How the ammonite shell gradually became curved

Ammonitesare the collective name for animals of the subclass Ammonoidea, class Cephalopoda, phylum Mollusca, an extinct group of marine organisms. They arose in the Early Devonian, survived multiple mass extinction events, and finally at the end of the Cretaceousexited the stage of nature along with the dinosaurs. These creatures, which once dominated the Paleozoic and Mesozoic oceans for more than three hundred million years, can now only be glimpsed in their former glory through the surviving fossils.

Ammonite fossils. Image: Takkk / wikimedia

Ammonites, coming from myth and legend

As for ammonites, today's non-specialists do not necessarily know many detailed concepts, after all, they have been extinct for 65 million years. For ancient people with even greater imaginations, there was even less way to give a reasonable explanation for the various ammonite fossils appearing in the strata. Over time, ammonites acquired a layer of magical color.

In Egypt there was a god called Ammon, whom the Greeks called Zeus Ammon, and he hada pair of goat-like horns on his head. Mesozoic ammonite fossils are very abundant in Europe, and people found that the excavated ammonites closely resembled the ram's horns on Ammon's head. They believed that the ammonites here were very likely transformed from the horns on the head of the god Ammon, so they used the name of the god Ammon to name this kind of stone. This is the origin of the name of the subclass Ammonoidea.

The god Ammon with ram's horns. Image: Dan Mihai Pitea / wikimedia

Stepping inside an ammonite

Setting aside the mythological embellishment, let us seriously step into the inner world of the ammonite and learn about the life of this predecessor. Ammonites had a wide variety of shells; regardless of outer shape, they all went through a process from small to large. The shell that a small ammonite first possesses is called theprotoconch, and as the ammonite keeps growing, the soft body inside continuouslysecretes new shell wall forward, while simultaneously secreting transversesepta。

behind. The life cycle of an ammonite. Image: De Baets.et al./Paleontology Online (2016)

The septa divide the shell space into many small chambers,and the soft body of the ammonite lives only in the outermost chamber, which is called the living chamber. The many small chambers inside are called gas chambers. The gas chambers can be filled with gas and water, used to control the ammonite's floating and sinking. Scientists found through calculation that if the gas chambers contain only gas, the ammonite would float at the sea surface. If the gas chambers are loaded with 20% by volume of seawater,the ammonite can suspend itself in the sea like a submarine。

One can see individual gas chambers. Image: John Alan Elson / wikipedia

The exquisite ornamentation of suture lines

When thinking of ammonites, the impression in everyone's mind may still be the appearance of their close relative the nautilus, and it is generally hard to state the specific differences between them and the nautilus. Indeed, some ammonites, like the nautilus, haveconical(fossil),horn-shaped(fossil) orspiralouter shells, and inside the outer shell they also have gas chambers separated off. Looking only at the outer shape, it seems very hard to distinguish them.

Palau nautilus Nautilus belauensis. Image: Profberger / wikipedia

The key to distinguishing ammonites from nautiluses lies in the suture lines. As mentioned earlier, ammonites and nautiluses have many gas chambers inside their shells, and these gas chambers are separated by septa. The line where the septum contacts the inner surface of the shell is the suture line; this is a feature of the shell's interior, and we can observe it only when the outer shell's surface layer has been peeled off. Compared with the suture lines of the nautilus,the suture lines of ammonites are generally more complex and ornate。

Cross-section of a nautilus (left) and cross-section and outer surface of an ammonite fossil (right). Image: Chris 73; Ra"ike / wikimedia

However, the suture lines of ammonites were not so exquisite and complex from the very beginning. If we place ammonites from different periods together, we may be able to discover the pattern of evolution within them - the suture lines of early ammonites were relatively simple, and as time went on the suture lines became more complex. In the long river of evolutionary history, more complex suture linesincreased the strength of the ammonite shell, adding an important weight to the ammonite's ability to survive in the sea.

A species of the order Goniatite that appeared in the Early Devonian. Image: Ghedoghedo / wikimedia

The ceratitic ammonite Ceratitic that appeared from the Late Permian to the Triassic. Image: Drow_male / wikimedia

The Cretaceous ammonite Cleoniceras. Image: Drow_male / wikimedia

As a kind ofcreature that dominated the oceans for more than three hundred million yearsand preserved its vitality through several mass extinction events, their trick was not merely increasing the complexity of the suture lines. What also evolved along with this was the morphology of their shells.

Current research holds that the subclass Ammonoidea evolved from the subclass Nautiloidea through the subclass Bactritoidea. During the constant process of evolution, the shape of their shells was also changing. If you could return to the era when nautiluses appeared and watch their evolutionary course all the way through, you should be able to seehow their shells were "bent" step by step。

Succession of shell morphologies in ammonites, with the outer shell coiling step by step. A. Lobobactrites, B-C, Cyrtobactrites D-F Anetoceras (Anetoceras), G-H Anetoceras (Erbenoceras). Image: palaeos.com

Being "bent" could exchange for an advantage in movement speed. Research shows that as the degree of shell coiling increased, swimming speed also continuously increased.

The true face of ammonites

Fossils of ammonite shells are very widely distributed, and large numbers of ammonite fossils have been found all over the world, but their tissue samples are very rarely preserved. Therefore, scientists could only reconstruct ammonites based on the structure of their shell fossils and by referring to the appearance of the modern nautilus. The reconstructed ammonite has the cephalopod'siconic arms, horny beak and funnel. The reconstructed form is between the nautilus and the modern suborder Octopoda. As for whether their true face was indeed like this,I'm afraid one would have to ask the dinosaurs。

Tentative reconstruction of the soft tissue of a Jurassic ammonite fossil. Image: Dzik, J /Acta Palaeontologica Polonica (1981)

A reconstruction of an ammonite. Image: Nobu Tamura / wikipedia

Ammonites, widely distributed and considerable in number, are not onlya treasure pursued by all kinds of paleontology enthusiasts, but also a capable assistant for geologists.

By calculating the strength of the ammonite shell structure, geologists can infer the water depth data of the place where the fossil was found. In addition, because ammonites had fast evolution rates, short individual lifespans, exist in many marine sedimentary rocks, are relatively common and easy to identify, and have a worldwide distribution, they can be used forprecise geological dating, and can distinguish geological time intervals of less than 500,000 years.

The largest ammonite Parapuzosia seppenradensis, with a diameter estimated to reach 2.55m. Image: Gunnar Ries / wikimedia

Ammonites have been gone for 65 million years, but the fossils left behind by these former marine creatures still point out to us "latecomers" the road they once traveled, andtell the stories of those seas turning into mulberry fields。

This article is the 227th article of the 4th year of Species Calendar, from Species Calendar author @Sundayhao.