Interesting ancient creatures: 438 million-year-old "Changxing fish" fossils were found thousands of miles apart in Zhejiang and Xinjiang, proving the continental drift hypothesis

Produced by: Science Popularization China

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

Producer: China Science Expo

Editor's note: In order to unveil the mystery of scientific work, the China Science Popularization Frontier Science Project launched a series of articles called "Me and My Research", inviting scientists to write articles themselves, share their scientific research experiences, and create a scientific world. Let us follow the explorers at the forefront of science and technology and embark on a journey full of passion, challenges, and surprises.

The research team I lead is a very young team that has grown up gradually after more than 20 years of running-in under the careful guidance of Academician Zhu Min. The team is mainly engaged in the research of the origin, evolution and related biostratigraphy and paleobiogeography of Chinese armored fishes.

Armored fish are a type of armored fish that lived more than 400 million years ago and are unique to East Asia. They are currently only found in the Silurian-Devonian strata in China and northern Vietnam. The discovery of armored fish fossils in China can be traced back to the beginning of the last century. In 1913, Mr. Ding Wenjiang, the founder of China's geological career, went to Yunnan alone to investigate while serving as the director of the China Geological Survey and the Institute of Geology. He collected fish fossils in the Xishan Village Formation of the Early Devonian in Cuifeng Mountain and Liaojiao Mountain (now called "Liaokuo Mountain"). These fish fossils were later identified by American paleontologist Grabau as Cephalaspis sp. , which is our armored fish today. However, it was not until 1965 that Mr. Liu Yuhai, a Chinese paleoichthyologist, made the first systematic paleontological description of China's armored fish fossils, becoming the first person to study armored fish. Over the past 100 years, through the efforts of several generations of Chinese paleontologists, China has named a total of about 100 species of armored fishes and established a subclass classification unit, making it one of the five major groups of "armored fishes" along with the Osteochelids, Heterochelids, Ecliptochelids, and Anthochelids.

In the late 1990s, with the retirement of Liu Yuhai, Pan Jiang, Wang Nianzhong, Wang Junqing and other older generation paleontologists who studied armored fishes, China's research on armored fishes also faced an embarrassing situation of a lack of successors, and urgently needed young people to inject fresh blood.

In 2002, I graduated from the Department of Earth Sciences at Shandong University of Science and Technology and was admitted to the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences to pursue a master's degree. I was fortunate to become the first student of Academician Zhu Min and began to take over the research on Chinese armored fishes. In 2006, I received a full scholarship from the Royal Society of the United Kingdom to pursue a doctorate at the University of Bristol, UK, under the tutelage of Professor Philip CJ Donoghue, a member of the Royal Society of the United Kingdom, and continued to carry out research on Chinese armored fishes.

In 2012, I returned to China after completing my studies and worked at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, where I led students to conduct in-depth research on the morphology, anatomy, systematics, and ecology of Chinese armored fishes . It took us two decades to successfully solve two century-old problems: the internal anatomy of the skull and the anatomy of the body behind the head of armored fishes. This provided the most critical fossil evidence for the theory of the developmental origin of the human jaw, the origin of the gills of the middle ear, and the origin of the fin folds of the limbs .

After the research results were published in Nature magazine as cover recommendations and cover articles in 2011 and 2022, respectively, they attracted widespread attention in the academic community and were considered a classic case of evolutionary developmental biology. They have been included in more than a dozen textbooks in Europe, America, Japan and other countries and regions, such as Evolutionary Developmental Biology, Evolutionary Neurology, Life History, and Vertebrate Paleontology. At the same time, I also guide students to carry out research on biostratigraphy and paleobiogeography of armored fishes.

Paleontological fossil evidence played a crucial role in proving the continental drift hypothesis . In 1910, German meteorologist and geophysicist Wegener discovered that the shapes of the two sides of the Atlantic Ocean complemented each other, which gave rise to the idea of ​​continental drift. He believed that the landmass of the world was a connected whole 200 million years ago, and later, due to the action of forces, it continued to separate and drift to its current position.

In order to verify this hypothesis, scientists have found a lot of evidence, one of the most convincing evidence is the fossil evidence from the ancient creature Lystrosaurus . Lystrosaurus is a mammal-like reptile that lived about 200 million years ago. It mainly lived on land and could not migrate across the sea. However, its fossils are widely distributed in the Late Permian-Early Triassic strata in Xinjiang, China, southern Africa, India, Antarctica and other places. Its wide distribution across oceans strongly indicates that these fossil sites may have been in the same united continent at that time - Gondwana.

China's armored fish also plays a pivotal role in proving the theory of plate drift! Armored fish are a type of fish that mainly live in lagoons, deltas, coastal and shallow sea environments near the edge of the ancient continent, and mainly live a benthic filter-feeding life. Most armored fish have flat heads and lack paired fins, indicating that they have weak swimming abilities and limited ability to spread and migrate. The vast land and vast oceans have become obstacles to their spread and migration. Therefore, armored fish have a strong indigenous color, and their distribution can be used to divide paleobiogeographic regions.

Recently, my students Li Xutong, Zhang Yumeng, Shan Xianren and others published a cover article in the Journal of Geology (English Edition), reporting an important progress in our research on the paleobiogeography of armored fish in the early Silurian. That is, we found traces of the armored fish Changxingyu in both the South China Plate and the Tarim Plate. This not only enriches our understanding of the morphology and diversity of Changxingyu, but also provides new evidence for the plate drift theory [1].

The cover shows a huge ripple structure discovered by the research team in the lower Silurian of the Tarim Basin in Xinjiang, indicating a shallow marine environment. Nianzhong Changxingichus and Gu's Changxingichus may have lived in the same shallow marine environment around the Tarim-South China joint plate about 438 million years ago.

(Image source: Cover of Acta Geologica Sinica)

Four generations of paleontologists relayed the discovery of Changxing fish in Changxing, Zhejiang

Changxing fish is the oldest armored fish that lived in the early Silurian period about 438 million years ago. They are very small, with a flat head armor of only about 3.5 cm, and the body should not exceed 10 cm. They have a pair of eyes, but only one nostril, which is a wide slit. The eyes and nostrils are both on the top of the head. They have no upper and lower jaws for chewing food, and no paired pectoral and pelvic fins. Therefore, they are likely to be a fish that lives on the seabed, is benthic, and has poor swimming ability. In terms of systematic classification, Changxing fish belongs to the family of Xiushui fish, a primitive group among early armored fish, and represents the primitive state of armored fish in many morphological characteristics.

Changxingaspis gui was first established by paleoichthyologist Wang Nianzhong in 1991. Its type species is Changxingaspis gui from the Silurian marine red beds in Changxing County, Zhejiang Province. The genus name Changxing comes from Changxing County, Zhejiang Province, where the fossil specimen was found, and the species name Gu was given by Wang Nianzhong to commemorate his mentor Professor Gu Changdong at Nankai University.

The discovery of Gu's Changxing fish in Changxing, Zhejiang (a) In 2003, the author collected fossils alone in the wild in Changxing; (b) Columnar diagram of fish-bearing strata in Changxing, Zhejiang; (3) After 20 years, the research team returned to the site of the discovery of the Changxing fish fossils in Xiaopu Town, Changxing, Zhejiang in 2023 (from left: Jia Liantao, Zhao Wenjin, Zhu Min, Gai Zhikun, Shan Xianren); (d) Ecological restoration map of Gu's Changxing fish. (Image source: provided by the author)

The discovery of Gu's Changxing fish is a research story spanning 40 years and completed by four generations of paleontologists.

As early as 1988, Mr. Pan Jiang from the China Geological Museum conducted field work near the Xiaopu Coal School in Meishan, Changxing County, Zhejiang Province, and discovered a fossil specimen suspected to be Changxing fish. However, due to the incomplete preservation of the fossil, Mr. Pan Jiang did not formally name it as a new species, and temporarily identified it as the undetermined species of the genus Xiushuisaspis ( Xiushuisaspis sp. ).

In 1977, paleontologist Wang Nianzhong was one of the first ten scientific and technological workers sent to France, Germany and the United Kingdom for training after my country's reform and opening up. He went to the Institute of Paleontology of the National Museum of Natural History in France to study the large fossil fishes of the Late Permian in Xinjiang under the tutelage of the famous paleoichthyologist J. Lehman. During the two-year training period, Wang Nianzhong not only completed the research on the Late Permian fish fossils, but also focused on the research on the jawless brain skull and fish microfossils that he was interested in.

Soon after returning to China, Mr. Wang Nianzhong proposed to Mr. Zhou Mingzhen, then director of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, that he hope to carry out two important research projects in China, one of which is the study of the skull of armored fish. Based on Mr. Pan Jiang's early discoveries, Mr. Wang Nianzhong focused his investigation on the Changxing area of ​​Zhejiang Province. After years of hard work, he finally collected a large number of armored fish fossils, especially a batch of beautifully preserved armored fish skull fossils.

In 1991, Mr. Wang Nianzhong officially published this batch of precious fossil materials and named two new genera and species, namely, Lehman Meishan Fish and Gu Changxing Fish[2]. In 2003, led by Mr. Zhu Min and Mr. Wang Nianzhong, I went to Changxing, Zhejiang to collect fossils and materials for my master's thesis.

After that, I went to Changxing, Zhejiang alone every year to look for fossils, and stayed there for several months. I remember that the research funds were very tight at that time, and I couldn't afford to rent an off-road vehicle in the wild. In order to save money, I stayed at the home of a fellow villager and borrowed a bicycle from him to ride to the fossil field to collect fossils.

From 2003 to 2007, I conducted five field geological surveys in Changxing District, northwest Zhejiang Province. "Hard work pays off", and after years of persistence in the field, I finally made a breakthrough here and collected a large number of fossils of Zhejiang dawn fish and Gu's Changxing fish. But later, because the research team focused on the study of the brain of Zhejiang dawn fish, the study of Gu's Changxing fish was temporarily shelved, and this shelving lasted for 20 years. It was not until 2022 that Zhang Yumeng and Li Xutong, undergraduate students from Jiangxi Normal University, joined the "Science and Technology Innovation Program" of the Chinese Academy of Sciences, and we restarted the study of Changxing fish. The program, in the form of a project, funds outstanding second- or third-year undergraduate students majoring in science and engineering from universities across the country to carry out scientific research innovation practice activities from June to December at the Chinese Academy of Sciences. Under the joint guidance of me and my master's student Shan Xianren, Zhang Yumeng and Li Xutong finally completed the study of Changxing fish in two years, and finally published the research results in the form of a cover article in the "Acta Geologica Sinica (English Edition)". Therefore, the discovery and research of Changxing fish is a scientific research story that has been passed down from generation to generation and completed by four generations of paleontologists. Today, my graduate student Shan Xianren has successfully been admitted to the University of Bristol in the UK to pursue a doctorate, and my undergraduate students Zhang Yumeng and Li Xutong have also been admitted to the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences to pursue a master's degree, continuing this scientific research story.

Fossils of Gu's Changxing fish and restoration of its morphology

(Image source: Reference 1)

Changxingaspis gui is a small armored fish with an oval head armor, about 3.5 cm long, and about twice the size of Xiushui fish. At the end of the head armor, Changxing fish has a pair of sickle-shaped inner angles that are very well developed, while the inner angles of Xiushui fish are almost undeveloped. Therefore, Mr. Wang Nianzhong does not agree with Mr. Pan Jiang to classify these new materials into the genus Xiushuisaspis , but represents a new genus and species of Xiushui fish family, and named it Changxingaspis gui .

What is particularly amazing is that the Gu's Changxing fish has preserved a beautiful skull structure. Through its restoration and research, the anatomical information of the internal skull of armored fish has been understood. This was the most comprehensive and in-depth research result on the skull of armored fish at that time [2]. However, due to the limitation of fossil materials, we know very little about the ventral surface of the skull of Gu's Changxing fish.

The new material of Gu's Changxing fish in this study is a pair of positive and negative molds of the complete head armor, which reveals more information about the ventral side of the head armor of Gu's Changxing fish, including the gill opening, the post-gill wall and the post-gill area. The study shows that, except for the base of the inner angle, the ventral part of the head armor of Changxing fish is much shorter than the dorsal part, and this part ends at the post-gill wall. Therefore, there is no slender post-gill area on the ventral side of the head armor of Changxing fish, which is different from the original understanding.

The first discovery of Changxing fish in the Tarim Basin, Xinjiang

The Tarim Basin in Xinjiang is located in the southern part of Xinjiang, in the core area of ​​the Silk Road. It was once a must-pass place on the Silk Road in history. At the same time, the Tarim Basin is also a large superimposed craton oil and gas basin with an area of ​​about 56×104 square kilometers. It is my country's strategic energy base in the 21st century.

At the end of the last century, the Eighth Five-Year Plan and Ninth Five-Year Plan scientific and technological research projects on "Oil and Gas Resources in the Tarim Basin" made breakthrough progress in the study of paleontological fossils in the Tarim Basin. However, in terms of paleontology, since most of the armored fish specimens in the Tarim Basin are broken and incomplete, the overall research level of armored fish in the region is still very low. After 2007, related research in the region stagnated.

After returning to China after completing my studies in 2012, I restarted the field survey of ancient fishes in the Silurian System of the Tarim Basin in Xinjiang. In 2018, I reported the discovery of the earliest multibranched fish, the Serratus serrulateus, in the Silurian System of the Tarim Basin in Xinjiang. This discovery pushed the origin of multibranched fishes back from the Early Devonian to the Llandoverian Period of the Silurian Period, a date pushed forward by about 20 million years[3].

The sawtooth fish exhibits many characteristics that evolved in parallel with modern rays, and it is likely that this represents a new lifestyle for armored fishes: a semi-buried lifestyle. This new lifestyle allowed armored fishes to better hide themselves and avoid predators. Perhaps thanks to this new lifestyle, the family Archaeopterygii, to which the sawtooth fish belongs, has become the longest-lived lineage of the armored fish subclass [3].

In 2019, we organized another field survey and discovered new material of Zhang's western fish, which we classified into the Xiushui fish family. This discovery broke the traditional understanding that there were no Xiushui fish fossils in the Tarim Basin [4].

In 2020, we received funding from the National Natural Science Foundation of China. From 2021 to 2023, the research team overcame many difficulties during the epidemic and went to the Tarim Basin in Xinjiang for field surveys many times. They completed three large-scale field excavations and finally collected abundant armored fish fossils in the Tatarertag Formation in the Kelmati area of ​​Keping, Xinjiang.

After three years of excavation, indoor repair and comparative research, in 2023, we reported for the first time in the international academic journal "Paleogeography, Paleoclimate, Paleoecology" the discovery of a true armored fish, Rong's Jiangxia fish, in the Tarim Basin in Xinjiang. This is the first time that the research team has discovered the oldest true armored fish in the region since the discovery of the oldest multi-brachial fish in the Tarim Basin in 2018 [5]. These fossils are of great significance to the analysis of the Paleozoic paleogeography of the Tarim Basin!

The field team discovered a large area of ​​ripple structures in the Tataertag Formation in the Kelmati area of ​​Keping, Xinjiang, indicating that the area was a shallow marine environment 438 million years ago.

(From left: Shan Xianren, Lin Xianghong, Gai Zhikun, Zhao Ridong)

(Photo source: provided by the author)

The new species of Changxing fish studied and named this time, Nianzhong Changxing fish, represents the "first discovery" of the Changxing fish genus in the Tarim Basin. The species name commemorates Mr. Wang Nianzhong's groundbreaking work in the study of the brain anatomy and head carapace morphology of Changxing fish. The new species is clearly distinguished from Gu's Changxing fish by the characteristics of the middorsal pore width-to-length ratio of 3, the lateral transverse tube extending to the lateral edge of the head carapace, and the second lateral transverse tube bifurcating at the end.

Nianzhong Changxing fish fossil and morphological restoration

(Image source: Reference 1)

Changxing fish was discovered in Zhejiang and Xinjiang. What is its scientific significance?

Systematic paleontological research is also, to a certain extent, a journey to find relatives for extinct organisms. In particular, when several different species of the same genus are found, it can be said that the brothers and sisters of the species have been found, which has very important biostratigraphic significance, indicating that the two species have been geographically isolated for a short time and have just differentiated into two new species. Nianzhong Changxing Fish represents the first fossil record of the Changxing Fish genus in the Tarim Plate. Its discovery is equivalent to finding a brother for Gu's Changxing Fish, which is thousands of miles away, and has important biostratigraphic and paleozoogeographic significance.

In 2023, we collaborated with the team of Academician Rong Jiayu from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, to conduct field investigations and demonstrations on typical Silurian sections in the Jiangsu, Zhejiang and Anhui regions in eastern China. We eventually established this set of marine red beds as the Kangshan Formation. One of the key pieces of evidence for establishing the age of the Kangshan Formation is the evidence from the Changxing fish fossils.

The armored fishes from the Kangshan Formation in northwestern Zhejiang include Changxing fish of Xiushui fish family and the true armored fishes such as Aeolus, Meishan fish and Anji fish. Previous taxonomic studies have compared the true armored fishes such as Aeolus in the Kangshan Formation with the Chinese armored fishes in the Xikeng Formation based on "overall morphological similarities", and then believed that the Tangjiawu Formation and the Xikeng Formation are at the same level and their age is the late Landovile Terrechi of the Silurian.

Recently, the taxonomic conclusions based on phylogenetic analysis clearly show that the Aurochs and Meishan fish in the Kangshan Formation and the Qingshui fish in the Qingshui Formation constitute a monophyletic group, the Aurochs; the Angifish in the Kangshan Formation and the Angifish in the Qingshui Formation constitute another monophyletic group, the Angifish genus; both of the above monophyletic groups are more primitive than the Chinese Armoridae in the Xikeng Formation of the Upper Red Beds. Therefore, from the perspective of true armored fish fossils, the Kangshan Formation is considered to belong to the Lower Red Beds and can be well compared with the Qingshui Formation.

Paleogeography of the Tarim-South China joint plate and the distribution of Xiushui fishes in the Early Silurian

(Image source: References 1, 3)

However, Xiushuiichthyidae was also used as a symbol for the comparison between the marine red beds and the upper red beds of the Xikeng Formation in northwestern Zhejiang. With the deepening of fish biostratigraphic research in recent years, it was found that although the Xiushuiichthyidae has a longer stratigraphic extension (from the lower red beds of the Silurian to the upper red beds), the members of this family show different compositional features in different strata.

Among them, Xiyuichthys only appears in the lower red beds of the Silurian system (Qingshui Formation and Tatarertag Formation), while Xiushuiichthys only appears in the upper red beds of the Silurian system (Xikeng Formation). The discovery of Changxingichthys in the Tatarertag Formation of the Silurian system in Xinjiang proves that this genus also belongs to the lower red beds. Therefore, this study further supports that the fish-bearing marine red beds (Kangshan Formation) in northwestern Zhejiang belong to the lower red beds from the perspective of Changxingichthys.

In addition, Nian Zhong Changxingichthys is the third genus of armored fish shared by South China and the Tarim Basin after Jiangxiaichthys and Xiyuichthys, further supporting the hypothesis that the Tarim Basin and South China may have been very close about 438 million years ago, and may even have belonged to the same plate, namely the Tarim-South China joint plate, thus providing a geographical basis for the multiple close biological exchange events that occurred between South China and the Tarim in the early Silurian period.

References:

1. Li, XT, et al., New findings of Changxingaspis (Xiushuiaspidae, Galeaspida) from the Silurian of Tarim Basin and Zhejiang Province. . Acta Geologica Sinica (English Edition), 2024.

2. Wang, N.-Z., Two new Silurian galeaspids (jawless craniates) from Zhejiang Province, China, with a discussion of galeaspid-gnathostome relationships, in Early Vertebrates and Related Problems of Evolutionary Biology, MM Chang, YH Liu, and GR Zhang, Editors. 1991, Science Press: Beijing. p. 41-66.

3.Gai, Z., et al., New polybranchiaspiform fishes (Agnatha: Galeaspida) from the Middle Palaeozoic of China and their ecomorphological implications. PLoS One, 2018. 13(9): p. e0202217.

4. Liu, YH, et al., A reappraisal of the Silurian galeaspids (stem-Gnathostomata) from Tarim Basin, Xinjiang. Vertebrata PalAsiatica, 2019. 57(4): p. 253-273.

5. Liu, WY, et al., The first Eugaleaspiforme fish from the Silurian of the Tarim Basin reveals a close relationship between the Tarim and South China blocks at 438 mya. Palaeogeography, Palaeoclimatology, Palaeoecology, 2023. 628: p. 111774.