Unveiling the Dragon: How Squids Evolved to Be Delicious | Paleontological Rhapsody

Author: Wang Guanqun, Pan Haochen, Fang Xiang (Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)

The article comes from the Science Academy official account (ID: kexuedayuan)

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Last time, we talked about the last golden age of the armored cavalry ammonites among cephalopods (click here if you missed it!). This time, I will talk to you about the real demons among cephalopods that are now rampant in the oceans - Coleoidea.

What is a sheath? The typical representative is squid! Lao Wang is already at the barbecue stall: "Boss! Another skewer of grilled squid! I want it spicy!"

Spicy squid (Photo source: Peninsula Morning Post)

Some of these Cthulhu prototypes have now completely discarded their outer shells, while others retain their inner shells as body support. As for the armored groups, they are either like ammonites, leaving only records in thousands of stratigraphic books, or they are as decadent as Nautilus, with only two genera and six species left. Like gorgeous armor blasted into a museum by powerful firearms, they tell of their glorious past in the last bit of their reserves.

However, these detached sheathed clades had no intention of returning to the countryside to retire, but instead became increasingly sophisticated to fight against the vertebrates, their lifelong enemies. After the devastating mass extinction at the end of the Cretaceous period ended the golden armor legend of the ammonites, the sheathed clades finally ascended the throne and became the last pride of the mollusks in the ocean today.

Some illustrations of sheath-shaped organisms (Source: Artistic Forms of Nature (1904) by Ernst Haeckel)

My shield is so strong that nothing can penetrate it.

After the transformation of the shell by the cephalopod ancestors in the Cambrian period, the armor of cephalopods not only has a simple defensive purpose, but also has a unique buoyancy system, which allows cephalopods to fight their way out in the chaos of the Cambrian period. The epoch-making armor innovation created the glory of heavy armor for hundreds of millions of years. In the early Paleozoic era when predators just appeared, this highly functional armor was indeed much more useful than the shell of other mollusks that only sought self-protection. However, as time went on, the disadvantages of this armor became apparent.

Unique buoyancy armor system, the structure of the living Nautilus (Image source: Reference [2])

On the one hand, as cephalopods grow, their armor will gradually become larger, and the closed cone part responsible for controlling the specific gravity will become longer and longer, which makes the armor vulnerable at the end. As the Ordovician monsters, such as the endodontites, which are several meters long, began to run rampant in the sea, this disadvantage in defense became more and more obvious.

On the other hand, although cephalopods with functional armor have always been winners in life at the top of the food chain, the huge power advantage brought by the armor allows cephalopods to carry out dimensionality reduction attacks on other molluscs and become carnivorous predators. However, the maintenance and damage of power armor is still a big problem that cannot be ignored. For other mollusks, a broken shell means a decrease in defense in a certain aspect, which can still grow back after a while. Once the power armor of cephalopods is damaged, the leakage and imbalance of specific gravity are not so easy to repair. Over a period of time, the impact of this breakage on athletic ability may be fatal. The upright shell of the early cephalopods increased the probability of risk and also troubled the steering of the cephalopods' jet movement.

An Ordovician giant endodontite, showing the risks posed by its extremely long shell (Image source: Reference [3])

After discovering this problem, ammonites and nautiluses chose a temporary solution: turning their shells inwards and starting a spiral life. This method, similar to perming hair, did improve the stability and flexibility of the shell structure.

However, the existence of the shell is still a problem that cannot be completely solved. As predators, cephalopods soon found that the strong shield they once used to protect themselves became a burden, especially after the emergence and development of jaws of natural enemies, vertebrates. Not only did the shell often fail to provide protection, it also restricted the physical development of armored warriors, which in turn affected the evolution of predatory weapons.

As a result, the shield was no longer an absolute defense that could not be penetrated by anything, but became a double-edged sword, which caused these warriors to suffer in the years to come. The new cephalopods, the sheath-shaped species, chose a completely different strategy - they found a way to internalize the shell and use their own soft body to protect the shell that was once used to protect them (nesting doll warning!).

Figure 3. Early ammonites that began to perm in the Devonian period (Image source: Reference [4])

An unexpected sausage

However, the first to do so was not the Scaphoids, but a strange cephalopod from the Silurian period, Sphooceras. From the perspective of shell structure, Sphooceras is no different from ordinary orthorhombic stones. However, its adult body does not have a pointed pagoda top, but a rounded top. Researchers found that its juvenile body still has a protochonch (the first shell grown by the baby), but no trace of the protochonch has ever been found in its adult body. At the same time, the number of air chambers in its adult body does not increase like other cephalopods, but maintains a certain number. This means that this guy will automatically fall off the top part after the shell grows to a certain length, so as to ensure that his shell is not too long to affect movement.

What's even more bizarre is that when it grows up, it will wrap its shell with its mantle, which looks like a sausage. Since I can't find its Chinese name, I will just call it "sausage horn stone". As the "pioneer of internal shell reform" in the nautilus, the attempt of "sausage horn stone" was not successful. Although it solved a small part of the shortcomings of the shell, it was only a temporary solution. After the appearance of fish with jaws, it really became a sausage for fish to feast on.

Schematic diagram of Sphooceras's heroic act of amputation (Photo credit: Vojtěch Turek)

Reconstruction of the Silurian sausage Sphooceras (Photo credit: Franz Anthony)

The Age of Discovery: The Dragon Unsheathes

The invention of jaws in Silurian fish dealt a heavy blow to almost all of their marine rivals. The subsequent prosperity of Devonian fish prompted their rivals to innovate - some went on land to avoid the fish's sharp edge, while others made changes to their own structure.

Ammonites, a group of cephalopods, were born during this period, and used rapid growth and massive reproduction to transform themselves into "cephalopod cockroaches" (I just give birth to more than you eat~). However, vertebrates adhered to the creed of "no one can be left behind", and sent out landing troops in the late Devonian period, and developed amniotic eggs in the Carboniferous period (a structure that can prevent eggs from mechanical damage, water loss and microbial invasion, and is conducive to the exchange of gases between eggs and the outside world), facilitating normal reproduction on land and realizing the desire to conquer land.

Afterwards, this powerful force that had been tempered on land once again set its sights on the vast ocean. After making early attempts in the Permian period, they suffered a mass extinction. The world also ushered in a new life as the volcanic roars and magma waves at the end of the Permian period gradually subsided, announcing the advent of the Age of Exploration.

The Age of Discovery in the popular manga One Piece (Image source: Internet)

Land vertebrates, represented by reptiles, began to invade the ocean on a large scale in the Mesozoic Era. Ichthyosaurs, plesiosaurs, and mosasaurs appeared one after another, and turtles, tortoises, and archosaurs also began to go to the sea in large numbers, just like finding the one piece (the legendary wealth in the comic "One Piece") can become the Pirate King. This also made the Mesozoic ocean a veritable monster aquarium. These monsters that have experienced the hell mode on land are like no one in the simple mode in the ocean - their body size has increased dramatically, and their flexible jaws and strong teeth that have evolved on land to adapt to complex feeding habits have made all kinds of marine creatures feel the despair of "eggs fighting stones".

The carefully designed armor of cephalopods may be able to fight against the jaws of fish, but it is vulnerable to these dragons from the "other world" (land). The cephalopods are already one era behind in the technology tree, just like China, which had the world's first pure firearms unit, the Shenjiying, in the Ming Dynasty, but was blown to pieces by foreign guns and cannons in the late Qing Dynasty. After learning from the painful experience, the "insightful" sheathed creatures realized that it was time to take off their armor and "learn from the foreigners to defeat them."

Figure 6. The specialized Mosasaur in Jurassic World. Although its size is exaggerated, it still has no rivals in the ocean (Image source: The movie "Jurassic World")

The head can be cut off and blood can be shed, but the position in the food chain cannot be lost! Cephalopods spend their entire lives looking for ways to deal with the huge predation pressure of vertebrates. Compared with the "chrysanthemum sea tactics" of the ammonites, the calmer thecaeformes have a clear strategic idea, which is to strive to improve the attack and movement capabilities - the shell is not important at all, if it hinders my movement and attack, I will just throw it away.

Many advanced models have emerged on the road of innovation of the sheathed species. The earliest sheathed species is Belemnoidea, which appeared in the Carboniferous period. They are famous for their arrow-like internal sheaths (Gladius) (the English name comes from the short swords of the ancient Roman army). Among them, the Hematitida were the first to stuff the shell into their bodies, which was undoubtedly a brave attempt. However, if only the shell was internalized, it would be like the cephalopod Sphooceras (also named "sausage horn stone" by the author), joining the luxurious feast of marine sausage.

But in fact, after the scalyforms stuffed their shells into their bodies, the size of their shells was greatly reduced, and even the composition of the sheaths became lighter chitin, which made the scalyforms far more flexible than their compatriots. After abandoning the calcium shell that could not withstand the huge water pressure, the scalyforms gained the ability to explore the deep sea, which also laid the groundwork for the Octopoda and Decapoda in the family to escape the Cretaceous extinction.

The earliest known fossil of the sheath-shaped genus Gordoniconus beargulchensis is a belemnite (Image source: Reference [7])

Roman Gladius

Sheaths of several cephalopods a, Oegopsid (Oegopsidae); b, loliginid (Squid); c, sepiolid (Sepiotrix); d, vampyromorph (Vampyromorph) (Image source: Reference [5])

My spear is so sharp that it can penetrate anything.

After solving the problem of the "shell", the Scalyx began to devote all its efforts to weapons.

First of all, the evolution of the arms. Compared with the weak nautilus tentacles, the tentacles of the sheathed species are the real arms of Cthulhu. These tentacles have suction cups and even barbs, which no one wants to be hugged by. At the same time, the sheathed species increased the muscle strength of the tentacles, making these unicorn arms a delicacy on the human table in the future.

The second is the invention of the ink sac. In the Carboniferous sheath fossils, researchers have already discovered the ink sac, a "weapon for street fights without martial virtues." This dirty trick similar to spreading lime powder is still useful in the dangerous ocean. Of course, it also "scored again" in the future and became a delicacy on the table of humans, who have even less martial virtues. This is something that cephalopods never expected.

The barb of the Lightning Five Whip (Photo credit: colossal-squid-Museum of the New Zealand Te Papa Tongarewa)

Passaloteuthis bisulcata from the Jurassic period, with the ink sac in blue and the wrist hook marked in red (Image source: Wikipedia)

In terms of the power system, the sheathed species also invested a lot and made modifications. The muscle content of the mantle increased greatly after it began to wrap around the inner shell. At the same time, the sheathed species handed over the jet function to the muscle contraction of the mantle, which greatly enhanced the strength and sustainability of the jet, making the sheathed species, which used to be able to jump up and down like a Xiangxi zombie, become a horizontal underwater torpedo. In addition, the fins grown on both sides of the mantle of some types can also assist swimming well. This jet torpedo is at least as capable as vertebrates in terms of athletic ability.

The improvement of speed and power system also comes with a comprehensive upgrade of vision and sensory nerves to cope with faster and faster speeds and complex and diverse predation environments. The body, freed from the constraints of the shell, can also grow larger, so that cephalopods have more space to develop their intelligence, becoming the intellectual ceiling among invertebrates on Earth.

Deduction of the change in the jet mode of sheath-like mobile jets (Image source: Reference [7])

Not only that, some species of the scaly-shaped group have further practiced the low-level tricks of luminescence, camouflage, and poisoning to perfection, making themselves the immortals of the stars. Even the vertebrate leader of the martial arts world, when encountering such a martial arts master with amazing arm strength, a hand full of hidden weapons, extremely smart, a thousand-mile vision, invincible light skills, unparalleled poison skills (the four-character master has exhausted all his life's learning), and the art of disguise, and a handful of lime powder on his waist, he would have to weigh whether he was "two fists can't beat ten hands". However, this contradiction is not a problem within cephalopods at all - after the great change in weapons, the scaly-shaped group had long added their armored compatriots to the diet.

In this way, after nature once again displayed its divine power to complete the great cleansing at the end of the Cretaceous period, the ammonites, which could not hide in the deep sea and had difficulty developing shells under the erosion of acid rain, completely perished (I can only say that the nautilus was lucky to survive). In contrast, after going through numerous hardships, the Octopoda and Decapoda of the Scalyformes, which inherited the "unyielding spirit" of cephalopods, continued to wave their terrifying tentacles to challenge the top of the food chain.

Obviously, victory belongs to them - they have successfully conquered the human stomach!

Color-changing octopus (Photo source: Sohu)

An ammonite that was eaten by its fellow cystids (Image source: Reference [6])

References:

[1] Kröger, B., Vinther, J., Fuchs, D., 2011. Cephalopod origins and evolution: a con-gruent picture emerging from fossils, development and molecules. Bioessays 33, 602–613.

[2] Zhang Yuandong, Zhan Renbin, Wang Zhihao, Yuan Wenwei, Fang Xiang, Liang Yan, Yan Kui, Wang Yujing, Liang Kun, Zhang Junpeng, Chen Tingen, Quan Zhouwan, Ma Xuan, Li Wenjie, Wu Xuejin, 2021. Atlas of Ordovician Strata and Standard Fossils of China. Hangzhou, Zhejiang University Press.

[3] Klug, C., De Baets, K., Kröger, B., Bell, MA, Korn, D., Payne, JL, 2015. Normal giants? Temporal and latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change. Lethaia, 48: 267–288.

[4] De Baets K., Klug, C., and Korn, D. 2011. Devonian pearls and ammonoid−endoparasite co−evolution. Acta Palaeonto− logica Polonica 56 (1): 159–180.

[5] Fuchs, D., Iba, Y. The gladiuses in coleoid cephalopods: homology, parallelism, or convergence?. Swiss J Palaeontol 134, 187–197 (2015). https://doi.org/10.1007/s13358-015-0100-3.

[6] Klug, C., Schweigert, G., Tischlinger, H. et al. Failed prey or peculiar necrolysis? Isolated ammonite soft body from the Late Jurassic of Eichstätt (Germany) with complete digestive tract and male reproductive organs. Swiss J Palaeontol 140, 3 (2021).

[7] Klug, C., Landman, NH, Fuchs, D. et al. Anatomy and evolution of the first Coleoidea in the Carboniferous. Commun Biol 2, 280 (2019).