Trilobites With Fossilized Eggs Found for the First Time Ever

A study team has reported the uncommon discovery of two egg-bearing trilobite fossils found in Northern New York State, according to a new paper in the journal Geology.

Determined to be around 450 million years old (Ordovician period), the eggs are spherical to elliptical in shape, almost 200 micrometers in size and bunched up around the head region. The team’s report said the round structures are “too large to be microbial fossils” and not where you would typically find fecal pellets or geological deposits.

“Thus, the structures more readily fit an egg interpretation,” the paper said. “Their size, though small, is within the size range known from other modern and fossil arthropods. Notably, the eggs are somewhat smaller than the earliest growth stage of trilobites.”

The ancient arthropods and their eggs were fossilized within black shale, which likely happened due to a sudden event such as an undersea mudslide.

“They would have had to be buried quickly to have been preserved,” study author Thomas Hegna, a paleontologist at Western Illinois University, said in a news release.

The fossils were discovered by amateur paleontologist Markus Martin, who found them upon cracking open a few rocks he had collected. Martin then used an air abrasion system to get through rock layers and find the trilobite specimens.

“After Markus showed me the pictures of what he found we had a ‘eureka’ moment,” Hegna said. “My first thought was ‘What else could they be?’ People have found trilobites before, but never found the actual animal and eggs together.”

Hegna and his team used a micro CT scanner to create a “flipbook of slices” that cut through the preserved trilobites and eggs.

“We digitally dissected the fossils,” Hegna said. “The CT scans help us see if the eggs were attached to the body without disturbing the fossil. It helped verify the egg’s replacement relative to the trilobite.”

Before this discovery, very little was known about this early phase of trilobite development.

“By knowing more about their reproductive biology, we expand our knowledge about trilobite autecology and can begin to address long-standing research questions about trilobite mating behavior and reproductive strategies,” the team said in their paper.

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Image credit: Thomas A. Hegna et al, doi: 10.1130/G38773.1

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