The Xitun vertebrate fauna has been added to the new Ding, the real armor fish huge Xitun fish

Ecological restoration map of huge Xitun fish (painted by Shi Aijuan)

Huge Xitun fish fossil photo (photo by Gai Zhikun), 3D reconstruction and head beetle restoration (feng Mingjuan painting)

Qujing, Yunnan is known as the “Kingdom of Ancient Fish” and “the birthplace of human beings 400 million years ago”. Since the 1980s, the Xitun Formation near Xitun Village, Xicheng Subdistrict, Qilin District, Qujing, has gradually become a “Mecca” sacred site for the study of ancient vertebrate fossils. However, to date, no definitive reports of the important taxa of armored fish, true armored fish, have been seen in the main production layers of the Xitun vertebrate population.

Recently, Sun Haoran, a master’s student jointly trained by the Institute of Geology and Geophysics of the Chinese Academy of Sciences and the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, under the guidance of Zhao Wenjin, a researcher in the early vertebrate research team, published the latest research results on the true armor fish in the Xitun vertebrate group online in the Journal of Paleontology.

The study reported a new genus of true armor fish found by the research team in the Lower Devonian Xitun Formation in the Qujing region of eastern Yunnan: Xitunaspis magnus, which was the first discovery of true armor fish in the Xitun Group in qujing area, and the related research not only filled the geological gap between the Xishan Cun Formation and xujiachong formation of true armor fish in qujing area in the early Devonian Xishan Village Formation and enriched the fish diversity of the Xitun vertebrate fauna. It also helped scientists further clarify the phylogenetic relationships between various groups of true armor fish, enriched the understanding of the radiation evolution of the armorfish subclass in the early Devonian, and provided key fossil evidence for exploring the evolution of the sensory tube system (also known as the lateral line system) in armor fish.

The Xitun Formation of the Lower Devonian System under Qujing was the first to discover conclusive true armor fish

The original definition of the famous Xitun vertebrate fauna in eastern Yunnan was a pre-Devonian fish fauna characterized by the radiation evolution of the early types of armorfish suborder multigills and South China fish, shield fishes, Yunnan fishes and teleost fishes, and in recent years, with the continuous discovery of new fossil materials and the deepening of new understanding of existing fossil production layers, the armored fish in this fauna also include some true armor fish produced in the Xishancun Formation under the Xitun Formation of the main fossil strata and the Xujiachong Formation above, such as the Zhang’s True ArmorFish The giant Yunnan armor fish, roaming fish and Liu’s sickle fish are produced in the Xishan Village Formation, and the Jade Sea Wing Hornfish and Xu Jia Chong True Armor Fish are produced in the Xu Jia Chong Group, but no exact true armor fish have been reported in the Xitun Group so far.

The new discovery of the giant Xitun fish in the Lower Devonian Xitun Formation in the Qujing area represents the first conclusive true armor fish found in the fossil production layer of the main body of the Xitun vertebrate fauna. The genus name of the Xitun fish indicates that it was found in the Xitun Formation near the village of Xitun in Qujing, and the species name is derived from its larger head armor.

The fish has the typical characteristics of the true armorfish family, including a longitudinal fissured middle dorsal hole, no inner horn, middle dorsal canal development, and only 3 pairs of lateral transverse tubes protruding from the lateral dorsal tube, which undoubtedly belongs to the true armor fish family, but its large and thick head armor and more primitive sensory tube system are significantly different from other true armor fish.

Fossil evidence revealing the phylogenetic relationships of various groups of eumorphal orders

Armorfish is a unique and indigenous jawless fish in East Asia, except for the basal taxons (including the Xiushui fish family, Hanyang fish family, and Dayong fish family), which are mainly divided into three monophyletic groups – true armor fish, polygill fish and South China fish order, the latter two are often classified into the multigill superorder because of the large number of gill sacs.

Among them, the euthenomorphal order as one of the three main branches of the armorfish suborder, its internal groups are generally considered to belong to 4 family groups: Akebono, Chinese armorfish family, trinacid fish and true armor fish family, and 1 undefined family of taxa (including Yunnan armorfish, bayfish), but there are still great uncertainties in some evolutionary locations, such as the monophyleticity of the Akebonosidae family, the attribution of Yunnan armorfish and the phylogenetic relationship between true armorfish and other armored fish, which have not been fully resolved. The urgent need to combine the in-depth research work on some transitional types of fossil new materials to answer, the discovery of Xitun fish with obvious mosaic characteristics provides some key fossil evidence to solve the above problems.

Based on the phylogenetic relationship analysis of armor fish, combined with the new understanding of some characteristics of Xitun fish, the research team constructed a new matrix including 66 features, of which the new three new features were the presence or absence of the central tube, the thickness of the head nail, and the presence or absence of remnants of the middle transverse tube. The latest phylogenetic analysis results show that xitun fish genus and shieldfish genus and true armorfish genus together form a monophyletic taxon, the true armorfish family, and the relationship between the three xitun fish and shield fish is more closely related; Yunnan armorfish genus and mandarin fish genus should be a secondary taxon between the Chinese armorfish family, the trinacle family and the true armorfish family; the true armor fish family is still the most diverse branch of the true armor fish family, now including 3 genera and 6 species. The two species of shieldfish from the Silurians of Yunnan and Chongqing represent the earliest true armor fish that recovered after the extinction of the Akebonos and the Chinese armorfish family at the end of the Preludesic family, and the survival time of this family can be extended from the Silurian Rhodorian (~427 million years ago) to the Early Devonian Prague period (~407 million years ago).

To explore key evidence for the evolution of the sensory tube system of the armorfish suborder

The sensory tube system of armored fish usually appears as a grid of two pairs of longitudinal tubes and different numbers of transverse tubes, and has an evolutionary trend from complex to simple, but the evolution from the original mesh-like sensory tube system to the later simplified sensory tube system is not very clear. The discovery of the sensory tube system in Xitun fish has primitive and derived mosaic features provides new key evidence for the evolution of the sensory tube system of armor fish.

Combined with the results of the latest armor fish morphology and phylogenetic analysis, the research team made a systematic combing and comprehensive analysis of the sensory tube system of the armor fish subclass, and came up with the following understandings: In the basal dry armor fish such as Hanyang fish, Changxing fish and Dayong fish, the transverse tube is very developed and the longitudinal tube is reduced, which should be the original feature of the armor fish sensory tube system, and the scientists named it the Hanyang fish-type sensory tube system.

Compared with basal armor fish, the longitudinal sensory tubes and transverse sensory tubes on the dorsal side of the head of the order Euthanoaceae are well developed and usually show a tendency to decrease transverse tubes and increase longitudinal tubes; the U-shaped dorsal tubes are very developed in the Chinese armorfish family, the true armor fish family and the trinacle family, and the genus Ichthyosaurus, which is often considered to be a derivation of the order true armorfish; more than one mid-transverse tube is often considered to be the original feature found in the basal armor fish and the Chinese armor fish. The sensory tube system of the Akebonoceae family (without the middle back tube) and the Chinese armor fish family (with three pairs of middle transverse tubes) should represent an ancestral sign of the true armor fish; the sensory tube system of the true armor fish family and the trinple family has a developed middle dorsal tube and dorsal contact tube, that is, a typical true armor type sensory tube system, which may represent a detachment of the true armor fish order; Xitun fish can see 1 pair of middle transverse tube remnants, which may be the result of the incomplete development and degeneration of the first pair of middle transverse tubes. The middle transverse and central tubes that are not completely degenerated occur not only in basal armor fish, but also in multigill and South China fish with multi-gill sensory tube systems.

According to this, it can be considered that the evolution of the armorfish suborder sensory tube system generally has a tendency to simplify, which is manifested in the reduction of transverse tubes and the slight increase in longitudinal tubes, and the true armor fish type and the multi-gill type sensory tube system are all evolved from the Hanyang fish type sensory tube system, and then develop independently.

The research has been funded by the Strategic Pilot Science and Technology Special Project (Category B) of the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the Key Research Program of Frontier Science of the Chinese Academy of Sciences. (Source: China Science Daily Cui Xueqin)

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