Scientists still debate the causes of the Cambrian explosion, but the most widely accepted theory is that oxygen in Earth’s atmosphere began slowly increasing around 550 million years ago. Oxygen provided a much more efficient way to metabolize food, giving animals more energy to mobilize and hunt prey. The emergence of predators kicked off these escalating arms races, and then we essentially witnessed an explosion of different ways of doing business. Among the most bizarre: priapulids (penis worms): worms that turned their mouths inside out, revealing a throat lined with hairy teeth. These worms, also known as cactus worms, are mostly extinct today, but were widespread during the Cambrian. The fossilized worm found in the Grand Canyon (which was then closer to the equator and therefore had a tropical climate) represents a previously unknown species. Due to its relatively large size—about 10 centimeters—and distinctive teeth, it has been named Kraytdraco spectatus, after the fictional krayt dragon from the Star Wars universe. This particular penis worm appears to have had a gradient of hundreds of branched teeth, used to scoop food into an extensible mouth.
