A rare new fossil discovery by University of Cincinnati professor Carlton E. Brett depicts an extinct group of ancient arthropods in a surprising new way, reports PhysOrg.com

Trilobites are a group of extinct marine arthropods related to lobsters, spiders and insects that were one of the most successful species ever to roam the planet. Though they've been extinct for more than 250 million years, fossils of the ancient sea bugs are generally easy to find because the animals had a hard, easily preserved exoskeleton.

But like other arthropods alive today, trilobites had to occasionally shed their hard exoskeletons as they grew and matured. Thus, fossils revealing these creatures during their tender molting periods, such as the fossils recently analyzed by Brett, are rare.

So what did the ancient trilobites do after they molted, or "stripped?" It turns out they took the opportunity to congregate into mass swarms and mate.

"It's an orgy," said Brett of the scene depicted in the new fossils.

The remarkable behavior was fossilized during a massive hurricane event that swept over the trilobite swarm hundreds of millions of years ago. Sediment that was stirred up from the storm smothered the animals so quickly that they were preserved as is. These conditions are so unique that they might be compared to the way ancient Roman life was cut short and recorded at Pompeii by volcanic ash.

"We find molted pieces lying immediately adjacent to each other," said Brett. "This is proof that the sediments were not significantly disturbed after burial."

Aside from discovering the orgy behavior, Brett and his colleagues also found evidence that trilobites undertook mass migrations, much in the same way that some modern arthropods do. Fossils depicting the ancient animals show them forming long chains, as if a snapshot was taken mid-migration.

"The recent discovery of rows of more than a dozen specimens provides the oldest evidence of migratory queues similar to those seen in modern crustaceans," Brett told PhysOrg.com.

It's unclear what led to the extinction of trilobites, but they disappear from the fossil record during the Permian mass extinction event that wiped out much of Earth's marine life. Many scientists believe that their closest living relatives might be horseshoe crabs.