Ammonoid

Ammonoids inhabited the seas in abundance during the Devonian period 416 million years ago. They are sometimes also referred to as Ammonite, which is the colloquial term used for this large and diverse group of creatures. However, ammonites are a suborder of ammonoids and appeared only about 200 million years ago, in the Jurassic period. As per the taxonomic classification, Ammonoids fall under the Kingdom Animalia, Phylum Mollusca, Class Cephalopoda, Order Ammonoidea. 

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Classification

Ammonoids can be classified partly based on the structure and ornamentation of the septa comprising their shells' gas chambers. Based on these and few other characteristics the subclass Ammonoidea can be divided into three orders and eight known suborders. Almost all nautiloids exhibit a gently curving suture (the intersection of the septum with the outer shell) but the suture line in ammonoids was folded and formed saddles (peaks) and lobes (valleys).

Orders and Suborder of Ammonoidea

The three orders and their suborders have been listed in the order from most primitive to the most recently derived.

• Goniatitida (Devonian to Permian) - with round saddles, pointed lobes

• Anarcestina (Devonian only)

• Clymenia (upper Upper Devonian only)

• Goniatitina (Devonian to Upper Permian) 

• Ceratitida (Carboniferous to Triassic) - with round saddles and serrated lobes

• Prolecanitina (Upper Devonian to Upper Triassic)

• Ceratitina (Permian to Triassic) - includes the true ceratites

• Ammonitida (Permian to Cretaceous) - have folded saddles and lobes, fractal patterns

• Phylloceratina (Lower Triassic to Upper Cretaceous)

• Ammonitina (Lower Jurassic to Upper Cretaceous) - This suborder includes the true ammonites

• Lytoceratina (Lower Jurassic to Upper Cretaceous)

• Ancyloceratina (Upper Jurassic to Upper Cretaceous) - These are the heteromorph ammonites

Extinction 

Ammonoids appeared in the Paleozoic (416  million years ago), thrived during the Mesozoic, and faced extinction 66 million years ago at the end of the Cretaceous Period. It was also the period when the mass extinction of dinosaurs and many other kinds of land and sea animals happened. Ammonoid fossils are found in sedimentary rocks around the earth. They are more commonly found in the Cretaceous rocks of western Kansas, Pennsylvanian, and Permian outcrops of the eastern state.

Appearance

Ammonoids were squid-like creatures that lived in a shell and they are closely related to the modern-day squid, the octopus, and the chambered Nautilus. All of these animals belong to the class cephalopods. While the shells of most ammonoids were coiled in the same plane giving them the appearance of a cinnamon roll, the shells of some ammonoids were straight or erratically coiled. The exterior of these shells was covered in a variety of ribs, different colour patterns, spines, or nodes.

The modern Nautilus is the closest living relative of the ammonoid in its outward appearance but it can be associated more with the subclass Coleoidea. Ammonoid fossil shells are usually planispirals but there were some helically spiralled and non-spiralled forms also found. These were known as "heteromorphs".

The ammonoids greatly varied in size throughout their existence on earth. They were one of the most diverse animals during the Cretaceous period. Most Paleozoic ammonoids were the size of a golf ball or smaller but many ammonoids also had diameters up to 10 feet. 

Variations in Shape

Most ammonoid fossils are shells that are a planispiral flat coil, but some shells are partially coiled, partially uncoiled, and partially straight (e.g Australiceras), almost straight ( e.g baculites), or coiled helically/ superficially like that of a large gastropod (Bostrychoceras and Turrilites ). During the early part of the Cretaceous, the partially uncoiled and uncoiled shell forms started to diversify and are called heteromorphs. Nipponites are a good example of the most absurd-looking form of heteromorph. They look like irregular whorls tangled together. Although they do not have any evident symmetrical coiling, a three-dimensional network of connected "U" shapes can be detected on closer inspection of their shell. Nipponites are found in rocks of the upper part of the Cretaceous in the USA and Japan.

The ornamentation on the ammonoid shells varies greatly. Some shells are smooth and do not have any features on them except for growth lines. These shells resemble that of the modern Nautilus. In other shells, various patterns like ribs, spiral ridges, and spines can be seen. This type of ornamentation of the shell can be seen clearly in the Cretaceous ammonites. 

Ammonoid Shell Anatomy

This squid-like animal lived in the front chamber of its shell and the other chambers, called buoyancy chambers, regulated its position in the water column. The chambered part is called a phragmocone and it contains a series of progressively larger chambers, called camerae (sing. camera). They are segregated by thin walls called septa (sing. septum). The body chamber, which is the last and the largest chamber, was occupied by the living animal. When the ammonoid grew in size several larger chambers got added to the open end of the coil.

A thin living tube called a siphuncle passed through the septa and extended from the ammonoid’s body into the empty chambers. The ammonoid emptied the water of these shells through a hyperosmotic active transport process. This helped in controlling the shell buoyancy and enabled it to rise or descend in the water column. The buoyant shell of the ammonoids not only helped them swim well but also acted as a protection against predators. 

Life of Ammonoids

The diversity of the external shell in ammonoids indicates that they adapted to the marine environment in a big way. Little is known about the life of the ammonoids because their soft body parts were never preserved in any detail. By examining the ammonoid shells extensively and by using the models of these shells in water tanks, information about their way of life is being documented.

The fossils have led to the discovery that some ammonoids lived on the ocean floor all their life while others drifted through the water column with the currents and their fossils are often found in rocks that are laid under conditions where no ocean floor inhabiting life is found. Many ammonoids such as Oxynoticeras had a smooth streamlined shell and may have been energetic swimmers. The bottom dwellers may have been slow and less effective swimmers.

Ammonoids were formidable predators that preyed on fishes, crustaceans, and other small creatures. Several fossil ammonoids that were discovered had bites which reflected that they were prey for marine reptiles and larger vertebrates, such as sharks, fishes, and mosasaurs.

Study of Ammonoids

The abundance of ammonoids and their diversity has led to a vast and varied fossil record and it is used by geologists and palaeontologists for biostratigraphy. The fossils have unearthed a lot of information related to ammonoid paleobiology. Ammonoids experienced a rapid evolution and are studied extensively by the scientific community for several reasons. The fossils have helped in correlating the ages of sedimentary rocks found in different parts of the world. Geologists can match the ammonoid species contained within rock formations at different places. This helps in determining that the rocks were deposited at roughly the same time. 

Most of the ammonoid specimens from the Paleozoic era have been preserved only as internal molds and the outer shell which was made of aragonite did not survive the fossilization process. The suture lines can only be observed in the fossils that have internal molds as otherwise it is hidden by the outer shell.

Because of the rapid evolution of ammonoids during the Triassic, Jurassic, and Cretaceous periods, their fossils can be studied to establish zones representing less than a million years. When compared to a large geological period of 416 million years, this is a very fine resolution.

Sexual Dimorphism

A prominent feature found in the shells of the modern Nautilus is the shape and size of the shell that varies according to the gender of the animal. The shell of the male is slightly smaller and wider than that of the female. This sexual dimorphism explains the difference in the size between certain ammonoid shells that belong to the same species. The larger shell (called a macroconch) is of the female, and the smaller shell (called a microconch) belongs to a male. The reason behind the variation in shell size is attributed to egg production for which the female requires a larger body. This sexual variation can be witnessed in Bifericeras from the early part of the Jurassic period in Europe.

The sexual variation in the shells of ammonoids is a recent discovery. Earlier the macroconch and microconch of one species were often considered two closely related but different species found in the same rocks. When such "pairs" were consistently found together, it was conclusive proof of the fact that they were sexual forms of the same species.

Suture Patterns in Ammonoids

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There are Three Major Types of Suture Patterns in Ammonoidea:

• Goniatitic - There are several undivided lobes and saddles with typically 8 lobes around the conch. This pattern is peculiar to the Paleozoic ammonoids.

• Cercolouratitic - The lobes have subdivided tips which give them a saw-toothed look. There are rounded undivided saddles and the suture pattern is characteristic of Triassic ammonoids. This pattern is seen again in the Cretaceous "pseudoceratites."

• Ammonitic - The lobes and saddles have several fluted subdivisions that are usually rounded instead of saw-toothed. These ammonoids are the most important species from a biostratigraphic point of view. This suture type is typical of Jurassic and Cretaceous ammonoids but it goes back to the Permian period.

Conclusion

More than 10 thousand species of this marine predator thrived on Earth and it was considered one of the most diverse animals. Fossils of this now-extinct ammonoid have been discovered by scientists all over the earth, right from the glaciers in the Antarctic and the foothills of the Himalayas to the Great Plains of the North.