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.