Ammonoidea

Ammonoids lived in the sea a long time ago.

They had shells that curled like a ram's horn. Some shells were tight curls. Other shells were not curls at all.

Inside the shell were many small rooms. A thin tube ran through these rooms. This tube helped the animal move up or down in the water.

These animals are gone now. We find their shells in rocks today. They help us learn about the past.

Ammonoids were sea animals that lived long ago.

They are related to living octopuses and squid. Most ammonoids had shells that curled in a spiral. These shells look like the horns of a ram.

Inside the shell, there were many small rooms. We call the chambered part of the shell the phragmocone. Thin walls called septa divided these rooms. A tiny tube called a siphuncle ran through the walls. This tube helped the animal move up or down in the water. It did this by moving water out of the shell rooms. This helped the animal stay at the right depth.

Scientists study the shells to learn about these animals. They look at the suture lines. These are the marks where the walls meet the outer shell. These lines can look like peaks or valleys.

Ammonoids lived for a very long time. They first appeared about 410 million years ago. They went extinct about 66 million years ago. Some think a large impact from space caused this. This event changed the oceans and hurt small sea life.

Ammonoids were amazing sea creatures that lived a long time ago. They belong to a group called cephalopods. This means they are close relatives of living animals like octopuses, squid, and cuttlefish.

Even though they look like modern nautiluses, they are actually more closely related to squid. Most people call their fossils ammonites. These fossils are very helpful to scientists. They act as index fossils to help link different rock layers to specific times in history.

An ammonoid lived inside a special shell called a phragmocone. This shell was divided into many rooms called camerae. Thin walls called septa separated these rooms from each other. The living animal stayed in the very last, largest room.

To move through the ocean, the animal used a tiny tube called a siphuncle. This tube ran along the outer edge of the shell. The siphuncle helped the animal move water out of the empty chambers. This allowed the ammonoid to control its buoyancy. By doing this, it could rise or sink in the water.

Scientists study the shells to see how these animals grew. They look closely at the suture lines. A suture line is the place where the internal walls meet the outer shell. These lines can be very complex. They have peaks called saddles and valleys called lobes.

Some ammonoids had shells that curled in a tight spiral. Others were heteromorphs, which means they had shells that were not spirals at all. Some shells were even shaped like helices.

The history of these animals spans millions of years. The first ammonoids appeared about 410 to 408 million years ago. They went through many changes and survived several mass extinctions. One group, the Ammonitida, lived from the Jurassic until the very end.

Ammonoids were not all the same size. The smallest was a type called Maximites. Some were huge, like the Parapuzosia seppenradensis. This large ammonite could reach a diameter of nearly one meter.

Eventually, the ammonoids vanished from the Earth. They went extinct about 66 million years ago. This happened during the Cretaceous–Paleogene extinction event. Many scientists believe a large impact from space caused this. This impact may have caused ocean acidification. This change in the water likely hurt the small larvae of the ammonoids. While the nautiluses survived, the ammonoids did not.

Ammonoids were a diverse group of extinct cephalopods. They belonged to the subclass Ammonoidea. While they look similar to modern nautiluses, they are actually more closely related to living octopuses, squid, and cuttlefish. These living relatives belong to a group called Coleoidea. Most people refer to their fossils as ammonites. This name is most often used for the order Ammonitida.

The shell of an ammonoid is a complex structure. The chambered part of the shell is called the phragmocone. This part contains several chambers called camerae. Thin walls known as septa divide these chambers from one another. The living animal occupied only the last and largest chamber, called the body chamber. As the animal grew, it added newer, larger chambers to the open end of the coil.

To move through the ocean, ammonoids used a mechanism for buoyancy. A thin living tube called the siphuncle passed through the septa. This tube connected the chambers of the phragmocone to the living body. Through a process called hyperosmotic active transport, the animal emptied water out of the empty chambers. This allowed the ammonoid to control its position in the water column. By adjusting the gas and water in its shell, it could rise or sink.

Scientists distinguish ammonoids from nautiloids by looking at their internal anatomy. In most ammonoids, the siphuncle runs along the outer rim of the shell, known as the venter. In nautiloids, the siphuncle runs through the center of the chambers. Additionally, the septa of ammonoids have unique shapes. They often have bulges and indentations. The intersection where the septum meets the outer shell is called the suture line. These lines form patterns of peaks called saddles and valleys called lobes.

Ammonoids showed incredible variety in their physical forms. Most shells were planispirals, meaning they coiled in a flat plane. Some were involute, where the outer whorl covers the previous ones. Others were evolute, where the whorls do not cover each other. During the Cretaceous period, many heteromorphs appeared. These were ammonoids with non-spiral or helical shapes.

The history of ammonoids spans hundreds of millions of years. They first appeared during the Emsian stage of the Early Devonian, about 410 to 408 million years ago. They likely evolved from straight-shelled nautiloids called Bactritida. The group survived several mass extinctions, such as the end-Permian event. During the Jurassic and Cretaceous, they became very diverse. The order Ammonitida was a major group during this time.

Ammonoids varied greatly in size. The smallest known type was Maximites from the Upper Carboniferous. Some species were massive. The Titanites of the Jurassic reached 0.5 meters in diameter. The Parapuzosia seppenradensis of the Cretaceous was even larger. This species could reach a diameter of nearly 1 meter.

All ammonoids eventually went extinct. This occurred during the Cretaceous–Paleogene extinction event about 66 million years ago. This event was caused by the Chicxulub impact. Some scientists suggest that ocean acidification caused by the impact killed them. The larvae of ammonoids were likely small and planktonic. This would have made them very vulnerable to changing ocean chemistry. While nautiluses survived by laying eggs on the seafloor, the ammonoids vanished.