Ammonoids were marine animals of the Mollusca and Cephalopoda order that had a coiled external shell similar to that of modern nautilus.
Ammonoids used a hyperosmotic active transport process to pass a thin living tube through their septa and into empty shell chambers, helping them maintain buoyancy in the water column.
Origin
Ammonoids were an abundant group of invertebrates in marine ecosystems from Devonian times until their extinction during the Cretaceous period (66 million years ago). They can often found as fossils, formed when animal remains are buried by sediment and solidify into rock.
The name ammonoid derives from the Greek word ammon, meaning “horns of Ammon”. This moniker given to ammonite fossilized specimens with ram’s horn-like shells (Plinius the Elder, AD 23-79) preserved within them.
Ammonoids may be extinct today, but they remain an integral part of Earth’s history. Their evolution and distribution make them ideal index fossils – fossils that can be use to date or identify geologic periods or faunal stages.
Early Devonian ammonoids first appear in the fossil record. These creatures were closely related to bactritids, which distinguished by having a coiled shell with dome-shaped gas chambers connected by a tube known as the siphuncle to their body chamber.
However, as evolution progressed, bactritid shells became less circular and more oval. The septa connecting the phragmocone to the body chamber changed shape, while the siphuncle moved from centre to ventral side of the shell.
Ammonoids possess a distinctive suture line that runs through the center of their septa and camerae to connect their phragmocone with their body chamber. This suture line has distinctive folding that creates saddles and lobes–features which set it apart from nautiloids’ gently curving sutures.
Ammonoids grew more complex over time, as evidenced by several Triassic and Cretaceous ammonioids such as Beyrichites deleeni and Placenticeras intercalare.
Sexual dimorphism
Another pattern observed in ammonoids was their sexual dimorphism which remained constant over time. Male ammonoids had smaller, more delicate shells while the females boasted larger, stronger shells – this served to provide additional defense against predators.
These features were essential to their survival in the sea, where they could swim and float effortlessly without exertion. It is this remarkable ability that makes ammonoids such an influential figure in Earth’s history.
Evolution
The ammonoid has an impressive evolutionary history and it’s one of the world’s most successful mollusk groups. It serves as a key index fossil to date ocean floor layers and provide evidence for biostratigraphy, or biogeographic stratigraphy, worldwide.
Ammonoids are pelagic mollusks with a shell made up of gas-filled chambers. This shell allows them to float and swim efficiently with minimal effort.
Ammonoids are direct ancestors to cephalopods and squids, yet lack ink sacs like their close relatives. Furthermore, these unique cephalopods possess an externally shelled body chamber.
Ammonitella (Drushits and Khiami 1970) are composed of an initial chamber and subsequent whorl, each with a constriction at the aperture known as a nepionic constriction that appears in many derived ammonoids’ embryonic shells.
Morphology
Since the 1930s, researchers have extensively studied the evolution of ammonoid shell morphology. Of particular note are changes to its shape and coiling which likely caused by constructional constraints or adaptation to aquatic environments.
For example, several lineages experienced a decrease in the size of embryonic shells that occurred independently, convergently, and simultaneously. In some instances, this change also accompanied by increased coiling and an expansion of body chamber volume.
Another distinctive characteristic of ammonoid shells was the development of an umbilical lid, or lateral wall extension covering the umbilicus. This feature can observe across several lineages, even those which were extinct during major Devonian events.
This morphological trait could have developed as a means to enhance hydrodynamic performance, which is common among shelled cephalopods. Additionally, it’s possible that this feature was due to covariation or changes made due to constructional constraints.
Ammonoids distinguished by their shell, which consists of gas-filled chambers. While most have flat shells, some exhibit various shapes and morphologies. Some possess planispiral shells which move quickly through water while others possess wider, more open shells which move more slowly.
Ammonoids habitat
Ammonoids, or cephalopods with external shells, are one of the most diverse groups of animals on Earth. Their rapid morphological evolution and extensive fossil record have allowed scientists to study their biology and ecology in great depth.
Ammonites are sea-dwelling animals that lived in the oceans during the Jurassic and Cretaceous periods – also referred to as Mesozoic era for 140 million years. These periods occurred within the oceans during Jurassic and Cretaceous oceanic zones.
Most ammonoids had cylindrical shells lined with chambers and separated by thin walls known as septa. These walls shielded the shell from crushed and prevented the animal from escaping as it grew.
Each chamber tailored for the ammonoid’s body. As they expanded, new chambers would form and eventually cover the entire creature completely.
These shells were composed of aragonite, which is a mineral formed when molten rock cools and solidifies. After ammonoids died, their shells disintegrated while their internal moulds remained undisturbed in the sediment.
Fossil ammonite shells with holes have discovered, and these likely used as a form of protection from predators such as mosasaurs. Furthermore, fossil ammonite shells serve to tell the age and location of the rock where they discovered.
Ammonite shells
Scientists have recently observed that some ammonite shells are unique, enabling them to identify the species by shape and size. Furthermore, they can tell if the shell is flat or helix-shaped as well as examine any ribs, defensive spines or shell-strengthening ornamentation for identification.
The shape of a shell can provides us with insight into an animal’s behaviour and lifestyle. For instance, some ammonoid shells are wide, open-coiled, kinked or hooked – traits which aid them in navigating through water more efficiently.
Ammonoid shells are unique in that they possess a planispiral spiral shape, similar to that of a flattened helix. While this similarity with nautiluses makes them similar, ammonoids possess other distinguishing characteristics which set them apart from their modern-day relatives.
They had an intricate nervous system, including a central brain-like nerve ganglion and eyes that allowed them to distinguish shapes and colors. Some ammonoids even had sensory organs for hunting prey or avoiding predators – these organs can compare with living animals like birds or bats that possess similar sensory organs.
Ammonoids feeding
Ammonoids, an extinct group of marine shelled creatures, have been difficult to study until recently due to the scarcity of remains. But now, stunning X-ray images featured in science magazine offer new insights on how these coiled animals lived and perhaps why they perished.
Ammonoids, extinct relatives of squids, octopuses and cuttlefishes, were adept swimmers with flattened cylindrical or sleek shells that allowed them to float freely in or above water. They likely lived in open oceans where no bottom-dwelling life has found or they could have been slow-swimming bottom dwellers due to their flattened shells.
Their shells were remarkably diverse, featuring spiral ridges and ribs or even spines (the aptychus). The gas chambers in their shells, composed of septa, can folded to form saddles and lobes. Most ammonoids had a siphuncle: a narrow tubular structure connecting the phragmocone to living chambers which distinguished them from living nautiloids.
Cephalopods
Cephalopods distinguished by their chitinous parrot-like jaw apparatus that allows them to bite and swallow their prey. This is an integral part of their predatory lifestyle. Some ammonoids modified their jaws by adding radula – a rasping feeding structure.
They had ten arms that could grasp prey such as small fish, crustaceans, crinoids and some small orthoceras. Some of these ammonoids fed on plankton but may also have consumed other types of food sources according to the team’s findings.
Researchers examined three straight-shelled ammonites called Baculites that had preserved inside their shells, exposing their upper and lower jaws and the rasping feeding structure known as a radula. This toothed ribbon is use by mollusks to feed and transport food toward the oesophagus; researchers observed it in all three specimens studied. They say the radula serves as an example of an ammonoid’s buccal mass – a complex of delicate hard parts usually visible only when an ammonite shell breaks or sediment around it erodes enough for visibility.
Recommended readings:
- Where is the Heart Located in the Human Body?
- What is a Fossil?
- What is an Atom?
- The Periodic Table of Elements
- What is a Photon?
