Ammonoidea

Ammonoids, often called ammonites, are extinct sea creatures with coiled shells. They belonged to the subclass Ammonoidea and were a type of cephalopod, related more closely to modern octopuses, squid, and cuttlefish than to nautiluses, which they looked like. Ammonoids first appeared about 410-408 million years ago during the Emsian stage of the Early Devonian period and 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 very important for scientists because 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, but some had helically spiraled or unusual shapes called heteromorphs, especially during the Cretaceous period. Although 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 tightly coiled ram's horns. The ancient writer Pliny the Elder called these fossils ammonis cornua, meaning "horns of Ammon," because the Egyptian god Ammon was often shown wearing ram's horns. 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 can be grouped into different orders based on their shell features and fossil records. They include Agoniatitida, Clymeniida, Goniatitida, Prolecanitida, Ceratitida, and Ammonitida. These groups help scientists understand how ammonoids evolved over millions of years.

Different scientists sometimes group these ammonoids into fewer orders, such as Goniatitida, Ceratitida, and Ammonitida. The way ammonoids are classified often depends on the patterns and structures of their shells.

Evolutionary history

Ammonoids, a group of extinct coiled-shelled sea creatures, first appeared around 409–408 million years ago during the early Devonian period. They evolved from straight-shelled creatures called nautiloids. Over millions of years, ammonoids went through many changes and faced several mass extinctions that reduced their numbers, but they always managed to recover and diversify again.

By the end of the Cretaceous period, about 66 million years ago, ammonoids were still quite diverse with at least 57 different species. However, they, along with the dinosaurs, disappeared during the K-Pg extinction event caused by a massive asteroid impact. This event led to changes in the oceans that made it hard for ammonoids to survive, especially since their young were small and drifted in the water. Today, only nautiloids, which look similar to ammonoids, remain as their closest living relatives.

Description

Ammonoids, also known as ammonites, were a group of extinct, coiled-shelled cephalopods. They varied greatly in size, with the smallest reaching just 10 mm in diameter and some of the largest growing up to 2 meters (6.6 feet) across. Their shells were chambered, with each new chamber added as the ammonite grew.

These shells had a special structure called a phragmocone, made up of many small chambers divided by walls. A thin tube called a siphuncle passed through these chambers, helping the ammonite control its buoyancy in the water. This allowed them to move up and down in the water column. Ammonoids are closely related to modern octopuses, squid, and cuttlefish, but they looked more like today's nautiluses.

Soft tissue

Scientists have studied special fossils to learn about the soft parts of ammonites, which are hard to find. In 2021, they discovered a fossil showing some internal organs, including muscles that could pull the ammonite into its shell for safety. The body of the ammonite filled the largest part of its coiled shell, while the smaller sections were used for buoyancy, helping it stay afloat in the water.

Some ammonites, like Rhaeboceras and Hoploscaphites, had special hooks on their tentacles. However, these were not common among all ammonites. Scientists still aren't sure exactly how many arms ammonites had or what they looked like, and any pictures of them are just educated guesses until more evidence is found.

Paleobiology

One interesting feature seen in modern Nautilus shells is that males and females can look different. The male shell is usually a bit smaller and wider than the female shell. Scientists think the same thing happened with ancient ammonites. 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.

This idea was only recently accepted. Before, people often thought these different-sized shells were from two separate species. But because they are found together so often, experts now believe they were just males and females of the same species. Another clue to this difference is the width of the shell's coil, which helps tell male and female shells apart.

Paleoecology

Many ammonoids likely lived in the open water of ancient seas rather than near the sea bottom. Their fossils are often found in rocks that show no signs of life on the ocean floor, suggesting they swam in the upper 250 metres of the water. Some ammonoids, like Oxynoticeras, had flat, discus-shaped shells that helped them swim well, while others moved more slowly near the bottom.

Scientists think some ammonites ate tiny plants and animals floating in the water, called plankton, based on findings in their fossilized remains. 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 even 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 and 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 were made from worm dung and could protect against witches. In Nepal, ammonites from the Gandaki River are called Shaligrams and 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.