It’s unclear whether ammonites were very efficient swimmers, though. 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. The many chambers of their shells likely helped these cephalopods glide through the planet’s warm, shallow seas. They were connected to the shell by intricate lines known as sutures-the complexity of which varied tremendously throughout this animal’s evolution. The walls that separated each chamber helped protect the shell from being crushed. The animals constantly grew new shell material as they aged, but the their bodies always remained in the outer chamber. Most species, however, had coiled shells lined with progressively larger chambers separated by thin walls called septa.
Some ammonites had long, straight shells, while others had helix-shaped shells. Appearance and behaviorīased 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. 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. While many species of ammonites died out in that extinction event, scientists believe the survivors diversified explosively in the million years that followed. Meanwhile, true ammonites are a suborder of ammonoids that didn’t appear until about 200 million years ago, in the Jurassic period.ĭuring 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. Ammonoids are related to other cephalopods-such as squid, octopuses, and cuttlefish-and they were early relatives of the modern nautilus.
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.Īmmonite 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.
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.
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