End of the Line - The demise of the Trilobites
Trilobites existed for nearly 270 million years. Actually, not only did they exist… for the majority of their lengthy stay on Planet Earth, they thrived. These ancient arthropods filled the world's oceans from the earliest stages of the Cambrian Period, 521 million years ago, until their eventual demise at the end of the Permian, 252 million years ago, a time when nearly 90 percent of life on earth was rather suddenly eradicated. That cataclysmic event, the largest mass die-off in planetary history, has become fittingly known as the Great Permian Extinction, and also happens to serve as the end line for the entire Paleozoic era.
Trilobites evolved continually throughout their incredibly long march through “deep time” history. During that extended stay they inhabited virtually every available aquatic niche and eventually produced over 25,000 species that displayed a startlingly diverse array of sizes, shapes and surface ornamentations. Quite simply, by any measurable criterion, trilobites rank among the most successful creatures ever to exist on our world.
To put the trilobites' mind-boggling longevity into some kind of paleontological perspective, their reign lasted twice as long as that of the hallowed dinosaurs and more than a thousand times longer than our own human species has so-far managed to survive. Yet, despite their durability and obvious skill for adapting to ever-changing ecological conditions, it is now known that trilobites suffered a slow yet steady decline before eventually succumbing to a variety of environmental pressures.
But why did this happen? What caused these amazingly resilient creatures to dramatically disappear, leaving behind only their fossilized remains? Could it have been, as a number of scientists now suggest, something as simple as a subtle change in worldwide sea levels due to an early example of global warming? Perhaps it was something more sensational, such as a series of undersea methane explosions that served to effectively poison oceans around the globe. Or maybe their decline can be attributed to the rise of fast-swimming predators that viewed trilobites as little more than sushi-in-a-shell.
“Many people think only of the dinosaurs when it comes to a catastrophic natural event terminating a large segment of life on the planet,” said Bill Barker, a recognized trilobite authority living in Arizona. “The fact is that similar scenarios have probably occurred on numerous occasions throughout earth history.”
It is a relatively easy task to follow the rise and fall of the trilobite line through their quarter of a billion year-plus passage through time. Almost as soon as they emerged in the primal seas their diversity and global distribution was nothing less than astonishing. Olenellus and Redlichia biozones can be found within Lower Cambrian strata on every continent on earth, ranging from the Chengjaing deposits of China, to the Kinzer formation of Pennsylvania, to the Issafen layers of North Africa, to the Kangaroo Island outcrops of Australia. Indeed, the variance of trilobite species was never greater than it was soon after their initial flowering.
By the dawning of the Middle Cambrian some 510 million years ago, trilobites dominated the seas. But at the same time, the number and variety of trilobite predators -- including the legendary Anomalocaris, whose remains have been found everywhere from Chengjaing to British Columbia's famed Burgess Shale -- were also on the rise. Some observers might even state that from this time on, it was all downhill for trilobites, a long “slide” that would last for the next 250 million years!
Many scientists believe that throughout their history, trilobites served as both predatory and prey animals. While at the present time there is still only minimal fossil evidence to support a claim of this sort, such a deduction is, pardon us Sherlock, somewhat elementary. Fossilized trilobite carapaces sporting both healed and potentially lethal bite marks (many mirroring the general outline of a trilobite hypostome or “mouth plate”) are pervasive in certain locales, such as the famed Middle Cambrian Elrathia kingii beds of Utah.
It is only logical to surmise that some larger species of trilobites, such as the ubiquitous Olenoides, may have foraged upon, and possibly predatorily attacked, some of their smaller trilobite brethren. But as previously mentioned, it is also irrefutable that there were other, even more menacing beasties swimming through those primordial seas that were dead-set to do as much harm as possible to just about anything that shared their aquatic environment. Even as far back as the Cambrian, such behavior may have played a significant role in at least initiating the trilobites' gradual decline.
“There are some people who only collect trilobites showing predatory pathology,” said Terry Abbott, a trilobite collector based in Delta, Utah. “Bite marks or other natural injuries show that trilobites certainly interacted within their environment, both among themselves and with other pelagic creatures. Over time, that alone may have led to a lessening of their territorial dominance, especially as other potential predators emerged on the Paleozoic scene.”
Throughout the Ordovician and Silurian, both the variety and number of trilobites, while still incredibly impressive, fell sharply. By the beginning of the Devonian, 419 million years ago, trilobite species numbered in the hundreds rather than the thousands, and by the dawning of the Carboniferous (denoted as comprising the Mississippian and Pennsylvanian periods in U.S. paleontological parlance) their decline had become pervasive. By that time, only members of the relatively small, simplistically designed proetida order, remained to fill out the trilobites' remaining 100-plus million years on Planet Earth.
Whatever may have served as the root cause of their slow-yet-steady dissolution -- natural selection, undersea gas eruptions or even, as one recent theory postulates, problems caused by their outdated molting behavior -- there is ample evidence in the fossil record showing that the trilobite line was already in a precipitous decline well before their eventual demise. However, exactly which environmental pressures were behind that decline, and what the primary cause was that finally pushed these incredibly adaptable arthropods beyond the brink of total extermination, remains something of a Paleozoic mystery.
In all honesty, however, any of our previous mentions regarding predation practices, molting behavior and shifting sea levels positively pale in the shadow of the Pink Elephant in the room -- the strong possibility that something as dramatic as a meteorite strike at the end of the Permian may have served as the proverbial final straw that broke the trilobites' calcified exoskeleton.
So far scientists have yet to uncover the definitive “smoking gun” that signals the end of the Paleozoic -- the kind of evidence provided by the crater off of the Yucatan peninsula (and the subsequent worldwide Iridium layer) that is indicative of the damage that dealt the dinosaurs their final blow and dramatically marked the end of the Mesozoic. Yet paleontologists now believe that a worldwide disaster of epic proportions rocked the earth some 252 million years ago, in the process causing the largest mass extinction in the planet's history. Over 96 percent of all oceanic species and 70 percent of terrestrial life forms perished in that event's wake.
For a trilobite lineage already in sharp decline, such a momentous development would have been nothing less than a total catastrophe. While it seems highly unlikely that even at the peak of their evolutionary powers these arthropods could have survived such a cataclysmic ordeal, their diminished numbers and already antiquated lifestyle made them immediate candidates to join life's ever-expanding list of “failed experiments”.
Yet even as their species count -- and we must assume, their worldwide population -- precipitously declined in a post-Devonian world, trilobites continued to evolve to meet a wide variety of environmental pressures. The fact is that the trilobites from the Mississippian, Pennsylvanian and Permian periods were certainly not the biggest, baddest or boldest examples of their class. Yet even considering their diminutive size (usually an inch or less) and their modest, simplistic, generally ovate body plan -- a design that allowed them to burrow beneath the sea floor mud in order to avoid the constant threat of predation -- these trilobites still contributed significantly to their kind's lingering legacy. They may have been a far cry from the foot-long, spinose trilobite “monsters” that inhabited the Ordovician and Silurian seas, but in look, design and lifestyle, they were still very much quintessential trilobites.
Despite being in such steep species decline, the fossilized evidence of these last trilobites can be found in numerous spots across the face of the planet. The 360 million year old Mississippian-age outposts of Missouri, for example, have long been a favorite of collectors in search of such species as Ameropiltonia lauradanae and Comptonaspis swallowi. Over the last two decades, New Mexico has also emerged as a Mississippian hotspot, where 23 species have now been scientifically identified from the Caballero and Lake Valley formations, including the likes of Piltonia carlakertisae, Namuropyge newmexicoensis and Pudoproetus fernglenensis. These trilobites are often preserved in a dark brown or black calcite which contrasts dramatically against a reddish-pink matrix. Another important Mississippian-age trilobite location can be found in Antoing, Belgium, where for more than a century well-preserved examples of such species as Piltonia kuehnei, Witrydes rosmerta and Bollandia globiceps have been found in the hard black mudstone rocks of the area.
At the start of the Pennsylvanian, roughly 323 million years ago, trilobites were clearly continuing their decline in the fossil record, yet the proetid order still managed to produce an interesting array of species including such notable American varieties as Ditomopyge olsoni, Ameura major and the highly pustulated Brachymetopus nodusus. In recent years, a number of species from the remote Ulutau mountains region of Kazakhstan have invaded the world market, with many of these dolomitic specimens (including Ditomopyge kumpani and Griffithides praepermicus) being fossilized along-side other fauna, including brachiopods and crinoids, providing an interesting view of life at this stage of the evolutionary game.
By the time the Permian rolled around, 298 million years ago, the limited available fossil material provides bold evidence that trilobites were barely hanging on within their ever-changing aquatic world. Their size had shrunk to an average of a centimeter, and their speciation had reached a critical low. Yet despite their apparent difficulties, the distribution of such species as Paraphilipsia sp. and Acropyge multisegmenta was surprisingly cosmopolitan, with the generally disarticulated remains of these trilo types being found within sedimentary layers in such disparate locations as present-day Hungary, China, Pakistan, Russia and Japan.
“Post-Devonian trilobites are not the most exciting for collectors,” said Abbott. “Whether they're Carboniferous or Permian, they all tend to look basically the same. But they do tell an important story, especially about the decline and eventual end of the trilobites.”
Make no mistake about it, much as the eventual rise of mammals owes a great deal to the sudden demise of the dinosaur, dinosaurs themselves owe a great deal to the Great Permian Extinction, which effectively signaled the end of the Paleozoic and the beginning of the Mesozoic… the Age of Dinosaurs. It is not wild speculation to assume that life on our planet would look far different today if the numerous mass extinctions that dot the last half-a-billion year span of our shared history had not occurred. Who knows… with a few well-placed twists of fossil fate, perhaps some clever trilobite offshoot might have figured out how to traverse the tricky paths of evolutionary change and would now stand (or should we say crawl) as the dominant form of life on Planet Earth.