International Journal of Aquaculture, 2025, Vol.15, No.5, 255-265 http://www.aquapublisher.com/index.php/ija 257 Silurian strata of the Nianzhong Changxing fish found in the Tarim Basin, a large area of wave mark structure was also preserved at the same strata, indicating that the area was at the shallow sea tide zone at that time. Figure 1 Photographs of Jiangxialepis rongi sp. nov. (Adopted from Liu et al., 2023) Image caption: (a, b) the internal (a) and external (b) moulds of a nearly complete headshield, holotype, IVPP V30966.1. (c) close-up of the median dorsal opening, box region of Figure 2(b), showing the sawtooth margin. (d, e) the internal (d) and external (e) moulds of an incomplete headshield, IVPP V30966.2. Abbreviations: c, cornual process; ic, inner cornual process; md.o, median dorsal opening; md.r, median dorsal ridge; md.s, median dorsal spine; orb, orbital opening; pi, pineal opening (Adopted from Liu et al., 2023) 3 The Origin of Jaws and the Differentiation of Bone Fish and Cartilage Fish 3.1 The effect of jaw evolution on predation strategies The emergence of jaw is a revolutionary innovation in the history of vertebrate evolution. The jawed species (Gnathostomata) is named after it, including all vertebrates with upper and lower jaw bones. For jawless fish, food acquisition is mainly through filter feeding or adsorption, and the recipe and body size are greatly limited. And the evolution of the jaw makes active predation possible. Judging from the fossil records, the earliest jaws appeared around the end of the Ordovician and the beginning of the Silurian period. At that time, the jaws may have a simple structure but were sufficient to perform the predation function. A series of evolutionary steps are believed to lead to the formation of the jaw: including the evolutionary transformation of the first two gill arches, namely, the original gill arch evolved into a jaw arch structure that supports and moves the upper and lower jaws. This anatomical innovation significantly enhanced the ecological competitiveness of early fish. Jaws can actively hunt invertebrates and even other fish, allowing them to rise rapidly in ecosystems (Deakin et al., 2022). Changes in predation strategies in turn promote diversification of fish morphology: predation of large prey requires strong bite force and corresponding head muscle attachment, and predation of live prey requires more flexible jaw movement, which puts selective pressure on skull and muscle structure (Jobbins et al., 2024). There is evidence that after the rise of jaw species, jawless fish were threatened by new predators and their populations dropped sharply, and may eventually become extinct at the end of the Devonian. The bite marks on the jawless fish deck in the fossils are a direct record of this predation interaction. 3.2 Early differences between cartilage and bone fish Shortly after the appearance of Silurian, the jaws differentiated into two main branches: the cartilage and the bones. These two major groups were clearly separated from the end of the Silurian to the early Devonian period, and each evolved in different directions. Cartilage fish include the living sharks, rays and squids, and their bones are mainly composed of cartilage and lack real hard bones. Bonefish include most of the fish we are familiar with (such as carp, bass and other radial fin fish) and meat-fin fish (such as lung fish and quadruped ancestors), which have mineralized bone bones. Paleozoic fossils show that both jawed fish experienced significant radiation during the Devonian period (Schnetz et al., 2024).
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