Sexual Dimorphism / Trilobite Eggs


Sexual Dimorphism In Trilobites

     A number of years ago, an associate joked (we think) that he was about to begin writing a doctoral thesis exploring “Sexual Dimorphism in Trilobites”. Before we could wipe the somewhat quizzical looks from our faces, he quickly added that the subtitle of such a work was going to be  “Prove Me Wrong… If You Can!” 
     Whether or not he was pulling our proverbial leg, the bottom line is that despite countless hours of high-minded scientific research on trilobites, we currently know next-to-nothing about the sexual behavior of these ancient arthropods. And, in all honesty, that's a situation not likely to change any time soon. While such a titillating topic would appear to be ripe for scientific study, the truth is that virtually zero work has been done on the subject… and with good reason. This apparent lack of zeal is primarily caused by the fact that the available trilobite material -- or lack thereof -- would make such an effort little more than a well-intentioned exercise in fossil-fueled futility. 
     Paleontologists logically assume that from the moment trilobites first appeared on the Paleozoic scene some 521 million years ago, these primeval creatures must have been divided into males and females. They also surmise that the mass mortality trilobite plates that on occasion have been uncovered in locales ranging from Oklahoma to Russia to China, might very well provide evidence of early mating assemblages. Some scientists have even made passing references to the possible existence of trilobite egg sacks or mating claspers. And we do know that evidence has been discovered of Ordovician-age trilobite eggs among a few examples of the pyritized Triarthrus eatoni specimens found in upstate New York. Other than those rather meager informational tidbits, however, we are left virtually devoid of material that might provide either insight or substance on the topic.
   A few leading authorities, including the renowned British paleontologist Richard Fortey, have speculated that the fancy frills, spines and horns that adorn certain trilobite species may, in fact, represent a primal example of peacock-like sexual adornment. With a little imagination, one can readily picture male trilobites crawling along the ancient sea floor, brandishing an array of eye-catching bodily decorations (perhaps in a variety of bold colors) all done in an attempt to gain the notice of available females. Such behavior, however, is still highly speculative, and pushes us to examine an equally compelling question: if the males of the trilobite class exhibited such ornate ornamentation, then have we possibly misclassified, or at least misidentified, the females of the species? Judging by the material presented within the fossil record, such a conclusion seems to be a distinct possibility.
    But while we currently know little about sexual dimorphism in trilobites, there is still hope that we may eventually garner additional knowledge on the subject. Each year more trilobites that exhibit soft-body part preservation are being discovered in various locales around the globe. And with improved preparation techniques revealing previously unknown details of trilobite anatomy, perhaps one day we will uncover evidence that will shed additional light upon this most basic of trilo-topics. 
     The possibility exists, however, that we will never learn how to differentiate male and female trilobites. The world will probably keep spinning even if we don't. So when our doctorate-seeking friend asks the scientific world to “prove him wrong”, perhaps the best anyone can currently do is tell him that a few trilobite mysteries seem destined to remain hidden secrets of the Deep Time past.

Trilobite Eggs

     Hen's teeth… unicorn droppings… trilobite eggs… all apparently stuff of myth, mirth and legend. However, before we place each of these hallowed “rare as” punch-lines into a singular derisive category, let it be stated loudly and proudly that at least one of our entries -- and unquestionably the one that would most intrigue those perusing this section of The Trilobite Files - is very much the real deal. Make no mistake about it; trilobite eggs did indeed exist! 
     The fact is that evidence of trilobite eggs has long been sought yet never previously seen in the fossil record. Some scientists have even referred to any such irrefutable indicator of trilobite reproductive behavior as the Holy Grail of invertebrate paleontology. Famed French naturalist Joachim Barrande cited the subject of trilobite eggs as far back as 1872. The legendary Charles Walcott (perhaps best known for his discovery and exploration of British Columbia's storied Burgess Shale) wrote a subsequent paper in 1877, and spent much of his later career searching in vain for signs of trilobite eggs amid the myriad specimens he unearthed during his various excursions across the face of North America. 
     Walcott was confident that fossilized evidence existed of these primal arthropod ova. Yet despite his extensive efforts to uncover these elusive signs of trilobite reproductive practices, during his lifetime he was unable to find definitive proof of their presence. But now, nearly 150 years after Barrande and Walcott made their initial forays into trilobite egg research, a recent scientific study (conducted and written by paleontologist Thomas Hegna and amateur enthusiast Markus Martin) has finally offered unassailable confirmation of the long-standing beliefs shared by these paleontological stalwarts. 
    As seen in a small number of carefully prepared examples of the species Triarthrus eatoni that have been uncovered within the Ordovician-age Lorraine Shale of Oneida County, New York, it is clear that trilobites did indeed reproduce through the deposition of eggs. These beautifully fossilized 450 million year old trilobites, found in a long-renowned and oft-studied location known as the Beecher Trilobite Bed, are usually preserved in a vivid golden pyrite which contrasts dramatically against the surrounding black matrix. 
     These eye-catching Triarthrus trilobites have garnered world-wide appreciation not only for their distinctive appearance, but also for their unique soft-body part preservation, which frequently includes intricately fossilized gills, claws and antennae. Yet despite the recovery of thousands of complete specimens from this locale over the last century-plus, only in recent years has the clear evidence of trilobite eggs (some appearing as multi-cell zygotes) been found and confirmed. These landmark discoveries are the direct byproduct of a trilobite bounty unearthed during a 21st Century excavation that has recovered a fresh horde of soft-bodied Triarthrus specimens. 
    In all honesty, the trilobite eggs found on a precious few of these pyritized examples are easy to overlook, appearing as little more than diminutive, enrolled golden “dots” upon the dark matrix. Even under high magnification they are difficult to distinguish for all but the most highly trained eye. The 2016 scientific paper on the subject indicates that these eggs are usually found within the cephalic cavity of ventrally preserved Triarthrus specimens, and are often grouped in tightly-packed clusters, much like the eggs of many living arthropods (like the modern day female horseshoe crab).
     While the discovery of Ordovician-age trilobite eggs certainly ranks as an amazing scientific breakthrough, rather incredibly the finds being made at the Beecher Bed location don't represent the oldest eggs found in the fossil record. Indeed, some controversial evidence exists of Cambrian trilobite eggs. And back in 2004, Chinese scientists working in the Doushantou Formation of South China, found clear evidence of 600 million year old eggs, deposited by what is believed to be a primitive, coral-like animal. Yet most reading this would probably agree that tiny coral-like creatures are a far cry from trilobites. And while the concept that trilobites most likely produced eggs has long been an accepted part of Paleozoic theory, the recent evidence found in the rich Ordovician deposits of upstate New York seems to somehow make trilobites even more appealing and accessible to those who both study and appreciate these long-gone remnants of the Paleozoic seas.  

Image of Triarthrus eatoni eggs 1

 This pyritized Triarthrus eatoni trilobite from the famed Beecher Bed of upstate New York shows evidence of gills, claws and clusters of tiny eggs.

Image of Triarthrus eatoni eggs 2

This is a close-up view of the underside of the Triarthrus eatoni cephalon showing the small, circular eggs, some in a multi-cellular zygote stage.

Image of Triarthrus eatoni eggs 3

Here are a few more scattered eggs amid the claws and gills of the Triarthrus specimen.