For many types of animals, there are differences between males and females, sometimes it's size, sometimes color, and sometimes structure - either or both soft and hard body structures. Unfortunately, color and soft-body structures aren't usually preserved and even when it's preserved it doesn't leave enough information in the fossil record. For size and hard body structures, without other clues, which sometimes from living animals, we still can't know which sex is which. So, how can we tell the difference for trilobites? What if you found eggs? That would be a clue that could help in the determination. There is a good discussion on the Sexual Dimorphism / Trilobite Eggs page over an the AMNH's great Trilobite website.
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.
This pyritized Triarthrus eatoni trilobite from the famed Beecher Bed of upstate New York shows evidence of gills, claws and clusters of tiny eggs.
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.
Here are a few more scattered eggs amid the claws and gills of the Triarthrus specimen.
A paper from 2017 in the journal Nature describes trilobite eggs.
