International Journal of Aquaculture, 2025, Vol.15, No.5, 255-265 http://www.aquapublisher.com/index.php/ija 256 finally propose a prospect and summary of future research. Through this framework, we hope to fully present the complex course of fish evolutionary paths and how fossils and molecular biology can proof-develop the understanding in this field. 2 Evidence of Early Origin of Fish and Paleozoic Fossils 2.1 The discovery of the earliest jawless fish fossils (such as armored fish) The origin of fish can be traced back to the Early Cambrian period 500 million years ago. Primitive vertebrates that appeared at that time usually had no jaws and no paired fins, and were collectively called jawless. Typical early jawless fish include some ancient fishes that are covered with armor, such as members of the subclass of the Armor. Armors are also called "armor fish" because their heads and front of the torso are covered with bone decks. The earliest fossils of armor discovered so far are mainly unearthed from the late Ordovician to the early Silurian strata, indicating that jawless species were quite prosperous about 460 to 440 million years ago (Romano et al., 2018; Randle et al., 2022). For example, rich fossils of armor fish were found in the formations of the Silurgian Lando Verestrachi period (about 438 million years ago) in the Tarim Basin of China, including the Zhang family's Western Regions, Jiangxia fish and the recently reported new species of Nianzhong Changxing fish. These discoveries pushed the jaw fossil record forward by about 11 million years. Jawless fish have original morphological characteristics: their bodies are slender or flat, and they have no upper and lower jaw structures. They mostly make a living by filtering or sucking microorganisms. Although the "Haikou fish" and "Kunming fish" found in the Chengjiang fossil reservoir in the Early Cambrian period in Yunnan, China are not typical armored fish, they are regarded as the most primitive vertebrates. 2.2 Fish diversity in silurian and devonian The Silurian was a critical period in the history of fish evolution and was called "the dawn of fish". In the mid-Silurian period, the earliest records of jawed fish appeared. A variety of jawed fish that have been fully preserved in the early Silurian fossil repository in Chongqing, China, including representatives of primitive squid and cartilage fish. These findings suggest that in the early Silurian period, jaw species had begun to diversify and fill niches. By the end of the Silurian period and the subsequent Devonian period, fish experienced unprecedented radiation evolution, and the Devonian period was therefore often called the "Age of Fish". A large number of different types of jawed fish appeared in the early Devonian period: including the extinct sharks and rays of the charcoalis, and the two major branches of the radial fin and fermented fin (Lu et al., 2017). Fossil records show that fish have become the main vertebrate group in the Devonian marine and river ecosystems, occupying various ecological niches ranging from filter feeding, small claspers to top predators. In the marine strata of the mid-Devonian period, both jaw fossils of large predatory shield skinfish were found, and small intact skeletons of early radial finfish were found, representing two levels of predator and prey (Choo et al., 2017). With the rise of jaws, jawless fish gradually declined and became extinct during the Devonian period. 2.3 Correlation between fossil evidence and paleoecological environment Paleozoic fish fossils not only record the evolution of anatomical structures, but also reflect the changes in the paleoecological environment at that time. By analyzing the rock sedimentary characteristics and companion biota of fish-containing fossil formations, the environment and ecological habits of early fish can be inferred. The Silurian "lower red layer" strata in southern China is characterized by red sandstone and is interpreted as the sedimentation of the shallow sea or delta environment in the nearshore. The large number of armor fish fossils found in these lower red layers showed that the shallow sea area of the South China plate at that time was a prosperous area of armor fish. Fish fossils from similar eras also appeared in the red layer of the Tata Eltag Formation in the Tarim Basin in Xinjiang, including armored fish and spinyfish (Figure 1). This similar cross-regional biological combination means that in the early Silurian period, there may be biological exchanges between the South China plate and the Tarim plate, which are not geographically far apart (Liu et al., 2023; Li et al., 2024). Fossil evidence supports the “Tarim-South China Joint Plate” hypothesis that the two continents may be connected or adjacent during the Silurian period, thus sharing similar fish communities. In addition to geographical comparison, fish fossils can also reflect the paleoenvironmental conditions. For example, in the
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