Ammonites were ancient marine creatures that once filled Earth’s oceans. These shelled mollusks were prominent inhabitants of prehistoric seas for millions of years. Their distinctive forms and widespread presence across geological eras make them fascinating subjects.
Unraveling Ammonites
Ammonites were marine animals belonging to the class Cephalopoda, characterized by their chambered, spiral shells. Most ammonite shells coiled into a flat spiral, known as a planispiral, though some species developed unusual shapes like hairpin-like or even straight shells. The animal occupied only the outermost chamber, adding new chambers as it grew and regulating buoyancy by filling earlier chambers with gas or fluid through a siphuncle. This tube ran along the outer rim of the shell, unlike the central siphuncle of nautiloids.
Ammonites ranged in size from a few millimeters to 1.8 meters in diameter, with some incomplete specimens suggesting a potential diameter of 2.5 to 3.5 meters. They thrived in ancient oceans from the Early Devonian period, approximately 410 million years ago, until the end of the Cretaceous period. As predators, they likely fed on small plankton, slow-moving bottom-dwellers, and small fish, capturing prey with tentacles.
The Great Vanishing
Ammonites vanished from Earth’s oceans about 66 million years ago, marking their extinction at the end of the Cretaceous period. This event coincided with the demise of non-avian dinosaurs during the Cretaceous-Paleogene (K-Pg) extinction event. The primary cause for this mass extinction is attributed to the impact of a massive asteroid, estimated to be 10 to 15 kilometers wide, which struck the Yucatán Peninsula and formed the Chicxulub crater. This impact triggered a global catastrophe, including an “impact winter” that halted photosynthesis by blocking sunlight with dust clouds.
The asteroid impact caused tsunamis, earthquakes, and the release of enormous amounts of carbon dioxide. This environmental disruption resulted in severe cooling and ocean acidification, decimating marine plankton and collapsing the food chain for many marine organisms, including ammonites. While massive volcanic activity from the Deccan Traps also occurred, the asteroid impact was the main driver of the K-Pg extinction. Volcanism potentially aided recovery rather than causing the initial widespread extinctions. Ammonites, with their plankton-dependent larval stages and likely shallow-water habitats, were particularly vulnerable to these sudden environmental shifts.
Traces in Stone
Ammonites are no longer living, but their existence is documented through an abundant fossil record found worldwide. These fossils are commonly discovered in sedimentary basins, river shores, sea coasts, cliffs, and quarries across continents like North America and Europe. Fossilization typically begins when a dead ammonite settles on the seabed and is rapidly buried by sediment, preventing decay. Over millions of years, the aragonite of the shell can transform into more stable calcite, or minerals from surrounding sediments can fill the shell’s chambers or the void left by a dissolved shell, forming a cast.
Ammonite fossils are valuable to science as “index fossils.” Their rapid evolution and wide distribution mean specific species existed for relatively short, distinct geological periods, allowing paleontologists to accurately date the rock layers where they are found. This precision allows for the correlation of rock strata across different regions globally, making them a fundamental tool in geology. Some ammonite fossils, particularly from the Bearpaw Formation in Alberta, Canada, exhibit iridescence due to the mineralization of their aragonite shells, transforming into the gemstone ammolite.
Living Descendants
While ammonites are extinct, their evolutionary lineage extends to other cephalopods that inhabit modern oceans. The closest living relatives of ammonites are not nautiluses, as often assumed due to their similar external shells, but rather the coleoids, which include squids, octopuses, and cuttlefish. Despite having external shells, nautiluses belong to a separate subclass (Nautiloidea) and have distinct biological features, such as simpler septa within their shells compared to the complex septa of ammonites.
Modern cephalopods share characteristics with their ancient ammonite relatives, such as tentacles and complex nervous systems. The nautilus is a survivor of the K-Pg extinction event that claimed the ammonites, possibly due to its ability to inhabit deeper ocean environments, which were less affected by surface-level catastrophes. The larger size of nautilus hatchlings may also have contributed to their survival by giving them broader food options compared to the smaller, more specialized ammonite hatchlings.