Ecological restoration of Hongyu, accompanied by Ningxia fish and paddle fish in the devonian ancient lake (Brian Choo painting)
Photographs and sketches of Hongyu orthographic specimens (courtesy of Zhu Min)
Hongyu skeleton restoration diagram (courtesy of Zhu Min)
Classification location of Hongyu (courtesy of Zhu Min)
According to the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences: Nature Ecology & Evolution, a sub-journal of Nature Ecology & Evolution, the new progress made by Zhu Min's team in the field of fish landing research at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences was published online on September 4. They found a 370 million-year-old ancient fish in Qingtongxia, Ningxia, the Zhou's Hongyu, which is 1.5 meters long and has the characteristics of a eratope, a salamander and tetrapods, revealing that the characteristics of fish adapting to land life have appeared many times, and the fish landing process is far more complicated than previously understood.
The common ancestor of all terrestrial vertebrates (tetrapods), including humans, can be traced back to fish that landed in the Devonian period more than 300 million years ago. Fossil discoveries and in-depth studies over the past 100 years have painted a general picture of how fish landed. It is now known that tetrapods evolved from a branch of flesh-finned fish. Tetrapods and this flesh-finned fish are collectively referred to as tetrapod clades or tetrapods. In addition to terrestrial vertebrates, the tetrapod clade also includes a number of fish-forming members living in the water, which are known as tetrapod stems, the oldest representative of which is the Ostrichosa and Ken's fish found in the early Devonian strata of China. Quadruped stems flourished in the Devonian and Carboniferous periods, firmly occupying the niches of large predators in shallow waters, and evolving in the late Devonians, such as the Tiktaalik, morphologically very close to that of the earliest tetrapods. Members of the heterogeneous tetrapod stem group between Ken's fish and the hope salamander can be grouped into two branches: root-toothed fish and bony scale fish. Among them, the three-column fin fish in the bony scale fish, such as the famous true palm fin fish, are similar in many ways to the hope salamander, so the hope salamander and tetrapods are thought to have evolved from one of the three-column fin fish, while the root-toothed fish are in a more primitive position in the quadruped stem. Over the past 20 years, this evolutionary structure has remained largely stable despite the continuous discovery of new fossils.
The Devonian ancient fish discovered by Zhu Min and others in Qingtongxia, Ningxia, broke this structure and smeared a cloud of doubt on the picture of the fish landing. The section of Shixia Ditch at the junction of Qingtongxia and Zhongning in Ningxia reveals the classic Devonian fish-bearing strata in northern China, and the earliest tetrapod fossil in China, pan's Chinese salamander, has been found. In the summer of 2002, Zhu Min, Zhao Wenjin, Jia Liantao and three others accompanied Professor Per Ahlberg of Uppsala University in Sweden to investigate the section of ShixiaGou and accidentally found a row of fossil sections on the steep wall of the quarry. With the help of quarrymen, they removed the thick sandstone about a meter square from the steep wall and transported it back to Beijing. After nearly a year of meticulous repairs, they were pleasantly surprised to find that this was a well-preserved fossil of a large finned fish, with more than 20 sections preserved in the vertebrae alone. Unfortunately, the tip of the head was broken before it was discovered, and may have long since become part of the roadbed. After restoration, the specimen represents an individual body length of up to 1.5 meters. They named this ancient fish Hongyu chowi in honor of academician Zhou Mingzhen, an internationally renowned vertebrate paleozoologist and winner of the "Simpson Medal".
In the Late Devonian, there were many large finfish representing different branches of the stems of tetrapods in the waters of Europe, North America, and Australia, and true tetrapods and their close relatives, the salamander, has also emerged. After a detailed study, Zhu Min and others found that the Hongyu had a collection of characteristics of many taxa. The fish shows striking resemblance to hope salamanders and primitive tetrapods in the shoulder straps and the joint structure of the shoulder blade and forelimb bones. The occipital joint has appeared between the first vertebrae and the skull of the Hongyu, and the exoskeleton of the shoulder strap is disconnected from the skull, which is also different from other fish living in the water, and consistent with the land vertebrates, which shows that the Hongyu has been very adapted to shallow water shore life, not far from the real landing.
On the other hand, the fish has many characteristics of root-toothed fish, such as the contact of the temporal bone with the external temporal bone, the strong shoulder strap and a high collarbone spine, thin round scales, and the high front maxillary face leaf, which are enough to be included in this taxon. Roottooth fish are a mysterious group of large predatory fish, the largest roottooth fish body length of up to 7 meters, known as the "Carboniferous water monster". As early as two hundred years ago, huge teeth belonging to the root tooth fish were found in the Carboniferous coal seam near Edinburgh, Scotland, and were originally used as crocodile teeth. Some famous early paleontologists, such as Richard Owen, who coined the term "dinosaur," Huxley, the "fighting dog of Darwin," and Agassi, the "father of paleoichthys," have studied root-toothed fish and fought fiercely about it. In the last century, due to the similarity of the oleracian bones of the root-toothed fish to the tetrapods, the academic community once believed that the root-toothed fish was very close to the node of the fish landing, but more than 20 years ago, more and more complete fossils of the root-toothed fish were found in the Australian Devonian strata, shaking the "inbreed" status of the root-toothed fish. In the ensuing evolutionary framework of quadruped stem groups, root-toothed fish, although still in the tetrapod clade, have moved down to a more primitive position and are far away from tetrapods.
The mosaic features presented by Hongyu pose new challenges to this architecture. Are root-toothed fish or bony scale fish "close relatives" of tetrapods? The fish landing process regenerates doubts. Zhu Min et al. carried out meticulous and in-depth feature analysis of quadruped stem groups, and established a feature matrix including 33 quadruped stem group members, 5 external taxa, and 169 features. Based on Bayesian analysis and minimalist analysis, two different quadruped stem group phylogenetic relationship frameworks were obtained, the most important difference of which was the position of root-toothed fish. The results obtained by Bayesian analysis still support the original position of root-toothed fish in tetrapod stems. Under this framework, the mosaic characteristics of the fish reflect a large amount of convergence between the root-toothed fish and the hope salamander-tetrapod, and the characteristics of the fish adapting to land life have appeared many times. The results of the most minimalist analysis revived the central position of the root-toothed fish in the fish landing process, and the characteristics of the fish indicate the close relationship between these three taxa, and the root-toothed fish are the closest fish to the hope salamander and tetrapods.
Based on the available fossil data, it is difficult to choose between the two theories. In either case, however, the discovery of the Hongyu proves that the morphological differentiation and biogeographic divisional level of the tetrapod clades have been greatly underestimated in the past, and that the Devonian root-toothed and bony-scaled fish have undergone significant parallel evolution towards adaptation to landfall life, and these large shallow-water predators belonging to different tax groups have tried to expand to the land in the past, but only one has been successful.
This achievement was funded by the National Natural Science Foundation of China and the Frontier Science Key Research Project of the Chinese Academy of Sciences.