Molting Behavior / Trilobite Disarticulation
Molting Behavior
Considering the apparently endless array of trials and tribulations they faced during their lengthy stint swimming through the planet's primeval seas, sometimes it's hard to believe that trilobites endured in such a hostile environment for over a quarter of a billion years. Yet despite their lives of constant peril, perhaps their times of greatest vulnerability didn't occur when they encountered razor-toothed predators, undersea volcanic eruptions or mountainous mud slides… perhaps those moments came when they shed their hard outer shells and by necessity revealed their “softer” side to the world. It was then, during periods of molting -- when they jettisoned those old calcite carapaces and began growing larger, thicker new ones -- that trilobites were left virtually defenseless against the multitude of competitors which continually battled them for control of their aquatic niche.
Some scientists have postulated that trilobites were actually the first creatures ever to utilize the molting process to facilitate and assist in their growth. And the fact that some trilobites attained prodigious sizes, often over a foot in length, suggests that molting behavior may well have continued throughout their lives. The fossil record also indicates that molting emerged right along with the earliest trilobites, over 520 million years ago. By the time the first trilobites started filling Lower Cambrian seas around the world, they already possessed a calcified exoskeleton, as well as clearly delineated cephalic suture lines, which provided them with the ability to effectively shed their shells in order to grow.
It is known that trilobites -- like all arthropods -- discarded their carapaces regularly, perhaps two or three times a year, and the vast majority of recovered trilobite fossils are of these molted shells rather than of the actual animals themselves. Indeed, it is believed that many of the impressive trilobite mass mortality plates that have been retrieved from key fossil-bearing locations around the globe (including Homotelus from Oklahoma, Xenasaphus from Russia and Phacops from New York) may be the resultant byproduct of a particular trilobite species grouping together during times of molting. In retrospect, it makes perfect sense for a community to gather at times of peak vulnerability and peril for mutual protection. A number of paleontologists have even suggested that such moments may have also provided an ideal opportunity for trilobite mating practices to occur.
Apparently, in order to complete the molting process, trilobites needed only to anchor their tails on the surrounding seafloor, flex their bodies, and pop out through the easily detachable sutures on either side of their cephalon. Ahh, it all sounds so easy… so efficient… so evolutionary. And it probably was -- at least for a while.
The fact is that a recent scientific paper by Michigan State University paleontologist Danita Brandt indicates that it may have actually been their increasingly “antiquated” molting behavior that contributed to the eventual demise of trilobites. This report states that as time marched inexorably on, the “improvisational” method that some trilobites utilized in order to shed their shells, as well as their apparent inability to reabsorb the mineral-rich resource provided by their discarded exoskeleton, proved highly inefficient - and potentially devastating - for these ancient sea dwellers. And while such apparent morphological disadvantages were probably not the primary cause of the trilobites' demise, they may have played a significant role in the class' gradual but steady decline throughout the Paleozoic.
Of course, any creature that existed for as long as the trilobite may have naturally outlived some of their inherent evolutionary advantages. And if their molting behavior eventually became one of those obsolete characteristics, all we can say is that trilobites still had one “shell” of a ride through early earth history.
Trilobite Disarticulation
Back in those bygone Paleozoic seas, trilobites more-often-than-not found themselves as the preeminent life form within their chosen marine ecosystem. Especially during the initial stages of their lengthy run through geologic time, certain trilobite species - due to their impressive size and powerful external appearance -- frequently served as the biggest, baddest bullies on the tidal bay block. Oh sure, the occasional menacingly meandering Anomalocaris, Beckwithia, jawed fish or giant sea scorpion could put just about any trilobite in its proper place within the Paleozoic pecking order. But for a hefty segment of their quarter-billion year passage through deep time, trilobites rather effortlessly ruled over their aquatic domain.
Yet despite such dominance, and despite being covered in thick calcite carapaces which often allowed them to present the outward facade of primitive undersea battle tanks, trilobites were, in fact, rather delicately constructed creatures. Evidence of this apparent design “flaw” is pervasive throughout the fossil record, where disarticulated trilobite fragments outnumber the remains of complete specimens by a disproportionally lopsided margin. Indeed, in a number of fossiliferous biozones around the globe, including those found in British Columbia, Morocco and the Czech Republic, independent pygidia, cephala and thoracic segments virtually litter the sedimentary strata… with often nary an articulated trilobite to be found.
Of course, much of this trilobite “hash” can be directly attributed to the frequent and highly efficient molting process that these arthropods utilized from the moment of their initial emergence in the Lower Cambrian right through their demise at the end of the Permian. Apparently as soon as their shells were shed, many of those protective exoskeletons were quickly torn asunder either by unfavorable marine conditions or by fellow arthropods -- especially those seeking a quick dose of calcium gained by ingesting small pieces of these cast-off carapaces.
When this molting-derived “shrapnel” was ultimately combined with the shell residue generated by a legion of deceased trilobite exoskeletons, the resultant trilo-litter may have served to virtually carpet certain marine habitats with bits and pieces of arthropod anatomy. Thus encountering the fossilized evidence of disarticulated trilobite parts - the bane of most serious enthusiasts, though a surprisingly satisfying endeavor for many scientists primarily concerned with examining particular morphological features - is very much an essential and expected part of the trilobite collecting experience.
Indeed, many weekend warriors have returned from the field eagerly anticipating the eventual unveiling of a complete trilobite specimen …only to be harshly faced with a disjointed example missing everything from genal spines, to free cheeks, to entire cephalons. Truth is, for many card-carrying members of the paleontological community such missing parts present only a minimal distraction. In fact, many - if not most - scientifically recognized trilobite species have been initially identified and described from nothing more than a series of disarticulated fragments… with the majority of those random bits being gathered as the result of somewhat haphazard surface collecting practices.
The rather surprising fact is that under intense scientific study, disarticulated trilobite parts can often yield just as much important information as the most complete specimen. And while the vast majority of serious collectors and museum curators prefer to display only those magnificent, articulated examples of our dreams, fragmentary trilobite remains are a vital link in the fossil chain. They often serve as revelatory pathways through which we can all gain a better understanding of both the lifestyle and eventual demise of these ancient arthropods as they battled for survival in those long-gone Paleozoic seas.
A Dalmanites sp. from Gotland Sweden featuring an array of disarticulated trilobite parts