Ammonoidea

Ammonoids, often called ammonites, were sea creatures with coiled shells. They belonged to the subclass Ammonoidea and were a type of cephalopod. They are related to modern octopuses, squid, and cuttlefish, but not to nautiluses, which they looked like. Ammonoids first appeared about 410-408 million years ago during the Emsian stage of the Early Devonian period. They went extinct around 66 million years ago during the Cretaceous–Paleogene extinction event.

Over their long history, ammonoids evolved into more than 10,000 different species. Their fossils are important to scientists. They help match rock layers to specific times in Earth's past, acting as index fossils. Most ammonite shells were coiled in a flat spiral called a planispiral. Some had helically spiraled or unusual shapes called heteromorphs, especially during the Cretaceous period. Ammonoids are extinct and their soft bodies are rarely preserved. Scientists have learned a lot about them by studying their shells and testing models in water.

Etymology

The name "ammonite" comes from the spiral shape of their fossilized shells, which look like ram's horns. The ancient writer Pliny the Elder called these fossils ammonis cornua, meaning "horns of Ammon." Many ammonite genus names end in -ceras, a word from ancient Greek meaning "horn."

Main article: Ammon

Classification

See also: List of ammonite genera

Ammonoids are sorted into groups based on their shell shapes and fossils. They include Agoniatitida, Clymeniida, Goniatitida, Prolecanitida, Ceratitida, and Ammonitida. These groups help scientists learn about how ammonoids changed over many years.

Some scientists put ammonoids into just three big groups: Goniatitida, Ceratitida, and Ammonitida. How we sort ammonoids depends on the designs and shapes of their shells.

Evolutionary history

Ammonoids were a group of coiled-shelled sea creatures that lived long ago. They first appeared around 409–408 million years ago during the early Devonian period. They came from straight-shelled creatures called nautiloids.

Over millions of years, ammonoids changed a lot and faced times when many of them died out. But they always came back and evolved new forms.

By the end of the Cretaceous period, about 66 million years ago, ammonoids were still many and different. But they vanished during the K-Pg extinction event when a huge asteroid hit Earth. This changed the oceans and made it hard for ammonoids to live, especially their young. Today, nautiloids are the closest living relatives to ammonoids.

Description

Ammonoids, also called ammonites, were a group of extinct, coiled-shelled sea creatures. They came in many sizes, from very small, about 10 mm wide, to very large, up to 2 meters (6.6 feet) across. Their shells had many rooms, and they added a new room as they grew.

These shells had a special part called a phragmocone, made of many small rooms separated by walls. A thin tube called a siphuncle went through these rooms, helping the ammonite stay balanced in the water. This let them move up and down in the water. Ammonoids are related to today's octopuses, squid, and cuttlefish, but they looked more like modern nautiluses.

Soft tissue

Scientists study special fossils to learn about the soft parts of ammonites, which are hard to find. In 2021, they found a fossil showing some inside parts, like muscles that could pull the ammonite into its shell for safety. The ammonite's body filled most of its coiled shell. The smaller parts helped it stay afloat in the water.

Some ammonites, like Rhaeboceras and Hoploscaphites, had special hooks on their tentacles. But these were not common for all ammonites. Scientists do not know exactly how many arms ammonites had or what they looked like. Any pictures of them are just good guesses until more evidence is found.

Paleobiology

In modern Nautilus shells, males and females can look different. The male shell is usually smaller and wider than the female shell. Scientists think ancient ammonites were the same. The bigger shells, called macroconchs, were likely females because they needed to be larger to carry eggs. The smaller shells, called microconchs, were likely males.

People used to think these different-sized shells were from two separate species. But because they are found together often, experts now believe they were just males and females of the same species. The width of the shell's coil also helps tell male and female shells apart.

Paleoecology

Many ammonoids probably lived in the open water of ancient seas, not near the sea bottom. Their fossils are often found in rocks that show no signs of life on the ocean floor. This suggests they swam in the upper 250 metres of the water. Some ammonoids, like Oxynoticeras, had flat, discus-shaped shells that helped them swim well. Others moved more slowly near the bottom.

Scientists think some ammonites ate tiny plants and animals floating in the water, called plankton. Like modern squid and octopuses, they might have squirted ink to escape predators. Round holes in some ammonite shells were once thought to be from small sea creatures called limpets. But they are now believed to be bite marks from mosasaurs, large marine reptiles that hunted ammonites. Some ammonites lived and reproduced in special areas of cold, chemical-rich water called cold seeps.

Cultural significance

In medieval Europe, people thought fossilized ammonites were coiled snakes. They called them "snakestones" or "serpentstones". They believed these fossils showed the work of saints and had special powers, like healing. Some traders would even carve or paint a snake's head on the fossils to sell them as petrified snakes.

In other places, people thought ammonites could protect against witches. In Nepal, ammonites from the Gandaki River are called Shaligrams. They are important to Hindus as a symbol of the god Vishnu. There is also an architectural style called the Ammonite order that uses ammonite shapes in building designs.