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Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found, supposedly because these were not calcified, and this also is supposed to represent the original state.Early trilobites show all the features of the trilobite group as a whole; transitional or ancestral forms showing or combining the features of trilobites with other groups (e.g. Morphological similarities between trilobites and early arthropod-like creatures such as Spriggina, Parvancorina, and other "trilobitomorphs" of the Ediacaran period of the Precambrian are ambiguous enough to make detailed analysis of their ancestry far from compelling.

Effacement is believed to be an indication of either a burrowing lifestyle or a pelagic one.Effacement poses a problem for taxonomists since the loss of details (particularly of the glabella) can make the determination of phylogenetic relationships difficult.The first major crisis in the trilobite fossil record occurred in the Middle Cambrian; surviving orders developed isopygus or macropygius bodies and developed thicker cuticles, allowing better defense against predators (see Thorax below).The Phacopida and other "new" clades almost certainly had Cambrian forebears, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly.The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except the Proetids died out.

Trilobites disappeared in the mass extinction at the end of the Permian about By the time trilobites first appeared in the fossil record, they were already highly diversified and geographically dispersed.This site stores nothing other than an automatically generated session ID in the cookie; no other information is captured.In general, only the information that you provide, or the choices you make while visiting a web site, can be stored in a cookie.By the Ordovician mass extinction vigorous trilobite radiation has stopped and gradual decline beckons.Most Early Silurian families constitute a subgroup of the Late Ordovician fauna.Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichiidae suggests that a uniform trilobite fauna existed over Laurentia, Gondwana, and Siberia before the tectonic breakup of the supercontinent Pannotia between 600 and Tectonic breakup of Pannotia then allowed for the diversification and radiation expressed later in the Cambrian as the distinctive olenellid province (Laurentia, Siberia, and Baltica) and the separate redlichid province (Australia, Antarctica, and China).