This is Trilobite Tuesday post #13.
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Clues to animal behavior are very rare in the fossil record. However, a Moroccan fossil of 22 small trilobites might provide some of the earliest evidence. These trilobites lived about 480 million years ago. And, their lineup might be a display of complex social behavior long before it was expected. The details appear in a paper published in Scientific Reports back in October 2019.
Smithsonian Magazine has a nice report on the fossil here.
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.
(CNRS / ENS de Lyon / Université Claude Bernard Lyon 1)
