Ammonoidea

Ammonoids are extinct sea animals with coiled shells. They belong to the subclass Ammonoidea and are cephalopods, the same group as octopuses, squid, and cuttlefish (which make up the clade Coleoidea). Ammonoids are more closely related to those animals than they are to nautiluses (family Nautilidae), even though their shells look like nautilus shells.

The first ammonoids appeared during the Emsian stage of the Early Devonian, about 410 to 408 million years ago. The last ones disappeared during or soon after the Cretaceous–Paleogene extinction event, about 66 million years ago. They are often called ammonites. That name is most often used for the order Ammonitida, which was the only group of ammonoids left from the Jurassic until they died out.

Ammonoids were very varied over their long history, and scientists have named more than 10,000 species. They are excellent index fossils. Scientists have often used them to match rock layers to specific geologic time periods.

Most ammonoid fossil shells are flat spirals called planispirals. Some shells were coiled in other ways or not coiled at all. These unusual shapes are called heteromorphs, and they have been found mostly from the Cretaceous period.

Because ammonites and their close relatives are extinct, little is known about how they lived. Their soft body parts were almost never preserved. Even so, scientists have learned much by studying their shells and by using models of those shells in water tanks.

For other uses, see Ammonite (disambiguation). Not to be confused with Amniote.

Etymology

The name "ammonite," which the scientific term comes from, was inspired by the spiral shape of their fossilized shells. These shells look a bit like tightly coiled rams' horns. Long ago, Pliny the Elder called fossils of these animals ammonis cornua ("horns of Ammon"). He named them this way because the Egyptian god Ammon (Amun) was often shown wearing rams' horns.

Often, the name of an ammonite genus ends in -ceras. This comes from an ancient Greek word, κέρας (kéras), which means "horn."

Classification

See also: List of ammonite genera

Scientists sort ammonoids into six groups called orders. They go from the simplest to the more advanced: Agoniatitida (Lower Devonian to Middle Devonian), Clymeniida (Upper Devonian), Goniatitida (Middle Devonian to Upper Permian), Prolecanitida (Upper Devonian to Upper Triassic), Ceratitida (Upper Permian to Upper Triassic), and Ammonitida (Lower Jurassic to Lower Paleocene). Some classifications group them into just three orders: Goniatitida, Ceratitida, and Ammonitida. How ammonoids are sorted depends partly on their ornamentation and on the shape of their septa, the walls that make up the gas chambers in their shells.

The Treatise on Invertebrate Paleontology (Part L, 1957) divided ammonoids into eight suborders. These included the Anarcestina, Clymeniina, Goniatitina, and Prolecanitina from the Paleozoic, the Ceratitina from the Triassic, and the Ammonitina, Lytoceratina, and Phylloceratina from the Jurassic and Cretaceous. Later classifications sometimes treat these as orders within the subclass Ammonoidea.

Ammonoids are widely thought to have come from straight-shelled (orthocone) "nautiloids." These belonged to Bactritida during the early Devonian (Emsian), about 409 to 408 million years ago. Over time, their shells changed from straight, to curved, to loosely coiled, and finally to tightly coiled. The Late Devonian mass extinction (Kellwasser event) at the end of the Frasnian greatly reduced the variety of ammonoids, and only a few groups survived. Among them, some of the Tornoceratina, a subgroup of Goniatites, became the ancestors of all later ammonoids. Ammonoids recovered during the Famennian, when their siphuncle moved from the lower side of the shell to the upper side. Then the Hangenberg Event nearly wiped them out again at the end of the Devonian. During the Carboniferous, they went through cycles of growing and shrinking in number.

During the Permian, the Capitanian mass extinction event hurt some ammonoid groups badly, including Prolecanitida. The Ceratitida were less affected and became the most common ammonoids of the time. The end-Permian mass extinction again left very few ammonoids, but some groups survived. The end-Triassic extinction caused great harm to ammonites, and only a few survived. These included members of the family Psiloceratidae, which became the ancestors of later Jurassic and Cretaceous ammonites. Ammonites became very varied in the Early Jurassic. Ammonites with open or non-spiral shells, from the order Ancyloceratina, became common during the Cretaceous. Ammonites stayed very diverse until the very end of the Cretaceous. Then all of them died out during or shortly after the K-Pg extinction event, which was caused by the Chicxulub impact. Some scientists think ocean acidification from the impact played a key role. Young ammonites were small and drifted in the water, so they were badly affected. Nautiloids, like modern nautiluses, survived. This may be because they laid eggs in small groups and lived a long time on the sea floor, away from the direct effects of the impact.

Description

Further information: Cephalopod size

Ammonites came in many sizes. The smallest was Maximites from the Upper Carboniferous. Adults were only about 10 mm across. Some of the largest, such as Parapuzosia seppenradensis from Germany, could reach about 2 m across. The largest ammonite found in North America so far was Parapuzosia bradyi, which measured 137 cm across.

Their shells were divided into chambers. The part with chambers is called the phragmocone. Each chamber, called a camera, was separated from the next by a thin wall called a septum. Only the last and largest chamber, the body chamber, held the living animal. A thin tube called a siphuncle ran through the chambers. The animal used it to move water out of them, which helped it rise or sink. In ammonites, this tube ran along the outer edge of the shell. In nautiloids, it ran through the middle. The walls between chambers had bulges and dips. The lines where these walls met the shell, called sutures, came in three main types.

Most ammonites had shells coiled tightly in a flat spiral. In some species, the shell covered the earlier coils. In others, the older coils could be seen. Some shells were very different and are called heteromorphs. Baculites had a nearly straight shell. Turrilites and Bostrychoceras were coiled in a way that looked a bit like some gastropods. The strangest was Nipponites, which had a shell made of connected "U" shapes. It lived in the late Cretaceous in Japan and the United States.

Some ammonites have been found with plates of hard material near their shell opening. Scientists now think these plates were part of a jaw. The plates are called aptychus. They are rarely found still attached to a shell.

Main article: Aptychus

Soft tissue

Ammonites are sometimes found in rare fossil sites called lagerstatten, such as the Solnhofen Limestone. However, their soft body parts are rarely preserved. Until 2021, scientists knew of only a possible ink sac and maybe some digestive organs. In 2021, one specimen was described that showed some of the soft inside parts, including organs. Scientists used neutron imaging on a fossil found in 1998 and could see muscles. These muscles showed that ammonites could pull themselves back into their shells for protection. The soft body filled the largest part of the shell, at the end of the coil. The smaller chambers were walled off and could be filled with gas, which helped the animal float. The animal likely had a radula, a beak, and a crop for storing food. It was probably a meat eater, though many ammonites were likely filter feeders.

Scientists are still unsure how ammonites looked. A 2021 study found hooks on the enlarged tentacles of scaphitid ammonites, such as Rhaeboceras and Hoploscaphites. These hooks seem to be found only in that group. Artists have drawn ammonites with many arms, like a nautilus, or with fewer arms, like a squid. A 1996 study suggested they had 10 arms, like modern squid. Pictures of ammonites, such as craspeditid species and Kachpurites fulgens, are based on guesses. Paleontologist Mark Witton has said that all pictures of ammonites should be treated as very uncertain until new evidence is found.

Paleobiology

Modern Nautilus shells look different in males and females. The male shell is slightly smaller and wider than the female shell. This may explain why some ammonite shells of the same species come in two sizes. The larger shell, called the macroconch, is thought to have belonged to females. The smaller shell, called the microconch, is thought to have belonged to males. Females likely needed a larger body to produce eggs. A clear example is Bifericeras from the early part of the Jurassic period in Europe.

Scientists have only recently noticed this difference in ammonite shells. The two sizes were often thought to be two related but different species found in the same rocks. However, because the two sizes are so often found together, they are more likely males and females of the same species. Another clue is the width of the whorls, measured as a ratio to the diameter. This has been used to tell "male" (Largiventer conch "L") shells apart from "female" (Leviventer conch "l") shells.

Paleoecology

Many ammonoids probably lived in the open water of ancient seas, not on the sea bottom. Their fossils are often found in rocks formed where no bottom-dwelling life is found. They seem to have lived in the upper 250 metres (820 ft) of the water. Many, such as Oxynoticeras, were likely good swimmers with flat, disc-shaped, streamlined shells. Some were weaker swimmers and probably moved slowly along the bottom. A synchrotron analysis of an aptychophoran ammonite found tiny remains of isopod and mollusc young inside its mouth area. This shows that at least this kind of ammonite ate plankton. Ammonoids may have escaped danger by squirting ink, like modern cephalopods do. Ink is sometimes preserved in fossils.

Many ammonite shells have round holes that were once thought to be caused by limpets attaching to the shells. However, the holes are triangular. Their size, shape, and placement on both sides of the shells match the upper and lower jaws. This suggests they were more likely made by a medium-sized mosasaur biting the shell. Some ammonites seem to have lived in cold seeps and even reproduced there.

Cultural significance

See also: Ammolite

In medieval Europe, people thought fossil ammonites were petrified coiled snakes. They were called "snakestones" or, more often in medieval England, "serpentstones". People saw them as signs of the work of saints, such as Hilda of Whitby, whose story is told in Sir Walter Scott's Marmion, and Saint Patrick. The fossils were also believed to have healing or oracular powers. Sellers sometimes carved a snake's head onto the wide end of the fossil and sold them as petrified snakes. Other times, the head was just painted on.

Others believed ammonites, which they called "salagrana", were made of fossilized worm dung and could be used to ward off witches. Ammonites from the Gandaki River in Nepal are known as Shaligrams. Hindus believe they are a real form of Vishnu. The Ammonite order, created by George Dance the Younger, is an architectural order with ammonite-shaped volutes. It has been used a few times in British architecture.