Source: China News Network
BEIJING, Oct. 27 (Xinhua) -- After 30 years, the scientific research team of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (Institute of Paleovertebrates of the Chinese Academy of Sciences) combed and found a fossil of the largest true armor fish of 420 million years ago, the wandering fish, in the fossil material of armor fish collected from Qujing, Yunnan Province, the "ancient fish kingdom", and together with another newly discovered fossil, the latest research revealed that it has a gill hole (also known as an outer gill hole) like a living manta ray. It provides important information for discussing the life habits and evolution of armor fish.
In an interview, Gai Zhikun introduced the recovery of the roaming fish. Photo by Sun Zifa, a reporter from China News Service
This paper on the important progress of ancient fish fossil research has recently been published as a cover article in the professional academic journal Journal of Paleovertebrates.
Meng Xinyuan was interviewed to show a picture of the restoration of the roaming fish. Photo by Sun Zifa, a reporter from China News Service
Predation on the "arms race" has led to a growing head armor
Gai Zhikun, the corresponding author of the paper and an associate researcher at the Institute of Paleovertebrates of the Chinese Academy of Sciences, said that the roaming fish was officially published by researcher Zhu Min in the Journal of Paleontology in 1992, and the genus name "Fish" indicates that its head beetle is large and stupid, and the shape is cute, and the species name "Roaming" takes the meaning of its roaming and foraging in the water.
Roaming fish belong to the order True armorfish under the suborder armorfish in the system classification. The suborder Armorfish, an extinct group of "armor-clad" carapace fish, is one of the important taxa to understand the transition from jawless to jawed vertebrates, and is currently only found in the Silurian-Devonian strata of China and northern Vietnam. As one of the three main taxa of the armorfish suborder, the order Euthanasian has a high species diversity, with 16 genera currently named.
In an interview, Gai Zhikun showed two newly discovered fossils of roaming fish. Photo by Sun Zifa, a reporter from China News Service
Early true armor fish individuals are very small, is a class of small fish with a head armor of only about 2 centimeters, until 425 million years ago, Yunnan Qujing Guandi Formation appeared medium-sized true armor fish - long hole shield fish (head armor about 8 cm), and this discovery of 420 million years ago roaming fish head armor up to 16 cm, is currently known to be the largest true armor fish.
Gai Zhikun believes that the growing evolutionary trend of the head armor of the true armor fish may be related to the co-evolution of the invertebrate sea scorpion in the same period. With the emergence of predatory organs in the Silurian sea scorpion, its predatory activities have also intensified, and the predatory true armor fish are likely to have also launched a vigorous "arms race" between the predatory and the sea scorpion, which resists the predation of the sea scorpion by continuously increasing the head armor.
Restoration of the back (A) and ventral surface (B) of the roaming fish (painted by Yang Dinghua). Courtesy of Gai Zhikun
30 years later, combing found that "fish that slipped through the net"
In 1987, when Zhu Min was collecting dissertation data in eastern Yunnan, he found a fossil orthotype specimen of the wandering fish in the pale yellow argillary silt near the wall of the Southern City Gate in Qujing, which was a relatively complete head nail, revealing the key morphological characteristics of the back of the wandering head carapace, but the specimen had only one piece and did not preserve the morphological characteristics of the ventral surface of the head nail.
In 2018, Zhao Ridong led the team to harvest a full harvest when digging in the field of the Xishan Village Group of Qujing Noodle Shop Reservoir. Courtesy of Gai Zhikun
Gai Zhikun said that for 30 years, he has been trying to find new roaming fish fossils, but the fossil discovery site near the south gate of Qujing Ancient City has long become a cultural attraction in the city center with the process of urbanization, and it is impossible to carry out field excavations. Therefore, the research team carefully sorted out other undescribed armor fish specimens collected by Zhu Min that year, and accidentally found a "fish that slipped through the net" that roamed the fish.
Another key fossil of the wandering fish in this study was collected by the research team in the dark gray siltstone of the Xishan Village Formation near the Qujing Miandian Reservoir, which is a completely new location and new layer, which is a nearly complete head of the roaming fish.
Photo of a wandering fish fossil in the dark gray siltstone of the Xishan Village Formation of Qujing Miandian Reservoir. Courtesy of Gai Zhikun
The fossil material of the wandering fish is extremely rare and precious, and only three pieces have been found so far. Although the new material of roaming fish fossils found in this study comes from two different fossil points in Qujing, and the upper and lower layers are slightly different, they all have the typical characteristics of true armor fish.
It has a manta ray-like gill hole
Meng Xinyuan, the first author of the paper and a master's student at the Institute of Paleovertebrates of the Chinese Academy of Sciences, said that the new fossil material found in this study revealed for the first time the secret of the ventral surface of the wandering fish head nail: the head nail bends to the ventral surface to form an abdominal ring, and the central part of the abdominal ring surrounds a pear-shaped gill window, and the gill window is covered by a large exoskeleton abdominal nail, and between the abdominal nail and the abdominal ring, 6 pairs of linear and continuous gill holes are clearly preserved, and the gill holes are large and round, which is an important opening for the gill sac of the roaming fish to the outside world. The mouth of the wandering fish may be located at the end of the pear-shaped gill window, and its ventral carapace is large enough to form the inner edge of the gill hole.
Like manta rays (A), tuan fan rays (B), and black and white rays (C), the roaming fish has 6 gill holes on the ventral surface, which may be a convergence adapted to benthic life. Courtesy of Gai Zhikun
Gills, as the main gas exchange organ for fish, play an important role in feeding and breathing, she said. There is a big difference between the jawless and jawed gills, generally speaking, the gills without jaws are cystic, called gill sacs, while the gills with jaws are fissure-like, called gill fissures.
A Hanyang fish; B Aquake; C Yunnan armorfish; D multi-gill; E bone beetle; F star turtle; G heteromakingfish; H hagfish; I lamprey eel; K shark. Courtesy of Gai Zhikun
The gills of roaming fish have gill sacs, and their gill sacs are opened to the outside world through independent gill holes, which is different from the opening of the blind eel and the xenomorph through a common total gill hole, and the gill holes that are independent of each other are more likely to represent the primitive state of the vertebrate, and the total gill hole may be the result of convergent evolution.
Meng Xinyuan pointed out that the armor fish represented by the roaming fish are the same as the bone armor fish, their gills are located on the ventral surface of the head armor, and the gill holes in the abdomen of the armor fish are very similar to the living manta rays.
Photo of a wandering fish fossil in the pale yellow clay siltstone of the lower part of the Xishan Formation of The Southern City Gate of Qujing. Courtesy of Gai Zhikun
Manta rays, also known as manta rays, mostly lurk on the flat seabed with a sedimenty bottom, prey on small fish and crustaceans for a living, in order to adapt to this benthic life, its pectoral fin expands until it heals with the trunk, and gradually evolves into an extremely flat fan or round body, and it is similar to the flattened head of the armor fish. "This suggests that roaming fish may also roam the flat seabed with a sedimentary substrate, feeding on organic detritus on the seafloor in a quiet coastal environment." She said. (End)