three parts (tagmata): a cephalon (head), composed of the two preoral and first four postoral segments completely fused together; The thorax is fairly flexible—fossilised trilobites are often found curled up like modern woodlice for protection.
Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs. Contrary to popular belief, it is this longitudinal tripartite division into left and right pleural lobes and a central axial lobe that gives trilobites their name, not the division into cephalon, thorax and pygidium.
Although trilobites were only armored on top, they still had a fairly heavy exoskeleton, composed of calcite and calcium phosphate minerals in a protein lattice of chitin. Thus a single trilobite animal could potentially have left multiple well-mineralized skeletons behind -- further enhancing the apparent abundance of trilobites in the fossil record. During moulting, the exoskeleton generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other: many trilobite fossils are actually moulted exoskeletons rather than dead trilobites. In fact, trilobites were the first animals to evolve true eyes, about 543 million years ago;
Some trilobites such as those of the order Lichida evolved elaborate spiny forms, from the Ordovician until the end of the Devonian period.
Trilobites range in length from one millimeter to 72 cm (1/25 inch to 28 inches), with a typical size range of two to seven centimetres (1 to 3½ inches).
Sensory organs
Many trilobites had eyes;
The eyes of trilobites were made of calcite (calcium carbonate, CaCO3). Pure forms of calcite are transparent, and some trilobites used a single crystallographically oriented, clear calcite crystal to form each lens of each of their eyes. The rigid calcite lenses of a trilobite eye would have been unable to accommodate to a change of focus like the soft lens in a human eye would;
The trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in a single eye.
Holochroal eyes
Holochroal eyes had a great number of (tiny) lenses (sometimes over 15,000), and are found in all orders of trilobite.
Schizochroal eyes
Schizochroal eyes typically had fewer (and larger) lenses (to around 700), and are found only in Phacopida.
Abathochroal eyes
Abathochroal eyes had few (and small) lenses (to around 70), and are found only in Cambrian Eodiscina.
Development
An egg hatched to give a tiny larva called a protaspid, in which all segments are fused into a single carapace. Trilobite larvae are reasonably well known and provide an important aid in evaluating high-level phylogenetic relationships among trilobites.
Terminology
When describing differences between different taxa of trilobites, the presence, size, and shape of the cephalic features above are often mentioned. 2 - Detailed morphology of the cephalon
Origins
Based on morphological similarities, it is possible that the trilobites have their ancestors in arthropod-like creatures such as Spriggina, Parvancorina, and other trilobitomorphs of the Ediacaran period of the Precambrian. There are many morphological similarities between early trilobites and other Cambrian arthropods known from the Burgess Shale and other fossiliferous locations.
Extinction
The exact reason for the extinction of the trilobites is not clear, although it would seem to be no coincidence that their numbers began to decrease with the arrival of the first sharks and other early fish in the Silurian and Devonian periods with their strong, hinged jaw.
The closest extant relatives of trilobites may be the cephalocarids.(Lambert, 63)
Fossil distribution
Trilobites appear to have been exclusively marine organisms, since the fossilized remains of trilobites are always found in rocks containing fossils of other salt-water animals such as brachiopods, crinoids, and corals. Within the marine paleoenvironment, trilobites were found in a broad range from extremely shallow water to very deep water. Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents, and occupied every ancient ocean from which fossils have been collected.
Trilobite fossils are found worldwide, with many thousands of known species. Because they evolved rapidly, trilobites serve as excellent index fossils, enabling geologists to date the age of the rocks in which they are found.
A famous location for trilobite fossils in the United Kingdom is Wren's Nest, Dudley in the West Midlands, where Calymene blumenbachi is found in the Silurian Wenlock Group Limestone formation.
Spectacular trilobite fossils, showing soft body parts like legs, gills and antennae, have been found in British Columbia (Burgess Shale Cambrian fossils, and similar localities in the Canadian Rockies); New York State (Odovician Walcott-Rust Quarry, near Utica, N.Y., and the Beecher Trilobite Beds, near Rome, N.Y.), in China (Burgess Shale-like Lower Cambrian trilobites in the Maotianshan shales near Chengjiang), Germany (the Devonian Hunsrück Slates near Bundenbach, Germany) and, much more rarely, in trilobite-bearing strata in Utah and Ontario.
Trilobites are collected commercially in Russia (especially in the St. Petersburg area), Germany, Morocco's Atlas Mountains, (where a burgeoning trade in faked trilobites is also under way), Utah, Ohio, British Columbia, and in other parts of Canada.
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