Biggest Trilobite Sea Beasts Found ... in Swarms

Talk about ruining a good beach day.

Swarms of up to a thousand giant trilobites—extinct marine arthropods such as this 35-inch-long (90-centimeter-long) fossil specimen—roamed shallow prehistoric seas, new fossils show.

The 465-million-year-old fossils, found recently in northern Portugal, are of the largest trilobites ever discovered.

The trilobites may have clustered to mate and molt—shedding old exoskeletons as new ones grew in—as well as avoid predators, scientists say.

The benefits of swarming may explain why these distant relatives of horseshoe crabs were among the most widespread arthropods of the Paleozoic era (542 to 251 million years ago).

Even so, finding complete specimens bigger than 12 inches (30 centimeters) is rare—making the new find "remarkable," the study authors write in a recent edition of the journal Geology.

The critters lived at high latitudes near Gondwana—a huge southern supercontinent—and close to the South Pole during the Ordovician period (map of Earth during the Ordovician period).

This oxygen-rich, cold-water habitat may have contributed to these trilobites' gigantic sizes, the authors added.

But repeated, sudden, "lethal" influxes of oxygen-starved water may have led to the newfound trilobites' demise millions of years ago.