A Chart of Geological Time
(from a trilobite's point of view)
last revised 12 OCT 2014 by S. M. Gon III
Modocia typicalis appeared in the Cambrian Period The chart below depicts the geological periods during which trilobites existed. The presence of trilobites is one of the diagnostic features of the Paleozoic Era, the earliest era of the Phanerozoic Eon. The Paleozoic portion of the geological scale of eras at the left is expanded on the right as geological periods, and the time scale indicates how many millions of years ago (mya) each period persisted.

The first appearance of trilobites defines the start of Series 2 of the Cambrian (521 mya), and they can be found in strata up to the upper Permian (252* mya), after which trilobites (among a large number of marine organisms) went extinct in the great catastrophe that removed over 90% of all species on earth. The Great Permian Extinction marks the end of the Paleozoic and the start of the Mesozoic. Trilobites are one of the few major groups of organisms that span the majority of the Paleozoic Era. The greatest numbers of trilobite species occurred during the Cambrian and Ordovician periods, after which trilobite extinction trends exceeded radiation events. Toward the end of the Devonian most of the families and orders of trilobites were gone. There were much fewer species in the lone surviving order Proetida in the Carboniferous and Permian periods. Nevertheless, to have persisted for nearly 300 million years is a testimony to the successful design and adaptability of trilobites. Some scientists even hold out the faint hope that in poorly explored deep sea environments, trilobites may still exist, a holdover from truly ancient times.

*The chart above (and below) are based on the International Commission of Stratigraphy 2008 revision of the Geological Time Scale.
See www.stratigraphy.org for the full set of current charts in UNESCO and US standard colors. The 2014 iteration makes some adjustments
to the Era boundary dates: 541.0 for Precambrian-Cambrian, 485.4 for Cambrian-Ordovician, 443.4 for Ordovician-Silurian, 419.2 for Silurian-
Devonian, 358.9 for Devonian-Carboniferous, 298.9 for Carboniferous-Permian, and 252.17 for End-Permian. First trilobites remain at 521.

Trilobite Family Diversity over the Paleozoic Era

Although trilobites are the signature organism of the Paleozoic, first appearing in the Early Cambrian, their peak diversity was in the early Paleozoic, and they began a general decline in the upper Paleozoic (despite bursts of adaptive radiations in the Ordovician, Silurian, and Devonian periods), and that ended with their extinction in the Late Permian. By that time, they had dwindled to two families (Proetidae and Brachymetopidae, both in the order Proetida) and had long ceased to be a prominent feature of the marine biota. The chart below indicates changes in the diversity of the Trilobita over the periods of the Paleozoic Era, based on a figure (p 269) in the Treatise of Invertebrate Paleontology (1997) modified, especially in the Cambrian portion, via recent publications on biostratigraphy and trilobite persistence.

Chart of Geological Time
* Cambrian ages and stage names in international standards have not yet all been established.
All figures this page 2009 by S. M. Gon III created using Macromedia Freehand

Extent of the Trilobite Orders over Geological Time
Trilobite Orders over Geological time

The chart above gives an indication of the relationships of the trilobite orders, and their extent over geological time. The Cambrian origin and proliferation of trilobites is very apparent, as well as the loss of the orders Agnostida, Ptychopariida, and Asaphida in the Ordovician, the loss of the majority of remaining orders in the late Devonian, and the final extinction of the class in the Permian. The Ordovician extinction event is particularly apparent in the chart of family diversity (second chart on this page). Click on any of the order names immediately above this paragraph to learn more about each order.

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Walking Trilobite animation 2000 by S. M. Gon III