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The secrets of tooth loss in Mesozoic birds

In most mammals, including humans, it is common to develop two sets of deciduous and permanent teeth during a lifetime. This is called didentition. In contrast, multiple tooth replacements are common in most amphibians and reptiles.
In the colorful nature, birds are one of the most abundant vertebrates. With feathers, wings on their forelimbs and light, hollow bones, they are equipped with special equipment to fly freely in the sky. The beak of a bird is covered with a horny beak, making it a good tool for food and “grooming”.
Would you believe a paleontologist who told you that birds in the past not only had teeth in their mouths, but also periodically replaced them, like reptiles?
New evidence from fossils
A group of paleornithologists from the Beijing Museum of Natural History and the Los Angeles Museum of Natural History recently reported in the international academic journal Cretaceous Research a fossil bird from the Bohai Sea found in the Lower Cretaceous Jiufotang Formation (120 million years ago) in Liaoning Province, China. The dove-sized fossil of the ancient bird not only provides abundant anatomical information about the skull, but the researchers also found several very rare replacement teeth inside the teeth of the upper and lower jaws, providing new fossil evidence of anti-avian tooth replacement patterns.
In recent decades, paleontologists have discovered a large number of beautifully preserved Mesozoic bird fossils in the Jehe Biota in northeast China, including the primitive backbone birds represented by Jeheornis and Confuciusornis, as well as the antibirds and modern birds whose skeleton morphology is more similar to modern birds. They provide important paleontological information for studying the origin and evolution of birds. Bohai birds are the most abundant anti-avian clade in jehe biota. So far, six species of bohai birds have been studied and named, including Guo’s Bohai bird, Meng’s shenqi bird, Ji’s toothed bird, Han’s Zhou bird, Ma’s subbohai bird and Kuosi’s longtalus.
The data “reconstruct” the skull shape
Sorting out the “family tree” of ancient birds is the primary goal of exploring the evolutionary relationship between birds. The phylogenetic analysis was performed by adding morphological information from the fossil bird to a new feature matrix using anatomical observations and feature coding. The results show that the systematic location of the new specimen is within the framework of bohaiornithidae, indicating that the specimen is a member of bohaiornithidae.
Due to the limitations of fossil burial and preservation conditions, paleontologists knew little about the specific morphological characteristics of some bones in the previous study of Mesozoic birds. In order to further study the anatomical structure of the fossil, the researchers used computed tomography (CT) and computed tomography (CL) to perform high-precision scanning and data reconstruction of various body parts, including the skull, upper and lower jaw, respectively. The CT scan data revealed detailed features of the bone morphology of the bohai bird skull, especially in the occipital region behind the head, improving the researchers’ understanding of the evolution of the antiavian skull.
Teeth are an important tissue in the body of most vertebrates. They can be divided into homomorphic teeth and heteromorphic teeth. Tooth eruption and replacement is a common biological phenomenon in vertebrates, but different animals have different patterns of tooth replacement. In most mammals, including humans, it is common to develop two sets of deciduous and permanent teeth during a lifetime. This is called didentition. In contrast, multiple tooth replacements are common in most amphibians and reptiles. What’s more, sharks that replace their teeth for life don’t actually grow teeth in their mouths, but their scales.
The replacement pattern is different from humans
So how do different groups of animals replace teeth?
In our common elephants, for example, molars, commonly known as molars, erupt from the back of the gums and gradually push the front teeth, like a conveyor belt, constantly moving to create new teeth from the back. In contrast to mammals, reptile teeth are not replaced uniformly, but in a pattern of tooth replacement with alternating odd and even positions. For them, tooth replacement time is independent of wear, and the basic regulatory mechanisms of the replacement process have been difficult to understand.
Since the 19th century, with the discovery of archaeopteryx fossils, it has been known that primitive birds retain the same multi-toothed features as reptiles in their jaws, whereas in living birds, the teeth are all decayed and the jaws are covered with horn-like beaks. However, due to the scarcity of bird fossils, paleontologists have not had a detailed understanding of the tooth replacement pattern of primitive birds. So far, only archaeopteryx and a few Mesozoic birds, such as Twilight, Twilight, ichthyosaurs, have been recorded.
In this study, the researchers accidentally discovered tiny replacement teeth hidden inside the bohai bird’s teeth, indicating that it was in the middle of a process of replacing old teeth with new ones. The scanning data showed that the replacement teeth preserved in the upper and lower jaw of the Bohai bird fossils developed in odd positions and showed a reptilian alternating tooth replacement pattern. The widespread pattern of tooth replacement in reptiles and primitive birds suggests that a conserved control mechanism regulates tooth replacement rhythms. In addition, the researchers compared the Bohai birds with Archaeopteryx and three modern species with teeth and found that they all showed similar tooth replacement patterns, suggesting that this feature was shared by three major Mesozoic bird lineages.

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