Ancient trilobites may have released sperm and eggs from their heads

Trilobites—three-sectioned, crablike critters that dominated the early Paleozoic—are so abundant that they have become the gateway fossil for most collectors. But paleontologists have found little evidence of how the extinct arthropods reproduced—until now. Researchers studying a fossil specimen of the trilobite Triarthrus eatoni spotted something odd just next to the animal’s head: a collection of small (about 200 micrometers across), round objects (in light blue, above). Those, they determined, are actually eggs—the first time anyone had observed fossil trilobite eggs right next to the critters themselves. The structures were exceptionally well preserved, the eggs and exoskeletons of the trilobites replaced with an iron sulfide ore called pyrite. They came from the Lorraine Group, a rock formation that spans much of the northeastern United States and dates to the Ordovician period (about 485 million to 444 million years ago); it has long been a mecca for trilobite hunters because of the pyritization. The placement of the eggs is suggestive, the researchers report in the March issue of Geology : They hypothesize that trilobites released their eggs and sperm through a genital pore somewhere in the head—much like modern horseshoe crabs do today. One possible reason for the rarity of the find may be that the brooding behavior of T. eatoni was relatively unusual in the trilobite world: The species tended to prefer a harsh, low-oxygen environment, and may have kept a closer eye on their eggs than other trilobite species. But, the authors note, one idea this finding does lay to rest is that trilobites might reproduce via copulation—a titillating but little-regarded hypothesis based on the fact that trilobites are sometimes found clustered on top of one another. Instead, trilobites were most likely spawners—and, in fact, that clustering behavior may be another parallel to horseshoe crabs, which can climb on top of one another in competition to fertilize released eggs.