Recently, the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences published the latest research results on the armoured fish Jade Sea Winged Hornfish, which reported the new material of the Yuhai winged hornfish discovered by the Xujiachong Formation in the Early Devonian Bragg period of Qujing, Yunnan (about 410 million years ago), revealing for the first time the position relationship between the esophagus and the dorsal aorta of armoured fish, and solving the mystery of the asymmetry of the dorsal aorta and the right drift of the esophagus in lampreys and bone armored fishes [1].
The "vampire" lamprey in the water & the "surprised" bone turtle with a "surprised" face
Before revealing the mystery of the lamprey, the dorsal aorta and esophagus drifting to the right in bone armor, let's get to know our two protagonists.
The lamprey is a jawless vertebrate that lives in water and is semi-parasitic. It is eel-shaped and has no even fins, only dorsal and caudal fins. On the ventral surface of the front of the head there is a circular funnel-shaped inhaler that is used to adsorb to other fish. Inside the funnel is the mouth, on which special "file tongues" with teeth can protrude from the bottom of the mouth. While the lamprey sucks the catch, the filing tongue acts as a piston to draw the blood of the catch.
Figure 1 "Water vampire" lamprey A. Sea lamprey (Petromyzon marinus), side-looking (Credit: D. Pulera painting); b. Sea lampreys in the Great Lakes region of the United States (Photo: Gai Zhikun in Chicago, USA)
The lamprey's nostrils grow above the top of the head, and unlike other animals, the lamprey has only one nostril, and the single nostril is a very interesting phenomenon of roundmouth in living ridge propulsions.
Behind the nostrils is the pineal eye, which has a photosensitive effect and can be called the "third eye" of the lamprey. The eyes grow on either side of the head, and from the eyes back is a row of 7 small round gill holes, hence the name lamprey.
Although the nervous system of the lamprey is still relatively primitive, it feels very sensitive, as long as a fish swims within 30 meters around it, it will shoot straight at the other party like a sharp arrow, and instantly suck the fish body with a suction cup. Usually, it only takes 2 hours for the sucked fish to die because the blood is sucked dry, at which point the lamprey will let go and "grow away".
In the sea, even huge cetaceans are often "patronized" by lampreys. The cetaceans have nothing to do with these "hateful" guys but to suck their blood and feast. In this way, the lamprey is really a veritable water "vampire"!
The second protagonist of the story, the bone armored fish, is the most successful group of armored fish in the Silurian and Devonian periods, with about 200 species, mainly living in North America, Europe, Siberia and Central Asia.
Most bone beetle fish appear to have paired pectoral fins and a flexible tail fin (upper crooked tail). These characteristics suggest that bone beetles may be the most flexible and athletic group of armored fish.
Figure 2 A variety of bone armored fish (Image source: Yang Dinghua)
The entire back of the head of the bone armored fish is wrapped in a complete semicircular piece of bony armor, and the ventral surface is the location of the mouth and outer gill holes, surrounded by tiny pieces of bone or scales. There is a pair of orbital foramen (the position of the eye) at the front of the cephalic carapace, a single nasal pituitary foramen in the middle, and a small pinecone foramen at the back. The "mouth" that seems to be opened by surprise is actually their "nostrils".
The "nostrils" (nasal pituitary nostrils) of bone beetle fish are strikingly similar to those of living lampreys, so scientists initially thought that the two taxa might be directly related. However, the results now suggest that their similarity may be the result of parallel evolution.
Yo, the generation is wrong
A peculiar phenomenon occurs in jawless lamprey larvae and some bone armor fish: their dorsal aorta is unpaired and asymmetrical. That is, their aorta does not extend exactly along the midline of the body and will have a noticeable rightward drift behind the gill area.
The blood circulation route of fish is single-cycle. Single circulation refers to the hypoxic blood expelled from the ventricles, exchanged gas through the gills, merged into the dorsal aorta on the back of the body and under the spine, and then transported the polyoxygenated blood to various organs and tissues of the body. Oxygen-deprived blood that leaves organ tissue eventually returns to the venous sinuses of the heart before a new round of blood circulation begins.
The dorsal aorta is the first functional intraembryonic vessel to appear in vertebrates, appearing as two separate bilateral vessels in the trunk and undergoing a lateral translocation from the lateral to the midline, eventually fusing into a large blood vessel in the midline. Therefore, the evolution of the dorsal aorta can be regarded as a very ideal model to help us understand the mechanisms that control the establishment and remodeling of larger blood vessels in the body.
At present, the asymmetry of the dorsal aorta and esophagus drifting to the right is only found in the larvae and bone nail fish of lampreys, and was once one of the evidence supporting the close relationship between living lampreys and bone nail fish, and scientists speculate that lamprey is a descendant of the degeneration of the exoskeleton of bone armor fish.
With the development of cladic systematics, there is increasing evidence that the missing exoskeleton of lampreys may represent primitive characteristics of vertebrates, that is, lampreys are a more primitive jawless fish; Bone armor fish, on the other hand, have more progressive features of jaws, such as paired pectoral fins, bone cells, and upper crooked tails, and are the closest sister group to jaws.
Therefore, the asymmetry of the dorsal aorta drifting to the right in lampreys and bone armored fish may be the result of parallel evolution! In this case, what exactly is the situation of the Chinese armored fish, which is located between the lamprey and the bone armored fish.
Fig. 3 Distribution of esophagus and dorsal aorta in lamprey (a), bone armored fish (b, c), armoured fish (d, e) and living cartilaginous fish (f, g) (source: Meng Xinyuan)
The "star" of the continental armoured fish
In the past, due to the limitations of preservation techniques and fossil scarcity, scientists knew little about the ventral surface and internal anatomical features of armored fish. The new specimen of the jade sea winged hornfish clearly preserves the key morphological information of the ventral surface, and is the first key fossil to clearly reveal the position relationship between the esophagus and the dorsal aorta.
In the early 90s of last century, when Academician Zhu Min, a famous paleoichthyologist in the mainland, conducted a field geological survey in the Qujing area of Yunnan, he first discovered the fossils of the jade sea winged hornfish, and the fossil output layer was the Xujiachong Formation of the Prague period of the Early Devonian.
Figure 4 Fossil photo of jade sea winged hornfish (Image source: Photo by Gai Zhikun)
In 1992, Academician Zhu Min officially named the fossil "Jade Sea Winged Horned Fish" and published his results in the Journal of Vertebrate Paleozoology. The genus name indicates that it has a pair of wing-like horns that extend laterally, and the species name was given to Mr. Liu Yuhai, the first person to study armoured fish.
The jade sea winged hornfish belongs to the armoured fish suborder Euarmoured Fishes taxonomically and is a special group of Euarmoured Fishes with snout processes and lateral extension horns. Armoured fish is a genus endemic to East Asia, currently found only in the Silurian and Devonian strata of southern China, the northern edge of the Tarim Basin and northern Vietnam, and is a very indigenous taxa.
Figure 5 Ecological restoration of jade sea winged hornfish (Image source: Yang Dinghua)
Figure 6 Restoration of the jade sea winged hornfish (Image source: Guo Xiaocong)
Although the research history of armoured fish is only a short period of more than 50 years, under the efforts of Liu Yuhai, Pan Jiang, Wang Nianzhong, Wang Junqing and other paleoichthyologists, a taxonomic unit at the suborder level has been established, and more than 90 species have been found, and together with bone nail fish and heterocarage, it constitutes the three major groups with the richest species and the largest population differentiation in jawless species.
The "behind-the-scenes driver" of the aorta's drift to the right - the asymmetrical Cuvier duct
In the new specimen of the jade sea winged hornfish, two openings are retained on the midline of the posterior wall of the gills, and the opening near the ventral side is much larger than the opening near the dorsal side, so the former is more likely to be a passage to the esophagus, while the latter may be a passage to the dorsal aorta.
Figure 7 Fossil photo of jade sea winged horned fish (Image source: Photo by Gai Zhikun)
Previously, an opening was also preserved on the posterior wall of the gill of the Jiangxi Silurian true armoured fish Xikeng schizochromia, which was once thought to be a channel for the neural tube (Figure 7e). However, the three-dimensional reconstruction image of the cerebral CT of the cereus shows that the neural tube should be located dorsally on the top of the oral gill cavity, while the foramen is close to the ventral side of the oral gill cavity, so the hole cannot be a channel of the neural tube, but rather of the esophagus.
Comparative anatomical studies of early fish showed that the dorsal aorta of armoured fish was the same as that of jaws, also paired, and extended along the midline of the body, without significant rightward drift. Since the same condition is present in the cephalox Wenchang fish and jawless hagfish, this may represent the ancestral state of vertebrates.
This proves that the rightward drift of the aorta of lampreys and bone nail fish may be an evolutionary convergence caused by the asymmetry of the Cuvier duct. Cuvier duct (Cuvier duct) is an anatomical structure named after Georges Cuvier (1769-1832), a famous French zoologist and founder of comparative anatomy and paleontology in the 18~19th century, which refers to the blood of the left and right front main veins and the two posterior main veins all converge to a pair of transverse total main veins (Figure 8), including us humans, the left and right sides of the Cuvier duct are not symmetrical, and the right side is much larger than the left (Figure 8).
In the body structure of the lamprey, the left and right Cuvier ducts are asymmetrical, and the right side is thicker than the left. Since the esophagus of the lamprey is connected to the dorsal aorta by connective tissue, in order to prevent the esophagus from collapsing on the heart that is not protected by the pericardial cavity, the dorsal aorta and esophagus will drift to the right (Figure 8).
In bone armor fish, the heart already has the protection of the pericardial cavity, why does the dorsal aorta and esophagus drift to the right?
This may be due to the narrow space caused by the flattening of the head armor dorsoventrally of bone armor fish, and the compression of the thick Cuvier duct on the right, which ultimately leads to the curvature of the dorsal aorta and esophagus drifting to the right (Figure 8).
Figure 8 The left and right Cuvier tubes, including humans, are asymmetrical, and the right side is much larger than the left (Source: Wikipedia, the free encyclopedia)
In addition, the dorsal aorta and esophagus of bone armored fish share a semi-closed cartilage groove, which may represent an intermediate state between lampreys (without cartilage wrapping) and armored fish and jawed (cartilage completely wrapped) (Figure 9).
Figure 9 Evolution of some key characteristics in chordates (Image source: Meng Xinyuan)
epilogue
The re-study of the jade sea winged hornfish not only fills the gap of anatomical information of armored fish, helps to understand the internal anatomy of armored fish, but also has important significance for exploring the phylogenetic relationship between lamprey, armored fish, bone armored fish and jawed vertebrates, and understanding the origin and evolution of key characteristics of jawed fish. And this research is also a moving achievement achieved by generations of paleontologists for 30 years.
Note: The relevant results of this paper were completed by Meng Xinyuan, a master's graduate, under the guidance of researcher Gai Zhikun, and have been published online in the international academic journal "The Anatomical Record". Anatomical Records, an official publication of the American Anatomical Society, founded in 1888, focuses on the latest advances in morphology and anatomical research, including molecular, cellular, systematic, and evolutionary biology.
Bibliography:
[1] Meng, X.-Y., Zhu, M., Li, Q., & Gai, Z K* 2022. New data on the cranial anatomy of Pterogonaspis (Tridensaspidae, Galeaspida) from the lower Devonian of Yunnan, China and its evolutionary implications. The Anatomical Record, 1–14.
[2] Gai Zhikun and Zhu Min, The Evolutionary History of Jawless Animals and the Chinese Fossil Record, 2017
Produced by: Popular Science China
Author: Gai Zhikun Meng Xinyuan
Production: Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences
Executive Producer: China Science Expo