Photograph by Breck P. Kent, Nat Geo Image Collection
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Ammonites, which evolved about 416 million years ago, were once the most abundant animals of the ancient seas. Scientists have identified more than 10,000 ammonite species, such as Arnioceras semocostatum pictured here, and use their shells to date other fossils.

Photograph by Breck P. Kent, Nat Geo Image Collection

What are ammonites, and how did they come to rule the prehistoric seas?

Earth once hosted more than 10,000 species of these ancient marine predators. Find out how they lived, when they vanished, and how much we know about them today.

With squidlike tentacles extending from their distinctive multichambered shells, the extinct marine predators known as ammonites were once among the most successful and diverse animals on Earth. Scientists have identified more than 10,000 species from fossils found nearly everywhere on the planet where oceans once existed, from the Great Plains of North America to the foothills of the Himalaya and the glaciers of Antarctica.

Ammonite is actually the colloquial term for ammonoids, a large and diverse group of creatures that arose during the Devonian period, which began about 416 million years ago. Ammonoids are related to other cephalopods—such as squid, octopuses, and cuttlefish—and they were early relatives of the modern nautilus. Meanwhile, true ammonites are a suborder of ammonoids that didn’t appear until about 200 million years ago, in the Jurassic period.

During their long history, ammonites survived three mass extinctions—most notably the Permian extinction, a global warming that was brought on by volcanic activity about 252 million years ago, and that killed 96 percent of the planet’s marine species. While many species of ammonites died out in that extinction event, scientists believe the survivors diversified explosively in the million years that followed. Ammonites hunted the planet’s seas until they were entirely wiped out by the same cataclysm that claimed the non-avian dinosaurs about 66 million years ago.

Appearance and behavior

Based on the fossil record, ammonites came in a wide range of sizes and shapes, from smaller than an inch to as large as nine feet wide. Some ammonites had long, straight shells, while others had helix-shaped shells. Most species, however, had coiled shells lined with progressively larger chambers separated by thin walls called septa.

The animals constantly grew new shell material as they aged, but the their bodies always remained in the outer chamber. The walls that separated each chamber helped protect the shell from being crushed. They were connected to the shell by intricate lines known as sutures—the complexity of which varied tremendously throughout this animal’s evolution.

The many chambers of their shells likely helped these cephalopods glide through the planet’s warm, shallow seas. A thin, tubelike structure called a siphuncle pumped air through the interior chambers of the shell, which scientists believe helped provide buoyancy and move ammonites through the water. It’s unclear whether ammonites were very efficient swimmers, though.

Scientists believed that ammonites, like modern cephalopods, had soft body tissue with tentacles attached to their heads for catching prey. Fossil evidence indicates they had sharp, beaklike jaws to snare prey such as plankton, crustaceans, and other ammonites. They were also preyed on by larger reptiles and fish.

Evolution and extinction

As ammonites evolved throughout the Mesozoic era, between 252 and 66 million years ago, their shell structures grew smaller, more tightly coiled, and more complex. Early ammonites had simple suture lines traced across their shells, while the sutures on species from the Cretaceous period (145 to 66 million years ago) formed intricate patterns that may have given later ammonites greater buoyancy control.

The ammonites came to an end 66 million years ago, during the planet’s most recent mass extinction event. In the final days of the Cretaceous, a 7.5-mile-wide asteroid slammed into Earth and killed off more than three-quarters of all species on the planet. Some scientists hypothesize that the ammonites couldn’t survive the aftermath because of the sudden decline of their main food source: marine plankton.

Ammonite shells are used today as index fossils, meaning they can help date other fossils that are found in the same layer of marine rock. These cephalopods make for ideal index fossils because they are abundant, widespread, and their various species lived during distinct time periods that can be easily identified by their suture patterns. Ammonite fossils also reveal information about ancient climates, as the sites where they are unearthed must have once been covered by ancient seas.