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Trilobite Fossil Shows Animals Have Stood in Line for Hundreds of Millions of Years

A line of 480-million-year-old trilobites found in Morocco may be the earliest evidence of collective animal behavior

(CNRS / ENS de Lyon / Université Claude Bernard Lyon 1)
smithsonian.com

Waiting in line may feel like one of the more primitive aspects of modern life; think of all those people lined up for the latest iPhone, concert tickets or a trendy pastry. But following the leader is actually a pretty complex bit of collective social behavior that may have ancient origins, according to a new study of 480-million-year old fossilized trilobites.

Found in Morocco, the fossil depicts “conga line” of 22 small, spiny, blind trilobites of the species Ampyx priscus. Becky Ferreira at the New York Times reports that it is one of the earliest pieces of evidence for group behavior ever discovered. The study appears in the journal Scientific Reports.

Around 541 million years ago during a period called the Cambrian Explosion, lots of new animal species appeared in the world’s oceans, sporting evolutionary upgrades like skeletons and nervous systems. Prior to this period, there’s no evidence of group behavior in animals because Precambrian life lacked nervous systems, as the study’s first author Jean Vannier, a researcher at the University of Lyon, tells Ferreira.

During another biodiversity bloom called the Great Ordovician Biodiversification Event about 100 million years later, Ampyx priscus entered the scene. Though the little arthropods were blind, the trilobites were likely able to stay in line by sensing each other’s rearward facing spines or by using chemical cues, according to a press release.

The researchers hypothesize that the creatures intentionally formed the line while swarming across the seafloor during migration or during mating season. “Given the scale of the patterns seen, this consistent linearity and directionality is unlikely to be the result of passive transportation or accumulation by currents,” the researchers conclude in their study.

This isn’t the first line of the little trilobites ever discovered, reports Michael Greshko at National Geographic. In 2008, researchers uncovered a similar line of the prehistoric arthropods. However, those paleontologists suspected the trilobites were likely lined up in a burrow, avoiding detection by a nearby predator when they were buried and fossilized. Other researchers have suggested that ocean currents deposited the arthropods in a line before they were fossilized. Vannier says that when they excavated the fossil in Zagora, Morocco, they did not see any evidence that the animals were in a burrow. The fact that they were all in the same high state of preservation indicates that the trilobites were probably all buried in sediment at once—not gradually deposited by ocean currents over time.

The overarching point of the study, however, is that collective behavior is deeply engrained in animals and developed very quickly after early ocean creatures developed nervous systems and sensory organs.

“It shows that collective behavior is not a new evolutionary innovation that appeared a couple of million years ago,” Vannier tells Greshko. “Instead, it is much older, dating back to the first biodiversification events of animal life.”

According to the press release, moving in a coordinated group likely offered some sort of evolutionary advantage to the creatures, though it’s difficult to say exactly what. The team hopes to continue investigating the early days of collective behavior. Next, they’ll look at a group of similarly-aligned, shrimp-like creatures found in China dating back 520 million years, even earlier than the trilobites.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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