Finally solved! This pair of ancient fish brothers, who were separated for 438 million years, were found less than 200 kilometers apart

Produced by: Science Popularization China

Author: Shan Xianren Gai Zhikun (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences)

Producer: China Science Expo

Some time ago, the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences made another new discovery. This time, paleontologists discovered a new species of true armored fish - Jiangxialepis jiujiangensis. In this article, we will take you to explore the daily life of paleontologists and learn about the story behind the discovery of this new species. If you are a student who has just finished the college entrance examination, this may be a "study guide" for paleontology.

"Split kiss", the kind that is useful only when it is split

What kind of fish is the "Jiujiang Jiangxia fish" discovered this time? In terms of systematic classification, they belong to the genus Jiangxia fish of the subclass of armored fishes, the order of eusaruma, the family of Aeolus, and their typical feature is that the front end of the middorsal hole extends forward to the edge of the snout of the head armor and splits the snout, that is, "split snout".

An armored fish with a split snout - Xikeng split snout fish

(Photo credit: Guo Xiaocong)

"Split snout" may not sound healthy, but for armored fish, it may be a good thing. The middorsal foramen is the nostril of armored fish. The nostril of armored fish is directly connected to the mouth. Water flows through the nostril into the mouth and then into the gills for gas exchange. Therefore, the middorsal foramen is an important respiratory organ of armored fish.

The dorsal aperture of most armored fish opens to the dorsal side of the head armor and can only inhale water from above the head armor. However, the split snout allows the dorsal aperture of armored fish to have an additional opening facing forward and downward. In this way, during swimming, the dorsal aperture can simultaneously receive water from above, in front of, and below the head armor. Therefore, scientists speculate that the split snout can greatly improve the water intake efficiency of armored fish when breathing.

Dorsal holes of armored fish of various shapes

(Photo credit: Yang Dinghua)

Among armored fish, the split snout was first evolved in Jiujiang Jiangxia fish. Scientists have also found the split snout in the fossils of another armored fish, Xikeng split snout fish, but this armored fish did not develop the split snout until 3 million years after Jiujiang Jiangxia fish.

So what is the relationship between these two types of armored fish? Is Jiujiang Jiangxia fish the "ancestor" of Xikeng split-mouthed fish? The answer is: there is no relationship between them. Jiangxia fish and split-mouthed fish belong to two completely different evolutionary branches. The former belongs to the family Aurochsidae, representing the most primitive branch of the order Euarmorhynchus; the latter belongs to the family Sinensis, representing a more advanced branch of the order Euarmorhynchus. Although both belong to the order Euarmorhynchus, Jiangxia fish and split-mouthed fish evolved from two different fish ancestors, and there is no generational inheritance relationship between the two.

This phenomenon of two different lineages evolving the same traits due to living in similar environments is called parallel evolution. The "split snout" feature exists in both the family Aeolus and the family Sinocera, indicating that the "split snout" originated independently at least twice in the order Eucera.

In addition to the "slit snout", Jiangxia fish also evolved characteristics that scientists previously believed only existed in Polybranch fish and Huanan fish, such as the development of the middle dorsal spine and middle dorsal ridge at the back of the head armor. The middle dorsal spine and middle dorsal ridge are similar in function to the vertical tail of an airplane, which can keep armored fish balanced when swimming and prevent the body from tilting.

The emergence of this series of characteristics indicates that the Euarmored fishes had made various evolutionary attempts related to water intake efficiency, maintaining balance, etc. in the early stages of their evolution, showing a high level of morphological diversity.

Fossil photos of the Jiujiang Jiangxia fish (Jiangxialepis jiujiangensis)

(Photo credit: Photo by Gai Zhikun)

The discovery of Jiujiang Jiangxia fish not only enriches our understanding of the species diversity of true armored fishes in the Early Silurian, but also has very important significance for a deeper understanding of the radiation evolution and biopaleogeography of armored fishes in the Early Silurian.

How do paleontologists “do reasoning questions”?

Paleontological research is often like "doing reasoning problems". The Jiujiang Jiangxia fish fossils discovered this time have posed several difficult problems to paleontologists, but scientists are experts in finding answers from clues, and this time is no exception. They discovered the migration route of the Jiujiang Jiangxia fish. How did they discover it? Take a look at the reasoning process below.

Jiujiang Jiangxia fish belongs to the armored fish species. We will start our reasoning from the living environment and migration routes of armored fish species, and then further understand Jiujiang Jiangxia fish.

To understand the living environment of armored fish, paleontologists have summarized a two-step method.

Step 1: Observe the ecological characteristics of the fossil itself.

The head armor of armored fish is strongly flattened, with the mouth on the ventral side and the eyes and nostrils on the dorsal side. **Scientists speculate that they live on the bottom of the body based on the eyes being on the dorsal side of the head armor.** This is because the dorsal eyes can only receive vision from above the head armor, and the area below the head armor becomes a blind spot for the fish. Therefore, only living on the bottom of the body can enable them to detect predators faster and escape from enemies.

At the same time, **armored fish belong to jawless fish, **they have not evolved jaws that can bite, that is, "chins", so they can only survive by filtering food . The mouth located on the ventral side of the head armor indicates that armored fish mainly filter algae or organic matter in the bottom water or bottom mud. Through these ecological characteristics, it can be inferred that they are a type of jawless fish that live a bottom-dwelling filter-feeding life.

In addition, they are covered with a heavy exoskeleton and lack paired fins . However, being light and having paired fins is very important for fish, because the pectoral and pelvic fins are equivalent to the forelimbs and hind limbs of vertebrates, and their main function is to enable the fish to move forward, maintain balance, control direction, or "brake" in time while moving. Armored fish that have not yet evolved paired fins are not good at swimming, and coupled with the "burden" of the exoskeleton head armor, armored fish are not good at swimming.

Jiujiang River Summer Fish (above) and Houji River Summer Fish (below)

(Photo credit: Shi Aijuan)

The mainly benthic lifestyle and weak mobility limit the ability of armored fish to spread and migrate, and it is speculated that armored fish lived in shallow marine environments on the edge of ancient land.

Shallow seas receive a large amount of sedimentary materials transported by rivers and other external forces, as well as materials eroded by sea erosion. The sediment sources are very rich, and the organic matter content is much higher than that of calm deep seas. In addition, the shallow sea zone has sufficient sunlight and oxygen, which is very suitable for bottom-dwelling filter feeders such as armored fish. In fact, in modern oceans, most bottom-dwelling organisms also reproduce in large numbers in shallow sea zones, and their species and numbers far exceed those in other zones.

In addition, scientists speculate that armored fish can only migrate along ancient coastlines and cannot spread across oceans.

This is because migrating across the ocean is a difficult task, and fish need to meet a variety of conditions to accomplish this feat. For example, they need to have strong enough swimming ability to maintain their physical strength and speed during the long migration, and they also need to have sufficient food sources to cope with the challenges of long migration and food shortage.

Obviously, for armored fish that are "bulky" and "eat soil" for a living, it is unlikely to migrate across the ocean. Real conditions only allow them to stay in their "comfort zone" and spread along the shallow sea environment at the edge of the ancient continent.

After the first step of observation, it is time to look at the paleogeographic distribution . From the paleogeographic distribution of the early armored fishes of Trechi, it can be found that their paleogeographic distribution was obviously restricted by the marine environment and was basically distributed only on the edge of the ancient land.

Paleogeographical research shows that in the early Silurian period 438 million years ago, the South my country Plate, mainly the Yangtze River Basin, was a vast ocean known as the Yangtze Sea. The sea is further divided into the Upper and Lower Yangtze Seas, with a narrow and long shallow sea area between them - the Jiujiang Strait. Wuhan, Hubei and Wuning, Jiangxi, face each other across the Jiujiang Strait, with the closest straight-line distance less than 200 kilometers.

Paleogeographic map and locality of armored fish fossils in the early Llandoverian-Telychian of South China

(Image source: Shan Xianren modified from Chen Xu and Rong Jiayu, 1996; Rong Jiayu et al., 2012;)

The Wentang fossil assemblage of the Qingshui Formation of the Lower Silurian red strata in Wuning, Jiangxi Province has the highest diversity of ancient fish species, covering almost all genera and species of this assemblage in other regions. This phenomenon can also be inferred that the Wuning area in Jiangxi Province may be one of the centers of radiation and diffusion of armored fishes.

The Jiangxiasaurus discovered this time was found simultaneously in Wuhan, Hubei and Wuning, Jiangxi. This indicates that in the early Silurian period, armored fish may have completed the north-south migration across the Yangtze Sea through the Jiujiang Strait between the Upper and Lower Yangtze Seas, thus enabling armored fish to spread eastward and westward along the northern edge of the Yangtze Sea to Chaohu Lake in Anhui and Jingshan in Hubei respectively.

How about it, is this reasoning interesting?

Two brothers of Jiangxia fish lost for 438 million years

Why do paleontologists dig and dig in the "big pile of dirt"? Wouldn't it be boring to work around fossils all day? If you think of paleontology as a journey to find relatives for extinct creatures, this kind of research becomes very interesting.

Take Jiujiang Jiangxia fish as an example. In fact, the process of finding it is very tortuous.

Before the discovery of Jiujiang Jiangxia fish, its "brother" Houji Jiangxia fish had already been discovered in Wuhan, Hubei. Wuhan is located on the northern edge of the Yangtze Sea, while armored fish have always been believed to have spread only along the southern edge of the Yangtze Sea. This shows that there must have been an exchange and diffusion event of armored fish between the southern and northern edges of the Yangtze Sea, so scientists set their sights on Jiujiang, Jiangxi. In the early Silurian period 440 million years ago, it was the shallow sea area closest to Wuhan, and other clues about Jiangxia fish must have been found there.

The Jiujiang area in Jiangxi Province is a very important fossil producing area of ​​early Silurian vertebrates in China. As early as the 1980s, Mr. Pan Jiang, a famous Chinese paleoichthyologist, conducted field surveys in Taiyangsheng Town, Xiushui County, Jiujiang, and discovered jawless fossils such as the Chinese armored fish, split-nose fish and Xiushui fish for the first time.

In 2003, Researcher Gai Zhikun, who was then a graduate student at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, also came to Xiushui County for field investigations under the leadership of Academician Zhu Min, and collected a large number of new materials on armored fish.

But later, because the research team focused on the study of Silurian dawn fish in Changxing, Zhejiang, the research on armored fish in Jiujiang, Jiangxi was temporarily shelved, and this shelving lasted for 10 years.

In 2012, after returning from his studies in the UK, researcher Gai Zhikun restarted his research on Silurian armored fishes in Jiujiang, Jiangxi, and embarked on the journey of "looking for relatives" for armored fishes again. For several consecutive years, he went to Jiujiang's Xiushui area to conduct field surveys. In 2019, the research team drove to Jiujiang to conduct field surveys as in previous years, but because the Xiushui County Expressway was under maintenance, the research team had no choice but to get off the expressway in advance from the Lixi Town Exit in Wuning County.

However, it was this change of route that allowed the research team to unexpectedly discover a group of ancient fish fossils that were older than those in the Xiushui area at a construction site in Lixi Town. This group of fossils is located below the Xiushan fauna and belongs to the Qingshui Formation of the lower red strata of the Silurian period. It represents the earliest recovery and radiation evolution of global vertebrates after the mass extinction at the end of the Ordovician period.

Taking this as an opportunity, the research team has increased its field excavation efforts in the Wuning area of ​​Jiujiang. After finding a biological brother for the reticulated Anji fish in Anji, Zhejiang in Wuning, Jiangxi in 2022, this time they found a biological brother for the Houjijiang summer fish in Wuhan, Hubei at the same location, and the straight-line distance between the two is less than 200 kilometers.

Conclusion

Uncovering the life of Jiujiang Jiangxia fish is an interesting process of reasoning, and the discovery of this species itself is full of drama, but this does not mean that paleontologists can only rely on guesses and luck to complete scientific research. Hard work over time is their norm. How about it? Do you think the work of paleontologists is interesting? After the college entrance examination, will you consider applying for paleontology?

Note:

The relevant research in this article was published as a cover article in the international academic journal "Acta Geologica Sinica (English Edition)". The title of the paper is "A new species of the genus Jiangxiasaurus from the early Terechian period of the Silurian in Jiangxi Province and its biostratigraphic significance". Shan Xianren, a master's student at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, is the first author of the paper, and Researcher Gai Zhikun is the corresponding author of the paper.

(Note: Latin text should be italicized.)