(Source: Mikiko Yukisai )
(Source: Daohan Linxue )
Ammonites are an umbrella term for a group of extinct marine life that lived from the Middle Ordovician to late Cretaceous, so named because of the often chrysanthemum-like lines on its surface. Ammonite is usually divided into 9 orders about 80 superfamily, about 280 families and about 2000 genera, as well as many species and subspecies, etc., it is closely related to the Nautilus.
Ammonite fossils are well suited as standardized stones, and geologists can use them to determine the age of the formation containing ammonite fossils. The suborder Ammonites are probably closest to the extant cephalopods than to the Nautilus suborder Nautilus. Ammonites first appeared on Earth around the late Silurian to the early Devonian period, and eventually became extinct in the late Cretaceous period along with dinosaurs.

The shell of ammonite is a conical tube with calcium carbonate as the main component. The beginning of the shell tube is small, usually spherical or barrel-shaped, called the fetal shell. The vast majority of ammonite shells are centered on the shell and spiraled in a flat surface, with a few shells in straight shells, spirals, or other irregular shapes. Each of the shell rings of a flat spiral is called a spiral ring, and its periphery is the abdomen, and the side that surrounds the inner spiral ring is the back, and the shell surface between the abdomen and the back is the side. Ammonites periodically make ascending movements forward during their growth, and behind them are secreted by the mantle membrane next door to support the animal body, increase buoyancy and strengthen the shell. Therefore, the shell can be divided into two parts: the part where the animal body inhabits without the next door, called the living room; the part with a series of next door is the air shell, and the space separated by the two adjacent next doors is called the gas chamber, and the next door is a curved surface, usually with a complex shape, especially near the shell wall. The contact line next door to the shell wall is called the suture line, which is a very important symbol of ammonite classification. Each next door has a circular next door hole where the body tube is located. The body tube may play a role in linking the software to the shell and adjusting the buoyancy to make the shell sink and float. It is usually located at the edge of the abdomen, but a few categories of body tubes are in the dorsal or near-center position.

The degree of unwinding of ammonite shells is very different, and can be roughly divided into loose rolls, contact rolls, outer rolls, semi-outer rolls, semi-inner rolls and inner rolls. The shell is also diverse in shape: from thin plate to spherical, some are triangular spiral, some are straight rod or ring, the abdomen is pointed, flat or round, etc. The surface of the shell is sometimes smooth, sometimes there are different types of decoration, common decorations are growth lines, longitudinal spiral lines, horizontal ribs, tumors, thorns, grooves, ridges, etc., these decorations have many changes due to thickness, strength, density, direction and combination of each other. In addition, there are different types of color bands on the surface of the shell, but the vast majority of color bands cannot be preserved on fossils. The shell of ammonite is covered with a mouth cover. There are single flaps (single-mouth caps), double flaps (double-mouth caps), and some are fused from double lobes (closed caps). Usually the mouth cap is stored separately from the ammonite shell. Ammonite fossils with mouth caps are rare.
Ammonite's systematic classification signs include shell shape, degree of spiraling, shell surface decoration, body tube position, and suture line characteristics. Among them, the characteristics of the suture line are of particular significance. Each suture line can be divided into an outer suture line and an inner suture line. The outer suture is a suture line on the outer surface of the shell, and the inner suture is a suture line on the back surface. The basic elements of the suture are the leaves and saddle. The leaves are the parts where the sutures bend backwards, and the saddles bend forward. According to the location of the leaf and saddle distribution, it is called the ventral lobe (or abdominal saddle), the dorsal lobe (or dorsal saddle), the lateral lobe (or lateral saddle), and so on. A series of folds in which the sutures do not fully become separate saddles and leaves on the sides are called the rib system; a series of secondary saddles and leaves located between the ventral lobe and the first lateral lobe are called the occasional saddle and leaf.
There are two ways to proliferate leaves and saddles: (1) saddle cracking, which is partially split by the saddle to form independent leaves and saddles; (2) leaf splitting, which is partially split by leaves to produce new leaves and saddles.
According to the morphology of leaves and saddles, ammonite sutures can be summarized into 4 basic types: (1) ammonite type: leaves and saddles are complete, the number is small, and there is only 1 wide round leaf on the side, which is seen in Devonian ammonite; (2) ammonite type: leaves and saddles are complete, more numerous, and more pointed leaves, seen in Devonian to Triassic ammonite; (3) ammonite type: complete saddle, jagged leaves, seen in Carboniferous to Triassic ammonite; (4) ammonite type: leaves and saddles are strongly forked or toothed, seen in Permian to Cretaceous ammonite.
The characteristics of the suture line are the main signs of the division of the order and suborders, and also the important signs of the division of superfamily, family, genus, and even species. The position and orientation (anterior or posterior) of the fetal shell and tubes is also meaningful at the level of the order and suborder. Shell decoration plays an important role in determining genera and species, as well as certain families. The shape of the shell and the degree of spiraling are hallmarks of the division of genera and species.
Ammonite fossils are used to form calcite material, and the chamber is filled with crystals. Its varied pattern and beautiful spiral electric, especially suitable for viewing and collecting. In Europe and Japan, Korea and Taiwan, the small ammonite is made into jewelry and ornaments that are quite unique, and people wear it or give it as a gift.
Ammonite fossils have the characteristics of rich shell decoration, thick shell wall and beautiful appearance. Ammonite fossils were found in Dongyuan Zengkeng, with a total of more than 200 pieces, which is currently the largest number of ammonite fossils found in the country, the richest variety and the largest ammonite fossil group, of which there is a 42 cm extra-large ammonite fossil, which is the crown of the southern country. Due to their well-preserved appearance and peculiar ornamentation, these fossils look more like chrysanthemums growing in stone, and their shape is cute.
Ammonite fossils are found in shallow sea sediments and symbiosis with many marine fossils. Through research , it is speculated that ammonites inhabit the tropical to temperate zone at a certain depth, and the type with thicker shell walls and thicker shell decoration is the less active type; the shell wall is thinner, smoother and the spiky shell is the more active type that inhabits deeper water bodies.
There are a large number of ammonite fossils in the Everest region of Tibet in China, even readily available, because more than 200 million years ago, it was once the ancient Himalayan Sea, and due to the orogeny movement, the earth's crust rose and the seabed became a high mountain. Therefore, the ammonite living on the bottom of the ocean is presented on the ground, becoming a witness to the changes in the movement of the Himalayan mountain crust, and also providing favorable evidence for the restoration of the local paleo-ecological environment.
For a long time, many scholars have speculated that ammonites with a spiraling shell are straight-shelled rod stones that gradually bend and evolve into a angular shell of a angular shell. However, stratigraphic evidence suggests that the oldest angular shell ammonites and bow-shaped pole stones appeared almost simultaneously. Therefore, it has also been hypothesized that rod stones and pyrogenic ammonites evolved almost simultaneously from right-angled stones with ovate shells and abdominal tubes. It contains a variety of ammonites in the Paleozoic and Mesozoic strata of China, especially the Early Devonian ammonite group in Guangxi, the Permian ammonite in South China, the Triassic ammonite in Guangxi, Qinghai and Tibet, and the Jurassic and Cretaceous ammonites in Tibet.
Ammonite is the most effective standardized stone for estimating rock age, division and comparison of strata, which can be divided into quite fine ammonite belts, such as the Triassic, Jurassic and Cretaceous periods of the Mesozoic Era, each era can be divided into more than 30 ammonite belts, the average continuation time of each ammonite belt is between 100 and 2 million years, and in the Paleozoic Early Carboniferous late Serbhof formations in Western Europe, 30 ammonite belts are divided, and the average ammonite belt continuation time is not more than 500,000 years. The application of isotopes and other methods to determine the absolute age of formations is far from achieving this accuracy.
If you think the age of the Earth is 4.6 billion years, then 500,000 years is a very short period of time. Most of the Jurassic and Cretaceous periods were divided by ammonites. Ammonite fossils are widely distributed — sites where the same kind of fossils were found may be thousands of miles apart. This is because during the Jurassic Period, the Pancontinent began to split, providing a shipping route for ammonites to spread around the world.