Including humans, 99.8% of the living vertebrates on earth have jaws (upper jaw and lower jaw), collectively known as jawed vertebrates or jaws. The emergence and rise of jaws is one of the most critical leaps in the evolution of vertebrates from “fish to man”. Many important organs and body configurations of humans can be traced back to the evolution of jaws at the beginning. But exactly when, where, and how did this jump happen? These questions require paleontological evidence to answer.
In the early evolution of jaws, fish fossils were very scarce for a long time, and there was a gap that lasted at least 30 million years. Vertebrate paleontology giant Alfred Rommel once called it “a major and persistent gap in the history of paleontology“.
In the latest issue of “Nature”, the team of Academician Zhu Min from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, in cooperation with several research institutions, filled this gap with 4 papers. The research team found the “Chongqing Special Buried Fossil Repository” and the “Guizhou Shiqian Fossil Repository” in the strata of about 440 million years ago in the early Silurian period in Chongqing, Guizhou and other places, respectively, which filled the global early Silurian period. The gaps in the jawed fossil record provide solid evidence for answering a series of important scientific questions in the initial stage of the “from fish to man” exploration, rewriting all aspects of the early evolutionary history of jawed vertebrates.
▲The cover of the latest issue of “Nature” shows the three-dimensional artistic reconstruction of the newly discovered 5 new genera of Silurian ancient fish (Cartography: Heming Zhang)
Two of these papers reported findings from the “Chongqing Uniquely Buried Fossil Library“. As the only special buried fossil bank in the world that preserves the complete jaw fossils of the early Silurian period, this 436-million-year-old biota can be called the “dawn of fish”. This is another world-class fossil bank discovered in China after the Chengjiang Biota and the Jehol Biota. It provides a large amount of key evidence for exploring important nodes in the evolution of the tree of life, and transfers the fossil record of complete jaws to the forward by 11 million years.
Although the fish fossils in the Chongqing Biota are well preserved, the fish are usually very small, with a total length of 3-4cm. In these two papers, the research team used a variety of methods to conduct detailed research and repeated exploration, among which three fish fossils are of particular importance for understanding the early evolution of jaws.
▲Fossils from the Chongqing Special Buried Fossil Bank (Image source: original paper[1])
A jawed fish reported in the paper, Xiushanosteus mirabilis belongs to placodes. The placodes are between the jawless armored fish and the more advanced bony fish and cartilaginous fish, and have very important evolutionary significance for exploring the origin of jaws and jaws. Compared with the existing placoderm fossils, xiushan fish is much closer to the origin of the jaws in age, and combines the characteristics of several “plagioclase” categories. The features it shares with other later placodes are likely primitive features of jaws.
▲The Ecological Restoration Map of Miracle Xiushan Fish(Photo credit: Heming Zhang)
Another discovery was a cartilaginous fish called Shenacanthus vermiformis. The early fossil records of cartilaginous fishes are very scarce, and the well-preserved Shen’s stickleback has achieved a breakthrough in the discovery of zero large fossils of cartilaginous fishes from the Silurian-Devonian period in China. Not only that, this fossil is also the earliest known large fossil of cartilaginous fish in the world. Surprisingly, the stickleback of Shen’s has primitive features that are only found in placodes. This discovery means that the seemingly flexible and flexible sharks, rays and other cartilaginous fish actually have direct ancestors who “wear armor and armor”. .
▲Cricket stickleback (Image credit: Heming Zhang)
The research team also discovered the world’s first fossil of the armored fish Smart Tujiafish (Tujiaaspis vividus) with a completely preserved body. Through comprehensive research, the anatomical structure of the body behind the head of the armored fish was revealed for the first time. A pair of ventral fin folds running through the whole body were found in the abdomen of the flexible Tujia fish. This discovery provides crucial fossil evidence for the “fin fold theory” of the origin of paired appendages in vertebrates.
▲Smart Tujia Fish Ecological Restoration Map(Photo credit: Zheng Qiuyang)
Two other papers from the same period presented discoveries from the “Guizhou Shiqian Fossil Library” about 439 million years ago. This fossil bank contains a large number of well-preserved jawed microfossils.
Among them, the tooth spiral of the Qianodus duplicis is the oldest jaw-like tooth to date, the earliest fossil evidence of the tooth is before the It has been pushed back by 14 million years, and indirectly supports the classification opinion that the scattered scale and spine fossils preserved in the Ordovician and early Silurian belong to the jaws.
▲Double-column toothfish(Photo credit: Heming Zhang)
In addition, the discovery of spiny spines of Fanjingshania renovata indicates that as early as the early Silurian, primitive cartilaginous fish had evolved typical It has the histological features of teleost fish. The above-mentioned progress also brings to an end the long-standing debate over the taxonomic location of Ordovician and Silurian fish scales and spines.
▲Xinsha Fanjingshan Fish(Photo credit: Heming Zhang)
In general, the discovery of “Chongqing Special Buried Fossil Bank” and “Guizhou Shiqian Fossil Bank” is the first time in the history of paleontology that the large-scale display of Silurian fish, especially jaws It reveals the process of the rise of early jaws: no later than 440 million years ago, the major groups of jaws were already thriving in southern China; by the late Silurian, more diverse and larger jawed species Emerged and began to spread across the globe, starting the process of fish landings and eventual evolution into humans.