Beginning 400 million years ago, nautiluslike 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, when South Dakota lay at the bottom of a shallow inland sea. In addition to ammonites, they found fossils of other marine creatures such as clams, sponges, corals, and fish. Measurements of the ratio of carbon isotopes—types of atoms that have different numbers of neutrons in their nucleus and, therefore, have different weights—in the limestone suggest that the site was a spot where methane-rich fluids rose to the sea floor from deep within Earth's crust. These "methane seeps" served as sea-floor oases, with methane-consuming microbes at the base of the food chain and creatures such as clams and marine snails either feasting on the microbes or hosting their own methane-munching bugs that allowed them to take advantage of the energy source directly. These creatures, and the predators that prey upon them, would have had higher-than-normal proportions of carbon-13 isotopes in their tissues than animals feeding on prey in ecosystems where the base of the food chain is composed of plants and photosynthetic organisms.
Previous studies suggest that there were hundreds if not thousands of such ancient methane seeps in a swath of sea floor that stretched from what is now eastern Montana to south-central Colorado, says Landman. At the South Dakota site, he and his colleagues found fossils of juvenile and adult ammonites, and they even unearthed ammonites that had shell damage suggesting they'd been preyed upon and partially eaten. That's only circumstantial evidence that the creatures spent their lives at these sea-floor oases, he notes. But the team's detailed geochemical analyses of the ammonite fossils, including a series of samples from one individual, reveal that the creatures' shells contain much higher proportions of carbon-13 than those in ammonites of the same age found elsewhere in the region. That difference provides strong evidence that these ammonites spent much, if not all, of their lives at the methane seep, the researchers contend in a forthcoming issue of Geology. If the ammonites had been consuming prey in a food chain that didn't ultimately derive its energy from ancient methane—one based on photosynthetic organisms, for instance—then the proportions of carbon-13 in the creatures' shells would have been much higher than the team actually measured.
"This is a really nice paper, and I like how the researchers made their case," says Ruth Martin, a paleontologist at the University of Washington, Seattle. The ancient methane seeps acted like modern-day reefs, she notes. "There was a good supply of food, and this would have been a nice place for the ammonites to hang out. ... It all makes sense."
Royal Mapes, an invertebrate paleontologist at Ohio University in Athens, agrees. "Undoubtedly, the ammonites were there [at the seeps] to take advantage of an incredibly rich food source." And perhaps it's no surprise that researchers haven't previously suggested that ammonites and other marine predators were full-time inhabitants of sea-floor oases, he notes, because oceanographers first discovered thriving ecosystems based on chemosynthesis and not photosynthesis around hydrothermal vents only 35 years ago, and similar ecosystems around deep-sea methane seeps were discovered less than 30 years ago.