An orgy of trilobites (really)

Four hundred million years ago, trilobites were a spectacularly successful group whose hard shells have fossilized in the billions. We now know that they molted, changing shells as they grew, and that the molting period was the perfect time to get together with the opposite sex. Fossil aggregations show that gigantic numbers of trilobites gathered and simultaneously molted and then, well, went at at each other. One paleontologist explained, "We find trilobite beds that we can trace across distances of 80 miles (130 kilometers), all the effect of a single event. The numbers of individuals caught in those must easily be in the billions." So essentially they tossed off their "clothes" and engaged in the largest crustacean orgy in all of history.