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Trilobites first appeared in the geological column in the Cambrian era  (543 to 490 mya). They were an enormously successful group of animals, with some 5000 genera and over 30,000 described species. Many of the trilobites became extinct during the great late Devonian extinction (c 360 mya) . The last of the trilobites became extinct in the even greater Permian-Triassic extinction about 250 mya.

One of the most common trilobites Elrathia kingii is from the mid-Cambrian (c 500mya) and is extremely abundant. While trilobites are extinct, the allied arachnomorphs- the related chelicerates- still persist today. The arachnids are well-known members of the arachnomorph group [1].

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This trilobite is found in bedding planes in high density (up to 500 individuals per square metre) [2] and in different age groups (sizes vary from 4-40mm for complete individuals) [2].

They are found in settled strata (lacking the bioturbation of surface wave action) and were adapted to very low oxygen conditions. This appeared to have been an adaptation that served it well as other trilobites could not compete in such a low oxygen environment. This accounts for both the sediments it is found in, and the lack of competition. Elrathia kingii occur as single species associations of very high density.

References

[1] Cotton, T.J. and Braddy, S.J. (2004). The phyologeny of arachnomorph arthropods and the origin of the Chelicerata, Transactions of the Royal Society of Edinburgh: Earth Sciences, 94, 169-193

[2] Gaines, R.R. and Droser, M. (2003). Paleocology of the familiar trilobite Elrathia kingii: An early exacerobic zone inhabitant. Geology, 31(11), 941-944