Check out this 480 million-year-old conga line of arthropods

When I was little, my big brother would take me fossil-hunting on a quest for trilobites, marine arthropods that have been extinct for around 250 million years. Occasionally we'd find lone specimens but never a bunch of them in a conga line as seen above. Paleontologists at France's Université Claude Bernard Lyon 1 studied lines of nearly two dozen trilobytes from Moroccan fossil beds to gain insight into the origins of collective social behavior. From the New York Times:

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, (paleontologist Jean) 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.