Another ancient fish fossil was discovered in Chongqing, you are still too comprehensive!

Produced by: Science Popularization China

Author: Li Maokun

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

A fossil of a 436-million-year-old fish was discovered

Recently, a research result published in the Journal of Vertebrate Paleontology (Vertebrata Palasiatica) attracted everyone's attention. The protagonist of the result is a true armored fish from the special buried fossil repository in Chongqing - Miaojiaaspis dichotomus gen. et sp. nov. In the genus name, "Miaojia" comes from the Miao people, one of the main ethnic minorities in Xiushan Tujia and Miao Autonomous County. "Double fork" is used to describe the feature that its lateral transverse tube is forked at the end.

The double-forked Miaojia fish was discovered in the Huixingshao Formation of the Landovary-Telerian stage of the Silurian Period in the southeastern part of Chongqing (about 436 million years ago). The holotype specimen is an incomplete head fossil connected to the torso, which is now collected in the Chongqing Institute of Geology and Mineral Resources, with specimen numbers CIGMR V0125a and CIGMR V0125b.

Restoration of the dorsal side of the head shell of the double-forked Miao fish

(Image source: Reference 1)

Armored fish is armored fish, is there any difference between real and fake ones?

The two-forked Miao fish is a true armored fish.

Seeing this, many people must have this question: Are there false armored fish?

Unfortunately, there are no false armored fishes. But there are other armored fishes.

To explain their situation clearly, we have to start with the jawless fish.

Jawless fish are a group of fish without upper and lower jaws and with a single external nostril. Excluding living jawless fish (represented by hagfish and lamprey), extinct jawless fish can be divided into two major categories: Kunming fish and armored fish. Armored fish account for the vast majority of jawless fish and constituted the majority of vertebrates at that time.

Armored fish are one of the most prosperous groups of armored fish. They are at a key node in the evolution of early vertebrates and can provide key fossil evidence for the origin of jawed fish. Except for several basal groups, armored fish can be divided into three monophyletic groups , namely true armored fish, polybrachii fish and South China fish according to the order of differentiation time.

Armored fishes are a group with strong local characteristics. Currently, most of them are only found in China and northern Vietnam. True armored fishes are a monophyletic group among armored fishes. They have been separated from the basal group since 430 million years ago and have continued to the Pragian stage of the Early Devonian (about 410 million years ago). True armored fishes are generally small in size, with triangular head armor or snout.

Why Chongqing again?

The earliest discovery of jawless fossils in Chongqing can be traced back to 1962, in Shuiyuantou Village, Xiushan Tujia and Miao Autonomous County, Chongqing. A jawless fish fossil was found on a rolling stone in the area by the Hunan Provincial Survey Team, and was later named Eugaleaaspis xiushanensis by Mr. Liu Shifan. Since many features of Eugaleaaspis xiushanensis are very similar to those of Dunyu, it was renamed Dunyu xiushanensis in 2012.

In 2019, Professor Li Qiang accidentally discovered a complete jawed fish fossil, Bianchengichthys micros, near Biancheng Village, Hong'an Town, Xiushan County. The relevant research results were published in Contemporary Biology in 2021.

In 2022, Chen Yang and others described the first and oldest true armored fish material from the Huixingshao Formation in Yongdong Town, Xiushan County, Yongdongaspis littoralis. This new material fills the morphological gap between the Chinese armored fish and the eugaleaspid cluster, which is of great significance.

In September 2022, Nature magazine published four academic papers by Zhu Min's team and its collaborators as cover articles at the same time. Among them, the miracle Xiushan fish (Xiushanosteus mirabilis), the dynamic Tujia fish (Tujiaaspis vividus) and the vermiform Shen's stickleback fish (Shenacanthus vermiformis) are all from the Chongqing special buried fossil reservoir. The newly discovered double-forked Miaojia fish also comes from here.

Photos of the Tujia fish fossil

(Image source: Reference 2)

What is so special about the double-forked Miao fish?

Similar to other fossil materials found in Chongqing's special buried fossil deposits, the body of the two-forked Miaojia fish is small, with a head armor length of about 2 cm. In addition to the head armor, the two-forked Miaojia fish also retains part of the trunk. Along the ventral edge of the trunk, there are obvious overlapping V-shaped exoskeleton bone units, called ventral fin folds. This is largely the same as the ventral fin folds found in the smart Tujia fish, indicating that this structure may be widely present in armored fish.

In addition, according to the results of phylogenetic analysis, the two-forked Miao fish and the flexible Tujia fish formed a monophyletic group - Tujia fish family.

It also provides key evidence for the question of how paired fins originated.

There is currently sufficient fossil and embryonic evidence to show that our human limbs evolved from the paired fins of fish, but where did the paired fins come from?

Two hypotheses dating back to the 19th century have speculated on the evolution of paired fins: the gill-arch hypothesis, which holds that the gill arches are the predecessors of paired fins, and the fin-fold hypothesis, which holds that paired fins evolved from a pair of ventral fin folds running along the body wall in early fish evolution.

Two hypotheses on the origin of paired fins: A, branchial arch hypothesis; B, fin fold hypothesis

(Image source: Reference 3)

As mentioned earlier, the double-forked Miaojia fish discovered this time and the dynamic Tujia fish reported in 2022 have roughly the same ventral fin folds. These two ancient fish provide the most critical and direct evidence for the fin fold theory.

It is worth noting that the ventral fin folds throughout the body cannot actually move freely because they are not supported by muscles and endoskeleton. So what is the function of this structure? How can it be explained?

The research team used computational fluid dynamics simulation experiments on the agile Tujia fish. The three-dimensional virtual models of the agile Tujia fish with and without fin folds were analyzed at different flow rates and different elevation angles to analyze the function of the ventral fin folds. The results showed that except for the case of 0° elevation angle, this pair of ventral fin folds can provide the Tujia fish with considerable lift, and can even offset its own gravity. Such a function is very important for bottom-dwelling swimmers such as armored fish.

The story of "from fish to man" is becoming more and more complete

Thanks to the unremitting efforts of scientific researchers, the story of "from fish to man" is becoming more and more complete. Many unresolved issues and gaps in the evolution of vertebrates have been revealed and innovated due to new fossil evidence. The discovery of the bifurcated Miao fish is another fossil evidence of the paired fin theory, and it also suggests that the ventral fin fold structure may be widely present in armored fish.

Many people still have many questions and confusions about "From Fish to Man", and some even say that "From Fish to Man" is too absurd and a lie.

According to current research, the evolution from fish to man began about 500 million years ago. It is not easy to correctly understand and recognize this evolutionary process, and we need to recognize and understand evolution on a time scale of millions or tens of millions of years.

Fortunately, this huge time span has left a magnificent chapter of vertebrate evolution in the geological history. The evolution from "fish to man" is not instantaneous, but a step-by-step process from fish to amphibians, reptiles, mammals and finally to humans. I believe we will find more evidence to prove this evolutionary process, and we welcome more friends to pay attention to paleontology and join us in discovering the life and its stories that really existed hundreds of millions of years ago.

References:

[1] CHEN Yang, LI Qiang, ZHOU Zheng-Da, SHAN Xian-Ren, ZHU You-An, WANG Qian, WEI Guang-Biao, ZHU Min. A new genus of galeaspids (jawless stem-Gnathostomata) from the early Silurian Chongqing Lagerstätte, China. Vertebrata Palasiatica, DOI: 10.19615/j.cnki.2096-9899.240820.

[2] Gai, Z., Li, Q., Ferron, HG, Keating, JN, Wang, J., Donoghue, PCJ, & Zhu, M. (2022). Galeaspid anatomy and the origin of vertebrate paired appendages. Nature, 609(7929), 959-963.

[3] Pieretti, J., Gehrke, AR, Schneider, I., Adachi, N., Nakamura, T., & Shubin, NH (2015). Organogenesis in deep time: A problem in genomics, development, and paleontology. Proceedings of the National Academy of Sciences, 112(16), 4871-4876.