Ammonites, marine creatures with distinctive coiled shells, disappeared millions of years ago, leaving only fossilized remains. Their extinction was a major event, prompting scientists to investigate its causes and their geological legacy.
What Were Ammonites?
Ammonites were shelled marine mollusks in the class Cephalopoda, related to modern octopuses, squids, and nautiluses. They possessed an external, chambered shell, typically coiled in a flat spiral, which they used for buoyancy control. As these animals grew, they added new, larger chambers to their shells, sealing off the old ones with internal walls called septa. The animal lived only in the outermost chamber.
Ammonites inhabited marine environments, primarily shallower seas at depths ranging from 50 to 400 meters. Ammonites were predators, feeding on smaller marine life like crustaceans, bivalves, fish, and plankton, captured with tentacles. Their shells, often adorned with intricate patterns of ribs, spines, or tubercles, provided both structural strength and defense against marine predators like mosasaurs and ichthyosaurs.
Their Disappearance
Ammonites vanished from Earth approximately 66 million years ago, during the Cretaceous-Paleogene (K-Pg) extinction event. This period also led to the extinction of non-avian dinosaurs and many other plant and animal species worldwide. The primary cause of this mass extinction is widely attributed to the impact of an asteroid, estimated to be 10 to 15 kilometers wide, which struck the Yucatán Peninsula.
The asteroid impact triggered widespread environmental changes, including fires and a prolonged “impact winter” that blocked sunlight, halting photosynthesis. This global disruption led to a collapse of marine food chains, particularly affecting plankton, which were a food source for many ammonite species. The impact also caused rapid ocean acidification, a condition making ammonites, especially their plankton-dependent larval stages, particularly vulnerable. While large-scale volcanic eruptions from the Deccan Traps also occurred around this time, contributing to climate instability, the asteroid impact is considered the main trigger for the K-Pg extinction.
Where They Are Found Now
Today, ammonites are found exclusively as fossils in sedimentary rocks globally. Their shells, made of calcium carbonate, were well-suited for fossilization. When an ammonite died, its shell would sink to the seabed, where it could be quickly buried by sediment. This burial protected the shell from decomposition and scavenging.
Over millions of years, the accumulating sediment compressed and hardened into rock, while groundwater carrying dissolved minerals seeped into the shells. These minerals, like calcite, pyrite, or silica, gradually replaced the original shell material or filled the internal chambers, creating detailed mineral casts or preserving their intricate structure. Ammonite fossils are particularly common in rocks from the Jurassic and Cretaceous periods, the eras when they were most abundant. Their widespread presence and rapid evolution make them “index fossils,” allowing geologists to accurately date rock layers and understand ancient marine environments.
Modern Relatives and Legacy
Although extinct, ammonites share a common lineage with living cephalopods. The nautilus, with its external coiled shell, often resembles ammonites, but evidence suggests ammonites were more closely related to coleoids, including modern squids, octopuses, and cuttlefish. Nautiluses, coexisting with ammonites, survived the K-Pg extinction, possibly due to inhabiting deeper ocean environments and having larger, more protected hatchlings.
Ammonites hold scientific importance as fossils. Their abundance and rapid evolutionary changes over geological time make them tools for biostratigraphy, allowing scientists to precisely date rock layers, sometimes to within a few hundred thousand years. By studying ammonite fossils, paleontologists gain insights into ancient marine ecosystems, the impacts of past climate change, and the broader history of life on Earth. Their diverse forms and wide distribution provide a record of marine evolution before their disappearance.