The Orders Of Trilobites - Order From Chaos

Trilobites presented an almost dizzying array of sizes, shapes, spines and segments. Their body plans, while all following a fundamentally similar three-lobed symmetry, featured an incredible diversity of design. Some trilobite species reached lengths of nearly a meter…. others never exceeded a mere centimeter. Some had multi-faceted eyes sitting atop towering stalks… others had no eyes at all. Some were covered in inch-long calcite quills… others possessed outer shells as smooth as the proverbial baby’s bottom.

During their 270 million-year journey through evolutionary time, these incredible invertebrates generated over 180 scientifically accepted families – an impressive number featuring nearly 5,000 different genera, and 25,000 recognized species. Such almost unimaginable longevity and multiplicity has continually presented paleontologists with an intimidating, yet fundamental challenge; how to best categorize and distinguish one group of trilobites from another. Quite simply, the issue boils down to finding the most expeditious manner of classifying these primordial creatures, one that allows us to gain at least a fundamental understanding of which families, genera and species should be grouped together… and which subsequently produces a logical line of descendants.

Once we begin to tackle this daunting dilemma, we can then begin placing the resulting trilobites into taxonomic orders that are manageable and to some extent practical. By doing so, scientists have tried to create some degree of mentally digestible “order” out of this arthropod-induced chaos.

After all, some trilobite orders, such as the Lichida, apparently arose in the Upper 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 Proetida; 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 Redlichiida, 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 field, there is still much debate when it comes to the topic of how to best align the trilobite class. Treatises have been published on the subject, and papers written… and then rewritten. And even then, there is still often controversy, if not outright contempt, surrounding some of the taxonomic classifications that in recent years have been handed down from the Mount Olympus of trilobite research.

For the first two decades of the 21st century, it was generally accepted that there were nine distinct trilobite orders -- the groupings into which every trilobite can be placed within some sort of basic evolutionary pattern. These orders were:

Proetida, Asaphida, Phacopida, Lichida, Ptychopariida, Harpetida, Corynexochida, Redlichiida and Odontopleurida. Then in early 2020 it was proposed that there should be a tenth order, Trinucleida, which incorporated certain species previously assigned to the Asaphida line. And if that was confusing enough, there is arguably an eleventh 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 still find themselves being continually “tweaked” as new trilobite discoveries are made and additional information garnered. And as previously unknown species continue to emerge from freshly-opened repositories around the globe, the task of identifying and classifying these recent arrivals has only served to add further fuel to the Paleozoic fires surrounding what has already proven to be an incredibly complex and surprisingly challenging issue. Indeed, even within this distinctly invertebrate world, these far-from made-to-order trilobite orders seem destined to serve as a major “bone” of contention for many years to come.

Here is a look at a representative example from each of these Trilobite Orders: