Several months have passed. Our ammonites have grown and left the surface layers of water. Thousands of larvae were eaten by predators, but hundreds survived and now inhabit the coral reef barrier surrounding the lagoon of one of the islands. Here, among corals, submerged tree trunks, sponges, and sea lilies, there are many more hiding places—and more food for the growing mollusks. Now their spiral shells have become stronger and reach several centimeters in diameter.

The mollusk occupies the front part of the shell, growing its edges and forming a partition behind it. The partitions divide the inner chamber into compartments. The edge of the partition where it connects with the shell wall has a complex curved structure. This suture line forms a beautiful pattern on the fossilized shells of mollusks and serves as one of the main systematic features. A thin hollow tube runs through all the partitions, penetrated by blood vessels—the siphon. With its help, the ammonite can change its buoyancy by filling some compartments with carbon dioxide and others with water.

A Place in the Sun

As they grow stronger and more agile, ammonites move away from the reef wall toward the open sea. Forming large scattered shoals in the water column, they follow ocean currents, spreading over vast distances. Their numbers and diversity can rival those of fish, with each species occupying its ecological niche. Some ammonites hunt in the water column, relying on speed. Others stay closer to the shore, defending themselves from predators with massive shells. At the surface, plankton catchers glide, spreading mucus membranes between their tentacles. Their shells do not resemble regular spirals; they bend in different planes, giving the ammonite a whimsical appearance. Millions of years later, such ammonites will be called “heteromorphs.” The habitat of many species lies at different depths. By performing daily migrations up and down, they seem to be executing a complex dance in response to their biological rhythms.

The diversity of ammonites led to the emergence of true giants. Most of these mollusks were small—ranging from 1-2 to 30 cm. Far fewer species had large shells about half a meter in diameter. But standing apart is Parapuzosia seppenradensis. This ammonite had a colossal shell—2.5 meters in diameter. If unraveled, the shell would be over 10 meters long! The total weight of the giant reached 1.5 tons, with the mollusk itself accounting for 700 kg.

Ammonites were equipped with complex eyes and excellent vision. Most of them lived in the upper layers of water, where good vision played a crucial role. The eyes of ammonites have not survived in the geological record. However, genetic studies of cephalopods suggest that they were no worse than those of squids and octopuses. Another important sense was smell. The chemoreceptors of cephalopods are located on their tentacles. One could say they both smell and taste with their “hands.” The ability to discern smells must have helped ammonites navigate the open sea, find food, and locate a mate.

More than a year has passed. Our ammonites lead a solitary, perilous life in the open sea. Both predators and bad weather can cause their demise. Ferocious storms, turning the surface of the Volga Sea into a boiling cauldron of foam, force the cephalopod navigators to dive deep. Their shells are strong and can withstand great pressure thanks to the complex design of the suture line. Here, in the darkness, at a depth of several hundred meters, schools of belemnites glow with a cold light, and the eyes of terrifying ophthalmosaurs flare up like pale saucers.

The soft, juicy flesh of mollusks attracts many hunters, including fish and marine reptiles like ichthyosaurs. Ammonites washed ashore by a storm become easy prey for small dinosaurs and primitive mammals. But they themselves are formidable predators of smaller creatures. Fish, crustaceans, sea lilies, and other marine inhabitants become their prey.

The musculature of ammonites was more complex than that of nautiluses, due to the high mobility of these mollusks. The characteristic hydroreactive movement of cephalopods was carried out by a strong muscular funnel. However, this presents a problem—if the mollusk does not somehow secure its shell, the impulse from the water expelled by the funnel will simply push its body into the living chamber. On the penultimate coil of the ammonite’s shell, a “black layer” is well noticeable. This is a rough patch covered with melanin and other organic matter, which the mollusk used as a support. By gripping this layer, the ammonite became as one with the shell, able to control its movement.

Tentacles are the ammonites’ catching tool. Since no clear imprints of soft tissues have been found, it is difficult to judge their structure. However, it can be said with confidence that their original number was ten, as in all cephalopods. Whether their number decreased like in octopuses or increased like in nautiluses is still unknown. No traces of chitinous hooks, as found in squids, have been discovered. But the presence of at least one pair of long catching tentacles seems logical for the active hunters that ammonites were.

The life of cephalopods is rarely long. It lasts a few years and ends with the most important event—reproduction. By the time they reach sexual maturity, the behavior of ammonites changes. They form clusters; males become aggressive and may damage their rivals’ shells with their sharp beaks.

Once a mate is chosen, pairs unite, intertwining their tentacles. Mating, resembling an exotic dance, lasts more than a day. The male transfers a spermatophore to the female and dies of exhaustion.

A few weeks later, the females will rise to the very surface to spawn and also end their life journey. Millions of eggs will be released into the open sea, so that young ammonites may begin their lives among the plankton of the Volga Sea.

For 340 million years, ammonites inhabited the seas. They participated in forming complex ecological relationships in ancient communities. Many common species played a key role in the food chain, acting as both predators and prey. Their reproductive strategy, with a short life cycle and millions of offspring, was effective. It allowed them to evolve rapidly, adapting to the environment and creating new species. But it also made ammonites vulnerable.

The asteroid impact at the Cretaceous-Paleogene boundary triggered a chain of catastrophic changes that destroyed the Mesozoic ecosystems. Changes in the acidity of seawater led to the death of plankton and, consequently, the young ammonites. This brief, in geological terms, event proved critical for these cephalopods, causing their complete extinction.

Today, their shells are the pride of many museums and private collections. They help scientists peer into the distant past of our planet, to understand its history. And their living relatives (though very distant) — nautiluses — remind us to cherish and protect the environment.