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Rare evidence about the lives of an ancient group of arthropods has been uncovered in the U.S. of the study are published in the journal Biology Letters.

The cluster of cyclidan crustaceans, which has been preserved for hundreds of millions of years, shows that these animals might have had similar defense strategies to their living relatives.

Over 300 million years ago, a gathering of ancient arthropods was frozen in time.

Known as cyclidans, these ancient crustaceans would have had very little time to react before being buried by a mudslide during a tropical storm. The animals were buried so quickly that the group of 50 cyclidans fossilized where they stood, allowing paleontologists the rare opportunity to investigate their behavior.

Based on the animals' position and orientation, the team think it's likely that the crustaceans might have gathered together to shed their shells during a mass molting. Doing this in groups can help to defend against predators, as co-author Dr. Greg Edgecombe explains.

"Molting is the period of an arthropod's life when it's most vulnerable," he says. "While its soft shell is exposed, the animal is in danger from predators or, in some cases, even hungry cannibals from its own species."

"This is the first time we've seen this kind of behavior in the cyclidans, and is by far the largest aggregation of these animals discovered to date. They're behaving just like lots of living crustaceans do hundreds of millions of years later."

What are the cyclidans?

The cyclidans are an ancient group of marine arthropods. While they're often overshadowed by more successful relatives like the trilobites, these animals survived for over 250 million years between the Carboniferous and the Late Cretaceous.

With segmented legs emerging from underneath rounded shells, cyclidans looked not unlike an underwater beetle. Some of the earliest species were just a few millimeters in size and form fossils that look like a tiny bunch of grapes.

Over a period of millions of years they evolved larger species, and by the Triassic they were about as wide as a human hand. They then shrunk again, before being wiped out alongside the dinosaurs at the end of the Cretaceous.

While the first cyclidan fossils were named in the 1830s, it took until the 1990s for scientists to confirm what type of animal they actually were.

"The cyclidans have been compared to many different arthropods, including trilobites, horseshoe crabs, true crabs and copepods," Greg explains. "In 1997, well-preserved fossils showed that they had two pairs of antennae and front legs adapted for feeding, which are both crustacean characteristics."

"As a result, it's generally agreed by most scientists that the cyclidans are crustaceans. Larger species probably filled a crab-like role in the environment, while it's speculated that smaller species could have been scavengers, or even living attached to other animals."

One curious quirk of the cyclidans, however, is that they're almost always found on their own. While there's no reason to think that these animals didn't live together, just one fossil preserving multiple animals had ever been found and, even then, it was only four individuals.

With 50 cyclidans in the same slab, the newly described fossil raises the bar for these animals—and offers new details about their lives.

Caught in the act

. These rocks are renowned for preserving fossils in fine detail, including the remains of sharks, worms and even an octopus ancestor named after former US President Joe Biden.

They're the last remnants of a marine ecosystem that existed over 320 million years ago in a monsoon climate of alternating dry seasons and heavy rain. During the rainy season, vast amounts of sediment were swept into the bay and buried anything unfortunate enough to be caught in its path.

This process was so quick that the bodies of the animals were rapidly cut off from oxygen, which stopped their remains from decaying as quickly. As a result, their soft tissues fossilized rather than rotting away.

In the case of the cyclidans, details of their walking legs, antennae and gills can still be made out in the rock. This allowed paleontologists to identify the crustaceans as the species Schramine montanaensis and investigate what they were up to.

"There's no sign that the cyclidans were washed together as they're not arranged with a preferred orientation as you'd expect from a current," Greg says. "As a result, we can be confident that they've clustered together for a behavioral reason. That's not unexpected, as living crustaceans are known to aggregate for many different reasons."

The team were able to rule out the gathering being caused by communal scavenging on a carcass, as there was no sign of any food that the crustaceans would have been feeding on. The individuals in the rock also weren't fully grown, making it unlikely that they had come together to mate, like many living arthropods do.

Instead, the researchers think that molting is the most likely cause, as gathering in numbers could have helped to dissuade potential predators.

"It's hard to say what the predators could have been, but there are plenty of possible options preserved in the Bear Gulch rocks," Greg says. "Small fish and a wide variety of invertebrates could have all wanted to take a bite out of these cyclidans."

Uncovering more group fossils from Bear Gulch Limestone and other rocks around the world will be crucial to reveal more social behaviors among all kinds of ancient invertebrates. Finding out how these animals lived could help researchers to better understand the evolution of their relatives that are still alive today.

This story is republished courtesy of Natural History Museum. Read the original story