[Creative Cultivation Program] Uncovering the survival secrets of the "ugly and cute" Oriental fish 390 million years ago

Author: Meng Xinyuan and Gai Zhikun

Reviewer: Zhao Wenjin

Do you know how hard this ugly and cute ancient fish fought for its survival? Do you know how tortuous the process of discovering and studying this ugly and cute ancient fish was?

Figure 1 Ecological restoration map of Oriental fish (drawn by Yang Dinghua)

This ancient fish is called Oriental fish. It was recently published in the professional academic journal "Acta Vertebrate Paleontology" by Meng Xinyuan, a master's graduate from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, under the guidance of researcher Gai Zhikun. However, the fossil materials on which its ecological restoration map is based are not recently discovered.

In the 1980s, Wang Junqing and other older generation paleoichthyologists from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences discovered a large number of early ancient fish fossils during a field survey in Wuding County, Chuxiong Yi Autonomous Prefecture, Yunnan Province. Since the fossils were relatively fragmented, these fossil materials were piled up in the underground specimen library of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences after they were collected. They did not attract much attention at the time, so these fossils were sealed and dormant in the underground specimen library for more than 30 years.

It was not until 2014 that Mr. Wang Junqing of the Institute of Vertebrate Paleontology and Paleoanthropology accidentally discovered a strange, dusty fossil while reorganizing specimens. After washing it with water, he recognized it as a part of the head armor of a polygill fish, which was incomplete but had obvious characteristics. In this way, the long-forgotten Oriental fish fossil finally saw the light of day. It took nearly 10 years for this fossil to be officially published recently. It can be said that it has finally come out after much anticipation. The publication of the research results on this ancient fish can be regarded as a good explanation for Mr. Wang Junqing's field discovery 40 years ago, and it is also a high tribute to Mr. Wang Junqing, who has retired for many years.

Figure 2 Photo of the new specimen of Oriental fish (A, photographed by Gai Zhikun) and its restoration (B, drawn by Guo Xiaocong)

The most distinctive feature of the Oriental fish is its "multiple gills". It has up to 45 pairs of gill sacs, the largest number of gill sacs among all armored fish in history. The gills are the most important respiratory organs for armored fish. They are arranged symmetrically on both sides of the ventral side of the head armor and help armored fish get oxygen from the water. The more gill sacs a fish has, the better it can withstand an oxygen-deficient environment.

The cartilaginous fish that we often see now, such as sharks and rays, only have 5-7 pairs of gills, and the bony fish such as sea bass and yellow croaker that we eat daily also have only 5 pairs of gills. Similar to these living fish, the number of gills of early armored fish was also relatively small, with only 6-7 pairs of gill sacs, which may represent the primitive state of armored fish. In terms of systematic classification, Oriental fish belongs to the subclass of armored fishes, order of Polybranchius, family of Polybranchius, and the family of Polybranchius is named after the genus Polybranchius. However, this "polybranchius" now seems to be somewhat unworthy of its name.

The thing is, when the genus Polybranchus was first named, it refreshed our understanding of "multiple gills" (see the early armored fish mentioned above) because it had 12 pairs of gill sacs. So it was named Polybranchus. In 2018, we discovered a 438 million-year-old polybranch fish in the Tarim Basin in Xinjiang. It was the world's first armored fish that would half-bury itself like modern rays, and it only had 6 pairs of gill sacs. Compared with it, 12 pairs of gill sacs can indeed be considered polybranchs. However, as new fossils were discovered, things gradually got out of control...

About 411 million years ago, the "multiple gills" of polybranch fish began to develop, such as polybranch fish with 12 pairs and broad armored fish with 18 pairs, but the number of gill sacs was less than 20 pairs; about 408 million years ago, the number of gill sacs in some groups of armored fish had increased to more than 30 pairs, with Oriental fish as a typical representative, with as many as 45 pairs of gill sacs. Therefore, the number of gills of polybranch fish, which was originally named "polybranch", can be said to be dwarfed by that of Oriental fish.

Figure 3 Comparison of gill sacs of Oriental fish (A), broad turtle (B) and polybranch fish (C) (drawn by Guo Xiaocong)

So what does the increased number of gill pouches mean?

This new fossil points us in the direction of solving the mystery of the connection between the number of gill pouches in armored fish and the Devonian marine anoxic event. As mentioned above, the exponential increase in the number of gill pouches in armored fish was an evolutionary trend during the Silurian-Devonian period.

The reason is probably that during the Devonian period, especially 395 million years ago, the diversity of terrestrial vascular plants increased significantly, the sea level continued to rise, and the supply of nutrients was sufficient. In several time periods, widely distributed black shale/organic-rich deposits were formed, which consumed a large amount of oxygen at the bottom of the water body, causing the shallow marine environment to be in a short-term hypoxic state, thus leading to a major vertebrate extinction event.

Armored fish have always been believed to live in shallow seas or coastal environments. The low-oxygen shallow sea environment created obstacles for their survival. Even though some armored fish already had a large number of gill sacs to enhance breathing, most of them disappeared in this extinction event caused by lack of oxygen. Oriental fish has the largest number of gill sacs in history, which is likely to be the secret weapon that helped them survive in the low-oxygen shallow sea environment, allowing them to live for nearly 20 million years.

Figure 4 Phylogenetic relationships of armored fishes and changes in genus-level diversity during the Silurian-Devonian (illustration by Meng Xinyuan and Yang Dinghua)

The discovery of the new specimen of Oriental fish fills the geological gap in the Middle Devonian multi-branchied fish in Yunnan, and extends the survival period of the Oriental fish genus from about 410 million years ago to about 20 million years ago. It is also of great significance for studying the co-evolution of the number of gill sacs of armored fish and the Early Devonian marine hypoxia event.

The discovery of the new specimen of Oriental fish is also the second fossil record of armored fish in the Middle Devonian in my country so far. The first record is the near-mid-orbital fish from Bobai, Guangxi, which is very incomplete. The only armored fish fossil later than these two records was found in Zhongning, Ningxia, in the Late Devonian. This brings up another unsolved mystery: when did armored fish migrate to Ningxia and Guangxi? This mystery needs to be answered by further research in the future.

When it comes to the evolution of the number of gill sacs in armored fish, many people may hold the view that "in order to survive in an oxygen-deficient environment, the Oriental fish evolved a large number of gill sacs." In fact, this teleology is the most common misunderstanding of the theory of evolution. We must know that variation has no direction, but selection has a direction. Organisms cannot determine the direction. It is just that the Oriental fish that can adapt to the environment survived.

Although paleontology may seem irrelevant to our daily lives, it is both interesting and useful. Isn't it great that we can infer so many things from a small fish fossil?

This article is produced by the Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting