The Orders Of Trilobites - Order From Chaos
Trilobites feature an almost dizzying array of sizes, shapes, spines and segments. Their body plans, while all following a fundamentally similar three-lobed pattern, present an incredible diversity of design. Some trilobite species reached lengths in excess of two feet. Others never exceeded a fraction of an inch. Some had multi-faceted eyes sitting atop three-inch stalks… others had no eyes at all.
During their 270 million year trek through evolutionary time, these amazing arthropods generated more than 25,000 scientifically recognized species. Such unimaginable longevity and multiplicity has continually presented paleontologists with a daunting yet elemental challenge; how to best categorize and distinguish one group of trilobites from another. Quite simply, the issue becomes one of finding the best manner in which to classify these primordial creatures so that we can gain at least a fundamental understanding of which family, genus and species produced a logical line of descendents.
Once we begin to tackle this dilemma, we can then place the resulting trilobites into taxonomic orders that are manageable and to some degree practical. By doing so, scientists have tried to create some mentally digestible “order” out of relative chaos. After all, some trilobite orders, such as the Lichidae, apparently arose in the Late Cambrian, existed through the Ordovician and Silurian before ending their run through the Paleozoic in the Devonian… a span of 180 million years. Then consider the Proetidae; this order, consisting of thousands of species, produced members that first emerged in the Middle Cambrian and lasted all the way to the demise of the entire trilobite class at the end of the Permian, an impressive stretch of 250 million years. Or in sharp contrast, there is the highly important early order Redlichidae, which arose, peaked and vanished, all within a roughly 20 million year span of the Cambrian.
In all honesty, despite the best efforts put forth by the brightest minds in the paleontological world, there is still much debate and discussion when it comes to the topic of how to best align the trilobite line. Treatises have been published on the subject, and papers written… and then rewritten. And even then, there is still often controversy, if not outright derision, surrounding some of the taxonomic classifications that in recent years have been handed down from the Mount Olympus of trilobite research.
At the moment, it is generally accepted that there are nine distinct trilobite orders -- the groupings into which every trilobite can be placed in some sort of basic evolutionary and cladistic pattern. These orders are: Proetida, Asaphida, Phacopida, Lichida, Ptychopariida, Harpetida, Corynexochida, Redlichiida and Odontopleurida. Then there is arguably a tenth order, Agnostida, whose diminutive species are contested as being “true” trilobites by some members of the fossil community. Yet even these relatively well-defined classifications are still being continually “tweaked” as new discoveries are made and new information garnered.
Here is a look at a representative example from each of these Trilobite Orders:
Proetida: Bathyurus extans, Ordovician, Canada
Asaphida: Isotelus maximus, Ordovicain, Ohio
Phacopida: Drotops megalomanicus, Devonian, Morocco
Lichida: Dicranopeltis neureus, Silurian, New York
Ptychopariida: Yunnanocephalus yunnanensis , Cambrian, China
Harpetida: Dolichoharpes dentoni, Ordovician, Canada
Corynexochida: Olenoides superbus, Cambrian, Utah
Redlichiida: Olenellus gilberti, Cambrian, Nevada
Odontopleurida: Kettneraspis williamsi, Devonian, Oklahoma
Agnostida: Iagnostus interstricta, Cambrian, Utah