The “conservatives” in Cretaceous birds also have an alternative side

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Restoration of Zhu’s cratonic owl Drawn by Zhao Chuang

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Zhu’s cratonic plover specimen courtesy of Wang Min and Li Zhiheng

The Cretaceous period records how birds evolved from dinosaurs and developed unique body size characteristics. At this stage of evolution, most birds have evolved a large number of morphological characteristics similar to living birds, and are very different from the most primitive Archaeopteryx. Basal birds, on the other hand, are a “minority” that looks more primitive and fossils are rarer, limiting their research.

On January 3, Nature Ecology and Evolution published a study completed by the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (hereinafter referred to as the Institute of Paleovertebrate Paleontology), reporting on a new basal bird, the Zhu’s cratonic gull, which found that the “conservative” ones have struggled to break through the limits of evolution.

The cratonic owl appeared in the Rehe biota 135-120 million years ago, and when researchers saw it, they found that it was almost the same as the original theropod dinosaurs in skull morphology, and did not evolve the skull mobility that most birds have, which is more conservative in evolution. In contrast, its body bones already have a large number of progressive features of birds. Wang Min, author of the paper and researcher at the Institute of Paleovertebral Research, said that this shows that the skull and body of the cratonic owl have the characteristics of modular evolution.

The strangest feature of the cratonic owl is that it has a particularly long shoulder blade and a first hoof bone (equivalent to the innermost bone of the sole). Through comparative cladistics, the researchers traced the dynamic trajectories of these two bones during the evolution of dinosaurs and birds.

The shoulder blades are an important part of the flight structure of birds. Wang Min explained that the original shoulder blades were more prone to length changes in theropods than in birds, and the independent lengthening of the shoulder blades of cratonic gulls may be an attempt to adapt to flight, because the elongated shoulder blades can expand the attachment area of the muscles that control the downward flapping of wings.

“Not only that, in dinosaur-bird evolution, the first hoof bone is usually shorter and shorter, for example, the relative length ratio of the first hoof bone in birds is much smaller than that of primitive theropod dinosaurs, and the proportion of the first hoof bone of birds has been established at the beginning of its differentiation. However , the relative length of the cratonic gull’s first hoof bone is much longer than that of other birds and most dinosaurs , meaning that it is the result of independent evolution. ”

Wang Min said that this conclusion can also be confirmed by the change in the phylogenetic signal of the first homopod, because they found that the degree of influence of phylogenetic relationships in cratonic plovers is relatively high in theropods. In addition, they suggested that the abnormal growth of the first hoof bone may be due to the cratonic gull’s ecological habits similar to that of birds of prey.

“The unique shoulder blades and tarsal bones of the cratonic owl show that under the dynamic action of ontogenetic development, natural selection, and ecological function opportunities, some seemingly relatively conservative bones ‘get rid of limitations’ and undergo evolutionary changes.” Wang Min believes that this study has helped scientists enrich their understanding of the early differentiation of basal birds. (Source: Hu Minqi, China Science News)

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