by AMNH on
The extinct mollusks known as ammonites inhabited the planet for more than 300 million years—almost twice as long as dinosaurs—before disappearing in the mass extinction event more than 65 million years ago. As many as 10,000 species may have existed, ranging from tiny organisms that measured only a fraction of an inch across to formidable animals more than 2 feet (0.6 meters) in diameter, such as the spectacular 75-million-year-old specimen shown here, which is on view in the Museum’s Grand Gallery.
Like the modern-day pearly nautilus, ammonites had shells that contained a body chamber for the animal and a series of connected chambers to hold air, enabling them to remain buoyant in water. A thin tube ran through the chambers, allowing the removal of water and replacement by air. Diverse, widespread, and abundant in their marine environments, ammonites left behind an extraordinary number of fossils.
Curator Neil Landman, who is investigating whether ammonites were already in decline at the time of their extinction, has examined specimens from all 31 known genera from around the world that were alive at the very end of the Cretaceous period. He is also researching jaw and tooth structures of different species to determine their role in the marine food web. “The challenge in studying ammonites is that they all are found in rock,” says Dr. Landman. “We are learning to get more information from CT [computed tomography] scans, which offer a non-destructive way to look at internal structures.”
Dr. Landman recently discussed his research, and shared CT scans of ammonite teeth and jaws, in this Inside the Collections video about the recently added Mapes collection of marine fossils to the Museum's holdings.
The most commonly found ammonite fossils are coiled like rams’ horns. In fact, ammonites are named for the Egyptian god Ammon, who was sometimes represented as a ram. But many ammonites were elongated or tubular, others snail-like, with surfaces ranging from smooth to heavily ribbed, probably for protection from predation. Since soft body tissues rarely fossilize, shells, along with jaws, are the focus of most ammonite research.
Ammonite fossils have intrigued humans for centuries. In medieval Europe, the coiled fossils were known as “snakestones” and were thought to have curative powers against snakebites and poisons.
In North America, Crow, Sioux, and other Great Plains Indian tribes often found such fossils, relics of the sea that once covered the continent’s western interior, and used these “stones” to heal injuries and to bring good fortune in marriage, hunting, travel, and warfare.
See ammonites on view in The Power of Poison and in the Grand Gallery.
A version of this article appears in the Spring 2014 issue of Rotunda, the Member magazine.