When it comes to iconic fossil invertebrates, trilobites are top of the list. Within rocks from the Palaeozoic Era some 520 to 250 million years ago, they're found in incredible diversity: big, small, spiky, smooth and just plain bizarre. Their fossils have revealed incredible information about their habitats, lifestyles and even their senses. Yet one very important area of trilobite life remains a mystery: reproduction.

Upstate New York is famous for its well-preserved trilobites, including Triarthrus, a type of trilobite from the Ordovician Period (more than 440 million years ago), commonly found with its hard exoskeleton replaced by a shiny mineral known as pyrite. It was Triarthrus that NY resident Markus Martin was working on when he spotted tiny round objects beneath some of its head segments. Eggs!

"It was definitely a 'Eureka!' moment," Martin says.

The trilobite Triarthrus. See those tiny orbs just underneath its head? Those are eggs! Image: Thomas Hegna. 

Before long, he'd noticed the same little orbs in the same spot on multiple trilobites. "Trilobites are so iconic and their eggs have been long sought after, so this was definitely a real privilege to uncover," says Martin.

In fact, these could be the first definite trilobite eggs ever found. 

Thomas Hegna of Western Illinois University, the lead author on a new study detailing the find, explains why they're so rare. "What I suspect most trilobites did was lay their eggs and then leave them," he says. "Thus, most trilobites would only be associated with their eggs for a brief time."

The tiny spheres – at less than 0.2 millimetres across, they really are minuscule – are found in clusters underneath the head segment, or cephalon. The researchers suspect these animals may have released their unfertilised eggs from special networks within their heads, not unlike modern-day horseshoe crabs. 

"Trilobites are some of the most common and charismatic of Paleozoic fossils, yet specimens with the 'soft parts' are very rare," explains Derek Briggs of Yale University, who wasn't involved in this study. "And these are by far the most convincing examples of trilobite eggs yet discovered."

Some animals – like us humans – fertilise eggs (or egg cells) inside the body. But this requires the right machinery – and those bits appear to be missing in trilobites (no evidence of external genitalia has ever been found on them). The other option, practiced by many living aquatic invertebrates, is to release eggs and sperm into the water to be fertilised outside the body. Looking at these fossils, Triarthrus (and perhaps all trilobites) seems to fall into that group.

The eggs have another surprising feature, too: their size. Not only are they tiny, but they're also a good deal smaller than the earliest-known life stage of any trilobites – suggesting these creatures were born smaller than we thought. It's possible they even had an unknown soft-and-squishy early life stage that didn't fossilise well.

And then there's the fact that the eggs were found attached to the adult. This may be a sign of some exciting trilobite behaviour. "Triarthrus seems to have possibly brooded its eggs – keeping them with it while they developed," suggests Hegna. (There are arthropods around today that do this.)

Scientists have long been interested in the reproductive habits of evolution's earliest arthropods (the large group of animals that includes insects, spiders, crustaceans – and trilobites). But information is hard to come by. "The field is challenging because exceptional conditions are required to preserve the evidence," says Briggs.

Such exceptional conditions are always exciting to behold. These trilobites may now join the short list of ancient arthropods who have, against the fossilisation odds, been preserved carrying their eggs: from Ordovician crustaceans and the even older shrimp-like Waptia, to the bizarre Aquilonifer, which seems to have carried its young on threads that trailed out behind it like little underwater kites.

Hegna is hopeful that this discovery will help guide future research on trilobite reproduction. "We now know what to look for, and trilobite workers (myself included) can more critically appraise other trilobite fossils for the presence of eggs."


Top header image: Thomas Hegna