436-million-year-old fish fossil rewrites the story of vertebrate evolution

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Take a snap

• ONE 436 million years old Bony fish fossils have been successful in helping researchers better understand vertebrate evolution.

• Before this discovery, the scarcity of Silurian fossil records has created a large data gap.

• Another fossil discovery from 423 million years ago has solved the surrounding mystery ancient teeth.

I read Lulu Miller for the first time Why don’t fish exist? in 2024. Interspersed with his exploration of his personal quest, Miller describes the story of David Starr Jordan, who named and described more than 2,000 species of fish (an estimated 20% of the species known at the time). What fascinates me most is how complex the biology of “fish” actually is. Miller explains the fascinating biology of fish, such as lungfish being more closely related to humans than they are to salmon! In an effort to learn more about fish evolution, I researched everything related to fish I could find: bony fish are the evolutionary ancestors of most vertebrates, ray-finned fish are the ancestors of 99% of today’s fish, and lobe-finned fish are the predecessors of tetrapods such as amphibians, reptiles, birds, and humans.

Despite everything I’ve learned, I still have questions: What did the fish look like before the ray and lobe fins were separated? Were ancient fish different from each other? Do they share some of the same characteristics that we still see in fish today? In 2026, a team from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences published research on two bony fish fossils they discovered in South China. A fossil has a name Eosteus of Chongqingis the oldest nearly complete articulated fish fossil ever discovered. This rare discovery is revealing a host of new information regarding evolutionary change.

History of fish

Although some scientists believe that bony fish diverged into ray-finned and lobe-finned fish during the late Silurian period, the general consensus is that the divergence occurred during the early Devonian period from about 419 to 339 million years ago. In fact, the Devonian period is sometimes called the “Age of Fish.”

Ray-finned fish, part of the superclass Actinopterygii, have thin, webbed fins that extend over bony spines. They make up the majority of fish species in the world today, accounting for more than 30,000 species. If you’ve heard of catfish, sturgeon or tuna then you know of at least one type of ray-finned fish. Fish species with lobed fins such as coelacanths belonging to the superclass Sarcopterygii have fleshier fins. There are also significantly fewer lobe-finned fish than ray-finned fish.

Before this split occurred, bony fish were called bony fish. Although scientists still use the term today, osteichthyans now also include tetrapods in addition to bony fish.

A better understanding of ancient osteons could reveal how this common ancestor laid the foundation for the development of animal life. The problem is that the fossil record is relatively incomplete. As the IVPP authors point out in their first published study, “Osteichthyans…dominate modern vertebrate biodiversity, yet their pre-Devonian fossil record remains scarce and fragmentary.”

In other words, nearly all the fossils we found are from the Devonian period. This is probably because Silurian animals had softer bodies. Because soft tissues are not as well preserved as shells or bones, creatures with softer bodies are less commonly found in the fossil record. That doesn’t mean molluscs can’t fossilize, just that fossilization is less likely.

What Eosteus of Chongqing?

Although only discovered in 2019, Chongqing Lagerstätte, a fossil site in China, has become famous for its well-preserved fish fossils. In the past, fossils discovered in this area include a shark-like cartilaginous fish called Shenacanthus vermiformis has a large armored chest plate and Xiushanosteus is amazinga fish with armored jaws.

E. chongqingensis found in the same area. Because this tiny fish was only 3 cm (about 1.2 inches) long, it was certainly not one of the apex predators of its time. This fossil dates from 436 million years ago, predating other fossils and microfossils of the same type.

The researchers found that E. chongqingensis has a combination of many different characteristics, including:

• The fish’s body has general bones but no bony fin rays

• Caudal fulcrum, spines or ossified (hard/stiff) scales on the caudal fin

• A single dorsal fin at the back

• Anal fin spines are commonly found in placoderms (ancient armored fish such as Dunkleosteus) and cartilaginous fish

Necessary, E. chongqingensis There are many evolutionary traits that we now see organized into distinct lineages. More importantly, the scientists noted that features associated with bony fish, such as dorsal fins and body shape, appeared earlier than they had previously thought.

The authors note, “Bayesian inference and the 50% majority rule consensus of the maximum parsimony analysis place the new fish in the osteichthyan stem, while the strict consensus leaves its position unresolved.” In less scientific language, they are saying that some analysis puts E. chongqingensis as relatives just before the appearance of bony fish. However, this has not been fully proven.

Another discovery

IVPP researchers also identified the head and torso bones of another Silurian fish called Megamastax amblyoduslived about 423 million years ago. They found M. amlyodus fossil from Qujing, a city located in China’s Yunnan province. Not the same E. chongqingensis, M. amlyodus is quite large. At 1 meter, or about 3.3 feet, it was the largest Silurian vertebrate of that geologic time period. This could mean M. amlyodus is an apex predator.

Previously, researchers had found fragments M. amlyodus fossil. While this provides an overview of the fish’s existence and rough size, many questions remain about this creature. Researchers initially believed that M. amlyodus may be a fish with lobed fins.

After nearly ten years, the authors published their findings in the journal Naturehave found the right advanced 3D and image reconstruction techniques to use M. amlyodus‘ Articulated head and body material to learn more about its anatomy.

Megamastax amblyodus has a unique anatomy. First, the researchers discovered that its main blood vessels may be arranged similarly to those of cartilaginous fish. M. amlyodus also has more ancient anatomical features such as a large front maxilla. But perhaps the most interesting is Megamastax amblyodus actually shows more characteristics of early bony fish, including:

• The bones outside the shoulder or small bones in the neck/back of the head area

• Evidence of tooth resorption and replacement

• Two rows of cushion-like teeth rest on individual, blunt roots

The importance of teeth

M. amlyodus‘ Teeth are important because they show what the teeth of early bony fish looked like. According to the research team, “Phylogenetic analysis Megamastax in the osteichthyan stem, near the osteichthyan crown group node.” In other words, M. amlyodus was an almost direct precursor to the earliest stages of bony fish evolution.

But teeth also solved a scientific mystery. For decades, scientists in Europe’s Baltic region have found well-preserved tooth plaques in Silurian rock. It is now clear that these are not isolated fossils but are part of M. amlyodus‘ jaw. Additionally, these fossils show how widespread skeletal man was during the Silurian period.

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