Trilobites are well known from an extensive fossil record. Due in part to their massive diversity – and their hard exoskeletons – some 17,000 fossilized species of the arthropods have been discovered and described. Yet despite this, how the creatures reproduced has remained somewhat of a mystery. Now researchers have described how for the first time trilobite eggs have been found paired with a fossil of the creature itself.
Discovered in the Whetstone Gulf Formation in New York, the Triarthrus eatoni fossil is thought to be at least 450 million years old, and has been preserved by the mineral pyrite, meaning that the fossil is made from what is commonly called fools gold. This is not the first time that eggs of the animals are thought to have been found, but the importance of this fossil comes as they are still associated with the trilobite which was seemingly carrying them.
The eggs measure around 200 µm (micrometer) in size, meaning that they were too large to be preserved bacteria, while the location in the head of the critter meant they were unlikely to be fecal pellets. The only answer that seems to fit is that they are indeed eggs.
The placement of the eggs in the head of the trilobite may seem a bit weird to us, but it is not all that strange for arthropods. Even today, horseshoe crabs – which have remained unchanged for hundreds of millions of years – hold their eggs within specialized pouches on their head before releasing them at mass spawning events, and it has been proposed that previously discovered fossils of trilobites with swellings on the head show brooding pouches.
The researchers analyzing this latest discovery, the results of which have been published in the journal Geology, used CT scans to create a “flipbook of slices” through the preserved trilobite and its eggs, allowing them to see if the structures were truly separated from the rest of the head, which they were.
Very little is known about the early stages of trilobites, though evidence suggests that they grew by molting their exoskeleton, increasing in size by adding a new segment to their body each time. The size of the eggs are much smaller than the earliest known trilobite stages. These early stages have hard shells, which suggests that very early hatched trilobites lacked a hard exoskeleton, but developed them soon after emerging from the egg.