Without a "chin" and "upper jaw", they are miserable.

The Cambrian period began 541 million years ago, and the Earth's oceans suddenly became lively, and the entire biological world presented an explosive evolutionary scene. Invertebrates in particular showed a booming development, and ancestors of various categories appeared one after another.

At the same time, the earliest representative of vertebrates, the primitive jawless fish , appeared in the biological world for the first time. The appearance of this jawless fish not only added a new member to the marine world, but also heralded the entry of a new era of biological evolution . But they only dominated the ocean for a short period of time. Why is that?

Jawless fish appear:

Vertebrates show their "first stage"

The Kunming fish, which was born in the Cambrian explosion, is the oldest primitive jawless fish . The discovery of the Kunming fish not only pushed the entire evolutionary history of vertebrates, including humans, forward by 50 million years, but also rewrote and improved the theory of early vertebrate evolution, and had a significant and far-reaching impact on the evolution of vertebrates. Since then, vertebrates , as a new type of animal group that first appeared on the evolutionary stage , have become an important evolutionary force in the biological world.

Fengjiao Kunming fish. Image source: Shu Degan, 2022

The most distinctive feature of jawless animals is the lack of upper and lower jaws , but they are also very primitive in many other aspects.

Their swimming skills are not very good, and they mainly rely on twisting their bodies to keep moving forward.

Due to the lack of upper and lower jaws, their mouths are like suction cups and cannot chew food. They can only rely on sucking or even relying on the natural flow of water to send marine organisms or microorganisms into their mouths for food.

They lack true paired fins , and the axial skeleton is merely cartilaginous rather than truly bony.

The body of jawless fish is like that of fish-like animals. In animal classification, they are classified as jawless fish in the order Pisciformes. They are more primitive than true early fish in evolution. Therefore, jawless fish are the earliest vertebrates .

Armored fish:

The lucky ones of the end-Ordovician mass extinction

The Silurian and Devonian periods were the age of fish, when both jawless and jawed fish appeared in large numbers. However, in the competition with jawed fish, armored fish, although once brilliant, were ultimately short-lived. Although jawless fish account for a very small proportion of species among living vertebrates, fossil records show that this group once flourished in the geological period and became the main body of vertebrates at that time. These were the armored fish that rose strongly in the Silurian period.

Silurian armored fish. Image credit: Yang Dinghua

Armored fish may have appeared in the Middle and Late Cambrian, but they remained dormant for a long time during the Middle and Late Cambrian and most of the Ordovician, with only a few fossils found. Only fragmentary fossils were found in the Ordovician. However, in the Silurian-Devonian period , armored fish began to flourish , and scientists have discovered various jawless fish fossils from this period all over the world.

Scientists have discovered that these fossils are covered with bony armor plates, the structures of the head and the back of the head have not yet separated, they are not flexible in movement, the trunk does not have pectoral fins and pelvic fins, and the front of the body has bony plates or scales, so they are generally called armored fish.

How did armored fish emerge? The climate changed dramatically at the end of the Ordovician period, and the Hirnantian Ice Age occurred, which led to the first mass extinction of organisms in the Phanerozoic Eon, causing the extinction of a large number of invertebrates and leaving a vast ecological gap in the once bustling Paleozoic ocean. Armored fish turned a disaster into a blessing. After surviving the Ice Age, they finally experienced radiation evolution in the Silurian-Devonian period.

The golden age of armored fish has arrived!

Fossil data show that the Silurian armored fish have evolved into the following major groups: armored fish, bony armored fish, heteroarmored fish, flower-scaled fish and armorless fish , and the ocean has become the world of armored fish.

Entering the heyday of armored fish, more than 300 species of heteroclad fish, more than 200 species of bony-clad fish and nearly 100 species of armored fish evolved radiation. These "armored" fish vary in size, with small ones a few centimeters long and large ones dozens of centimeters long. There are also great differences in appearance: heteroclad fish have a spindle-shaped body and fan-shaped mouth blades around their mouths, which may be used to scrape food; bony-clad fish have a horseshoe-shaped head armor, paired pectoral fins, and special side areas on the head armor, which may be used to accommodate multiple organs such as sensory organs.

Armored fish have a variety of lifestyles . Most species live on the seafloor, filtering organic matter from the seafloor. Some species, such as cup-shelled fish, have thick scales but lack fins, only a crooked tail. More advanced fin-shelled fish have strong swimming abilities and can feed on the surface of the water. Most armored fish live in fresh water.

By the Devonian period, this large group of early vertebrates reached its peak, and fossils of various jawless fish-shaped vertebrates have been found all over the world. In 2021, my country also discovered a new genus and species of armored fish, the Triceratops, about 419 million years old, in Qujing, Yunnan.

Ecological restoration diagram of Liu's sicklefish. Image source: Guo Xiaocong

The prosperous history of armored fishes once again proves a fact: whenever a new phylum appears, its early stages of development are always very small and temporarily at a disadvantage in the struggle for survival . When we discover their existence, this type of animal is often in the stage of radiation and development. Therefore, in fossils, the early primitive representatives of a group are often lacking or very rare.

The former "king" was eliminated

The thick, uniform head armor of the armored fishes, their single nostrils, and their increasing number of gill arches were the reasons for their decline and eventual elimination during the rise of advanced jawed vertebrates.

Although armored fish appeared in large numbers in the Silurian and Devonian periods and flourished for a time, they did not have true paired fins and had poor mobility. The axial skeleton was still only cartilaginous rather than true bone (i.e., hard bone ). The front surface of the body had large and uniform bone plates or scales, which played a role in protecting the body like the armor of ancient warriors, but the clumsy armor greatly hindered the flexible movement of the body.

In particular, armored fish do not have upper and lower jaws, and their mouths, which are feeding organs, cannot open and close effectively. They can only rely on sucking or even the natural flow of water to send food into their mouths, relying on gills to filter food. Therefore, they have more gills than fish, and the gill area occupies a considerable proportion of the body , which results in an asymmetric body shape with a large head and a small tail . The lack of upper and lower jaws limits the active hunting ability of armored fish, and the food range is very narrow, so there is no "prospect for development".

Armored fish are jawless vertebrates, and are at the previous stage of the development of jawed vertebrates. However, armored fish have become highly specialized in evolution. For example, the form of nostrils, there is only a single nostril, or only an internal nostril. For example, the jaw arch in the gill arch has not developed in the direction of the jaw, but has degenerated in some species, and the gill openings that open to the outside world have developed into a common gill opening, etc. All these show that in the process of evolution, armored fish and jawless fish have deviated from the direction of evolution towards jawed fish .

The Silurian and Devonian periods were the heyday of armored fish, with a large number of species and a variety of forms. In the previous Cambrian-Ordovician period, armored fish fossils were rare, indicating that this was a period of weak development of jawless vertebrates. Therefore, armored fish may not be the beginning of the development of jawless vertebrates, but the result of the development of jawless vertebrates. Jawed vertebrates may have originated from the same ancestor as armored fish.

"Upper Chin" and "Lower Chin" are here!

The rise of jawed fish is a very important event in the history of fish evolution, and the appearance of jaws is the most revolutionary innovation in fish metabolism. Jaws are not only important feeding organs , but also powerful weapons for attacking enemies , and can also make sounds and absorb oxygen . All of these have laid a far-reaching foundation for the future evolution of jawed animals, especially the emergence of fish on land, which triggered the evolution of a series of vertebrates.

The recently discovered Chongqing peculiar buried fossil deposit and Guizhou Shiqian fossil deposit fully demonstrate the appearance of Silurian jawed fishes, indicating that at least no later than 440 million years ago, major groups of jawed fishes had flourished in South my country, which was still a vast ocean at that time; by the late Silurian period, more diverse and larger jawed genera and species appeared and began to spread around the world.

The fish ecology of the newly reconstructed Fanjing Mountain has been restored. Image credit: Fu Boyuan and Fu Baozhong

Although jawed fish appeared in the Silurian period, the primitive jawed fish in the late Silurian period and early Devonian period were mainly small placoderms and acanthidae. After the middle Devonian period, the situation changed greatly. At this time, not only placoderms such as granulosae had developed into huge and ferocious carnivores, but also more advanced fish such as cartilaginous fish, cartilaginous scaly fish, crossopterygians and lungfish had developed greatly.

Obviously, in the competition with the jawed fish, the armored fish pale in comparison. The armored fish, which may have been advanced and adapted to the environment, became backward and unfit. The advanced replaces the backward, and the adaptable eliminates the unfit. This is the natural law of biological development .

As the functions of jawed fish become more and more perfect and their groups continue to prosper, the ecological waters occupied by armored fish will inevitably be replaced by jawed fish . Under this state, armored fish can only go to decline and extinction. Even if the descendants of jawless fish have evolved to this day, they are only a very small group in the ocean. It can be seen how important the "upper jaw" and "lower jaw" are in the evolution of organisms!

Produced by | Science Popularization China

Author: Feng Weimin (Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)

Producer｜China Science Expo

Submitted by: Computer Information Network Center, Chinese Academy of Sciences

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