A Chart of Geological Time
(from a trilobite's
point of view)
last revised 03 February 2009
by S. M. Gon III
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 (251 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.
for the full set of current charts in UNESCO and US standard colors.
Trilobite Family Diversity over the Paleozoic
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.
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.