The "Nine-tailed Fox" who is unwilling to be a relative of human ancestors is not a good turtle

Starting from the birth of the first fish during the Cambrian explosion, fish have gone through an evolutionary history of nearly 520 million years.

At the southeastern end of the South China Sea in the Devonian period 410 million years ago, there was a group of cute fish:

They have an oval and flat head armor with many serrated spines on the edge of the head armor. A huge nostril and a pair of small eyes are all located on the back of the head armor, indicating that they are a group of bottom-dwelling fish that may not swim very fast. In addition to the downward-curved notochord lobe, their tail has nine thick finger-like forks, each of which is covered with neatly arranged scaly fin rays, indicating that they may be a group of agile swimmers. It is the protagonist we are going to introduce - the nine-tailed fox turtle !

Ecological restoration map of the nine-tailed fox turtle (drawn by Yang Dinghua)

01

Small fox turtle

Evidence for the evolution of the back of the head in early fish

As the name suggests, the nine-tailed fox turtle gets its name from the nine scaly fin rays at the end of its tail.

The Classic of Mountains and Seas: Southern Mountains states: "In the Qingqiu Mountain, there is a beast that looks like a fox but has nine tails." The tail of the Nine-tailed Fox Turtle is similar to the tail of the Nine-tailed Fox in the Classic of Mountains and Seas.

Description of the Nine-tailed Fox and the Human-faced Fish in the Classic of Mountains and Seas

As a very primitive jawless fish, the "nine tails" of the nine-tailed fox turtle is its species name, and "fox turtle" is its genus name. It belongs to the Duyun fish family of the Polybranch fish order of the subclass of armored fishes.

Armored fish are a type of vertebrate that is very closely related to our ancestors. In a sense, they represent a key link in the evolutionary tree of almost all vertebrate ancestors, including us. Therefore, many organs in our human head can actually be found in armored fish.

For example, our human middle ear originated from the first pair of gills with respiratory functions behind the eyes of armored fish; the downward fin folds on both sides of the lower body of armored fish are the earliest verifiable prototypes of our limbs; and the paired separate nasal sacs and pituitary glands of armored fish are the prerequisites for the development of our jaws for chewing food.

As the most complete and exquisite member of the large family of armored fish, the discovery of the nine-tailed fox turtle is of self-evident significance to our research on "how these ancestors came about."

The uniquely preserved "nine-fingered" tail fin of the nine-tailed fox turtle (Photo by Gai Zhikun, illustrated by Feng Mingjuan)

Morphological studies have shown that the nine-tailed fox turtle has a typical downward-slanting tail, which we call a downward-slanting tail. The lower lobe of the caudal fin is relatively thick and obviously tilted downward, which may represent the notochord lobe, which contains the notochord.

We know that jawed fish evolved from jawless fish. Systematic studies have shown that the downward-distorted tail of the nine-tailed fox turtle may represent the primitive characteristics of vertebrates. The downward-distorted tail may have appeared in the jawless Kunming fish and Haikou fish in the Cambrian period, and has been preserved in the extant cyclostomes, early conodonts, heterostomes, ectodonts and cyprids, and these early jawless fish have all retained this primitive characteristic. The true upward-distorted tail only appeared in the jawless bony armored fish and jawed fish. Therefore, armored fish represented by the nine-tailed fox turtle are the group that is currently known to retain the primitive characteristic of downward-distorted tail and are most closely related to jawed fish.

At the same time, the fossils of the Nine-tailed Fox Turtle also revealed for the first time that in addition to the notochord, the tail fin of the Nine-tailed Fox Turtle also has 9 finger-like forks, which may be connected by a fin network of soft tissue. This morphological feature is similar to the forked tail that is widely found in Heteropods and Cynomolgus fishes, indicating that the tail fin of the Nine-tailed Fox Turtle is a relatively primitive forked tail.

In addition, each finger-like fork is covered with multiple rows of neatly arranged linear scaly fin rays, a feature very similar to that of bony fish, indicating that there are strong radial muscles attached under the fin rays. This indicates that the nine-tailed fox turtle can make good use of muscle contraction to flexibly control the contact area between the tail fin and the water flow, thereby generating different propulsion forces!

The scaly rays in the caudal fin were once thought to be an advanced feature only possessed by bony fish, but their widespread appearance in armored fishes suggests that the origin of these scaly rays is much earlier than previously thought, and may have appeared before the origin of the jaw.

Restoration of the nine-tailed fox turtle and the agile Tujia fish and the significance of their tail fin evolution.

The evolutionary sequence of the tail fin of early vertebrates. Node 1: Origin of the tail fin; Node 2: Origin of the lower crooked tail; Node 3: Origin of the dorsal fin; Node 4: Origin of the scaly fin rays; Node 5: Origin of the upper crooked tail. (Drawn by Feng Mingjuan, Yang Dinghua, and Shi Aijuan)

02

The "Speedy Walker" near the South China Sea

So, since the tail of the nine-tailed fox turtle is so flexible, does this mean that it has very strong swimming ability?

Swimming ability is crucial to the survival of fish. It is an important means for fish to escape from enemies, predators, migrate, seek mates and avoid disaster environments. Among them, swimming speed is an important indicator for evaluating fish swimming ability, which can reflect the activity index of fish and its metabolic level.

According to the swimming time, the swimming speed of fish can be divided into three categories: cruising speed, long-term swimming speed and burst swimming speed.

Cruising speed refers to the speed at which fish can swim continuously for more than 200 minutes without fatigue. At this speed, fish mainly use aerobic red muscle as a source of power. It is the most commonly used swimming speed in fish daily life, but it is also the slowest swimming speed.

Long-term swimming speed refers to the swimming speed at which fish become fatigued after swimming continuously for 20 seconds to 200 minutes. At this swimming speed, fish gradually use more anaerobic white muscle to supplement power. This may be the most commonly used swimming speed during fish migration.

Burst swimming speed refers to the instantaneous swimming speed of fish that lasts less than 20 seconds. At this swimming speed, fish mainly use anaerobic white muscle as a source of power. It is the fastest swimming speed used by fish when hunting, escaping from enemies, being frightened or swimming in strong currents.

Although the swimming speed of fish is affected by many factors, and the speed of many fossil species and extinct species cannot be directly observed, the area and shape of the fish's tail fin have always been considered to be key indicators for testing the swimming ability of fish, and can be used to analyze the cruising speed and burst swimming speed of fish.

At present, based on the database of swimming speed records of living fish and geometric morphological measurements of the tail fin, the application of phylogenetic generalized least squares model (PGLS) and univariate linear regression model (LM) to predict swimming speed has developed into a new analytical method for evaluating fish swimming speed, and has been successfully applied to the analysis and detection of the swimming speed of early basal jawed fish.

The estimated cruising speed of the ancestors of the major groups of early Paleozoic vertebrates (An, Anaspid, armored fish; Pt, Pteraspidomorphi, fin-armored fish; Th, Thelodonti, flower-scaled fish; Ga, Galeaspida, armored fish; Os, Osteostraci, bony armored fish; Pl, Placodermi, armored fish)

The unique preservation of the tail fin of the nine-tailed fox turtle enables us to apply new methods and models to analyze and measure the swimming speed of the armored fish of the same group as the nine-tailed fox turtle.

Paleontologists integrated a database of swimming speeds of 61 living genera and species, including cartilaginous fish and bony fish, and 160 living fish. For 43 genera and species of extinct Paleozoic fish, they used the maximum likelihood method to conduct a comprehensive swimming speed model analysis of 4,500 phylogenetic trees of early vertebrates, including armored fish, while acknowledging the uncertainty of geological ages.

Analysis of the swimming speeds of these early fishes showed that armored fishes and tilapia were the fastest, while armored fishes, osteoderms, and heteroatheriums were slower.

This discovery negates the traditional "New Head Hypothesis" on the evolution of swimming speed of early fish. A very important inference of this hypothesis is that the evolution of early vertebrates is characterized by evolving towards more and more active food acquisition, that is, towards faster and faster swimming speeds, which eventually led to the origin of jawed fish.

Among armored fishes, the downward-curved tail of Heteroscelis is considered to be the primitive state of early vertebrates. Armored fishes, crocodiles and armored fishes have all maintained this primitive state, while it has evolved into an upward-curved tail in bony fishes and jawed fishes. This evolutionary trend is considered to be the main sign of the improvement of fish mobility and swimming speed.

The above calculations show that the cruising and swimming speeds of these early fish have no correlation with the phylogenetic tree, because the armored fish with the fastest cruising speed are relatively distantly related to the living jawed fish, while the armored fish and placoderms that are most closely related to the living jawed fish have the slowest cruising speed.

In general, the group of armored fish represented by the nine-tailed fox turtle is a fast swimmer with strong swimming ability. Its cruising swimming speed is even faster than some more advanced jawed fish. Our phylogenetic analysis of cruising swimming speed also shows that armored fish have better athletic ability and higher metabolic level than more advanced bony armored fish and armored fish.

Therefore, the upward deflected tails of bony armored fish and jawed fish are not a sign of adaptation to faster swimming speed. This shows that these jawless armored fish had developed a certain ecological diversity and occupied different ecological niches long before the origin of jaws. This diversity in ecological niches enabled these primitive armored fish to obtain some superior movement advantages over jawed fish, at least to some extent.

Therefore, we might as well boldly imagine:

410 million years ago, near the southeast coast of the ancient South China Sea, a group of nine-tailed fox turtles were swimming leisurely in the water, passing through clusters of semi-aquatic fern bushes, when suddenly a huge and ferocious carnivorous jawed fish chased them and tried to bite them to pieces. These nine-tailed fox turtles relied on their powerful swimming ability, waving their nine-tailed tail fins, and relying on the lift brought by the fin folds on both sides of their lower body to swim quickly, thus easily avoiding the pursuit of these powerful predators.

Author: Lin Xianghong, postgraduate student at the School of Paleontology, Yunnan University

Reviewer: Zhikun Gai, Researcher, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences