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Trilobite Classification - the nine Orders
Numerous galleries of trilobite images

The Paleozoic is often called the age of the trilobite. Trilobites radiated repeatedly, expanding in diversity and distribution beginning with and after the Cambrian Explosion, but also suffered periodic declines in major extinction events. Nine Orders of Arthropod Class Trilobita are recognized. Trilobites particularly flourished in the oceans of the Cambrian and Ordovician periods, beginning around 540 million years ago, with a diminishing number of families persisting until the Permian. The number of families actually peaked in the Late Cambrian when an extinction event removed many. The morphological diversity actually peaked in the Ordovician. Many more families were removed at the end of the Ordovician 440 million years ago during a great ice age where ice sheets advanced to the equator. The diminished number of trilobite families that survived to the Silurian radiated into new and exotic forms, and still more exotic spiny and pustulose forms in the Devonian. The Devonian was punctuated by periods of rising seas that disrupted the reef systems where the trilobites flourished forcing selective adaptation. The end of the Devonian saw the Frasnian-Famennian event where only Proteus survived into the Carboniferous. Despite reduced ancestry, with decent with modification ruling, these trilobites filled the same ecological niches such that adaptation led to a repeating of many of the forms of their extinct cousins. While the genetic path was assuredly different, the newly evolved forms had recognizable morphological similitude with those long extinct. Regrettably, trilobites never truly recovered in the Carboniferous, with but a handful of genera extant by the Permian. Failing to adapt to deep-water habitats, their vulnerability to climatic change remained and led to their disappearance prior to yet another great mass extinction at the end of the Permian. The age of the trilobite yielded to the age of the insect.

Trilobites occupy a special place in the hearts of collectors and professionals alike. To these cohorts: "trilobites are very pretty". Riccardo Levi- Setti in his book "Trilobites" describes them as the butterflies of seas. Richard Fortey's more recent book "Trilobite! Eyewitness to Evolution" wonderfully projects this professional paleontologist's love of trilobites. There are also many wonderful websites that reflect a labor of love of the webmaster; particularly among these is Sam Gon's site, "A Guide to the Orders of Trilobites" that is, at once, artistic and a distilation a huge amount of science into concise, non-recondite language and illustrations. I'm sure Sam's site has and will continue to increase the number of people who consider that trilobites are very pretty. The popularity of trilobites is due in large part to the magnificent preservation of their calcified exoskelton that is seen in a myriad of forms, many very exotic; the many trilobite forms evolved due to natural selection for features of trilobitearmament and stealth, in the ageless arms race between predator and prey.

Trilobites comprise a complex and huge clade of arthropods with estimates of number of species ranging from 10 to 15 thousand among the nine distinct Orders. The extinct trilobite represents a problem for classification, a problem unlikely to benefit from modern genomic science. Darwin was confident in his conjecture that trilobites descended from one Pre-Cambrian crustacean ancestor. But, the trilobite's position in the universal tree of life remains a mystery today, with debate remaining whether their closest extant cousins are, for example, the horseshoe crabs, the spiders or the scorpions. Classification requires following the tree of life back to points of branching. This we cannot do for the trilobite whose first appearance in the fossil record is in the Cambrian. When they appear, they are already diverse in form, and dispersed in geography, clearly indicative ofthe incomplete nature of the fossil record. The highly diverse soft-bodied and jointed-legged animals that were ostensibly produced in the Cambrian explosion, but must also have Pre-Cambrian ancestry intensify the mystery. Among the Cambrian fauna, the crystal eyes of trilobites are unique. In the eyes is a strong clue, since the fossil record indicates pre-sight neural tissue existed in forms of worms that also contain segmented morphology also retained in the trilobite forms. Mystery is allure for those with a scientific propensity, and the trilobite does not disappoint. Reading Fortey's book, you can almost feel the author's sadness that these magnificent animals that once dominated life on Earth, declined near the end of the Paleozoic with but a single order surviving to the Carboniferous, before it too faded away.

The scientific bible for Subphylum Trilobita are books within the prodigious Treatise on Invertebrate Paleontology, Volume O, published and sold by the Geological Society of America through the University of Kansas (usually just called "the Treatise" by trilobite aficionados). Trilobites are divided into 9 Orders. The original treatise, first published in 1959, covered 8 Orders. In a major effort, Trilobita is under revision. Harpetida has been recently split from the Ptychopariida. The first volume of the revised Treatise covering two Orders, Agnostida and Redlichiida, was published in 1997. We await this effort's completion for the final volume to incorporate the other seven orders.

Fortey RA 1990. Ontogeny, hypostome attachment and trilobite classification. Palaeontology 33:529-576.
Fortey RA. 2000. Trilobite! Eyewitness to Evolution. HarperCollins, London.
Fortey RA A. 2001. Trilobite systematics: The last 75 years. Journal of Paleontology 75:1141–1151.
Kaesler RL, ed. 1997. Treatise on Invertebrate Paleontology, Part O, Volume 1, revised, Trilobita. Geological Society of America and University of Kansas Press, Lawrence, Kansas.
Levi-Setti R 1993. Trilobites. University of Chicago Press, Chicago.
Fossil Mall Trilobites Across The Orders

Sam Gon's site, "A Guide to the Orders of Trilobites"