Trilobite Fossils Show Conga Line Frozen for 480 Million Years

You probably don’t think twice when you queue up at the grocery store or join a conga line at a wedding. But this type of single-file organization is a sophisticated form of collective social behavior. And as suggested by the children’s song “The Ants Go Marching One-By-One,” humans are not the only animals that appreciate the value of orderly lines.

But how far back in the history of living things on Earth does this behavior go? At least 480 million years, according to a study published on Thursday in Scientific Reports. It points to evidence of fossilized marine animals called trilobites lining up one-by-one during a time when complex life was still coming of age on Earth.

“Probably, collective behavior developed very early among various groups of arthropods,” said Jean Vannier, a paleontologist at the Université Claude Bernard Lyon 1 in France, and the study’s lead author.

Dr. Vannier and his colleagues examined specimens of Ampyx priscus found in Moroccan fossil beds, which preserve single-file lines containing as many as 22 of the small spiny arthropods. The fossils represent some of the oldest evidence of collective synchronized behavior in animals.

These trilobites lived during the Great Ordovician Biodiversification Event, a period defined by a dramatic increase in the variety and complexity of marine life. It was the evolutionary sequel to the first major diversification event, the so-called Cambrian explosion, which established most animal groups in the fossil record some 541 million years ago.

Before the Cambrian, there is “no evidence for group behavior” in animals, Dr. Vannier said, because Precambrian life-forms lacked sophisticated nervous systems.

Ampyx trilobites, in contrast, had an anatomy that could have enabled chemical communication and sensory stimulation. Though they were visually blind, the trilobites had long spikes protruding from their rear ends. These appendages clearly overlap and link individuals in the fossilized chains, and perhaps allowed tactile or pheromone signals to be exchanged.

The arrangement is reminiscent of single-file formations assumed by modern spiny lobsters during mass migrations.

Scientists have previously studied these linear clusters in early arthropod fossils, and offered a variety of explanations for the interesting patterns. Some have suggested that Ampyx trilobites ended up in lines when they sought shelter in seafloor burrows made by other animals. Researchers have also demonstrated that certain fossil clusters of trilobites were likely deposited by forces such as ocean currents.

Dr. Vannier’s team analyzed the sediment that preserves the Moroccan fossils. “We thought it was important to re-explore this topic with new eyes,” he said.

The exceptional preservation of the fossils implies that the trilobites died suddenly when they were buried by sediment, perhaps during a storm, rather than being gradually transported to their resting place by currents. The researchers also didn’t find any evidence of burrows in the sediment that might account for the linear pattern of these particular trilobites.

Almost all of the lined-up trilobites were sexually mature, which could be evidence that the animals were traveling en masse to spawning grounds. The nature of their death and fossilization also suggests it’s possible that they marched together to retreat from dangerous conditions during storms.

“These options are not mutually exclusive,” they wrote in the study. “Ampyx may have alternatively responded to environmental stress and reproduction signals by adopting the same behavior.”

Whatever the precise explanation of this behavior, the trilobite fossils offer a tantalizing glimpse into the early days of coordinated teamwork in animals.

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