Morphologically speaking, all trilobites were variations on a surprisingly similar theme. Despite existing for nearly 300 million years and producing over 25,000 scientifically recognized species, for the most part trilobites shared a strikingly singular body plan. And there was good reason for these similarities; even at a very early stage in the history of our planet, the trilobite design had already proven to possess a certain degree of evolutionary perfection.
Sure, not all trilobites looked the same. Some trilobites had eyes… some didn't. Others possessed long, flowing genal spines while others displayed little more than vestigial stumps. Certain species featured posterior appendages comprised of sharply pointed spikes, while others exhibited tail spines that occasionally exceeded the length of their entire bodies. Some trilobites were designed like a hydrodynamic rocket ship while others resembled nothing more than a primordial meatloaf. A few early species even featured a strange, multi-segmented opistothorax, which provided bold evidence of the trilobites' even more primitive, worm-like predecessors.
Indeed, as we've demonstrated in other essays throughout The Trilobite Files, during their lengthy trek through time, trilobites existed in an almost dizzying array of sizes and shapes. Perhaps no other creature in the entire history of the earth has ever displayed the diversity of design shown by these singularly distinctive arthropods. But at their heart (and yes, trilobites apparently did possess a primitive but effective cardio-respiratory system), they were all remarkably similar.
Named not, as is generally surmised, for their three main body segments -- cephalon (head), thorax (body) and pygidium (tail) -- but rather for the three lobes that longitudinally divided their dorsal exoskeleton, the fact is that whether they were Cambrian Olenellids or Devonian Phacopids, most trilobites presented a fundamentally analogous body design. Such characteristics as occipital lobes, anterior margins and facial sutures (which allowed early trilobites to shed their molting shell), were shared by the majority of trilobite species, as were such exotic-sounding features as axial rings, articulating facets and pleural spines.
Taken together, a listing of such morphological features provides nothing less than a plethora of tongue-twisting trilo-terms. But in their purest form, they also allow both the professional paleontologist and the amateur trilobite enthusiast a convenient and effective means of comparing and contrasting their favorite ancient arthropods. And by doing so, the majesty, mystery and mystique shared by these incredible creatures begin to reveal themselves in all of their Paleozoic glory.
As superficially similar as some trilobites may appear to certain contemporary creatures -- perhaps most notably, isopods and horseshoe crabs -- in all honesty, any and all such similarities are nothing more than shell deep. Indeed, appearances aside, the fact is that trilobites have no direct living relatives, and while they do share many of the key characteristics of the arthropod phylum, with everything from brine shrimp to wood lice, their primeval morphology marks their members as a totally unique line of the planet's family tree.
So when you look at the fossilized remains of a trilobite, whether it is a one-inch Cambrian Elrathia kingi, or a foot-long Ordovician Isotelus maximus, please consider that lurking under those imposing calcite carapaces were creatures that represented one of our planet's first and most successful experiments with life. Quite simply, from their initial moments on the planet some 521 million year ago, few animals were ever as evolutionarily “perfect” in their morphological design as the amazing trilobite.
Arctinurus boltoni illustration courtesy of Ben Cooper
This Olenellus fowleri from the Lower Cambrian rocks of Nevada presents an early example of the “classic” trilobite body design… as well as a long opistothorax which hints at the species' more primitive origins