Beginning 400 million years ago, nautilus-like creatures known as ammonites - which sported dozens of tentacles and lived in spiral, conical, or helical shells - roamed the open ocean in search of fish and other prey. At least that’s what paleontologists have long assumed. But a new study finds that some members of this ancient group—relatives of octopi, squid, and cuttlefish—were far more sedentary creatures, spending most of their lives at spots where methane bubbled up from the sea floor.
Ammonites were one of the most long-lived groups of prehistoric animals, only dying out 65 million years ago, when they succumbed to the same mass extinction that claimed the dinosaurs. Scientists have typically found their fossils—shells that range from thumbnail-size to 2 meters across—in rocks derived from sea-floor sediments that contain no bottom-dwelling life, indicating that the creatures inhabited the open ocean and then sank to the barren sea floor after they died.
But new analyses of fossils unearthed in southwestern South Dakota dispel the notion that all ammonites were nomadic. Researchers led by Neil Landman, an invertebrate paleontologist at the American Museum of Natural History in New York City, studied fossils embedded in a 13-meter-high, 20-meter-wide chunk of limestone that formed almost 75 million years ago..