How did dinosaurs evolve into birds? Chinese scientists have found that limb bones evolve more slowly and change less

Beijing, June 6 (Zhongxin Net) -- How did the land overlord dinosaur evolve into a bird with wings in the blue sky hundreds of millions of years ago? This is undoubtedly the most "shocking" event in the evolutionary history of vertebrates, involving a large number of structural changes in the skeletal system, muscular system and epidermal derivatives, which are either the result of convergence or homologous action, but most of them are related to the origin of flight.

The latest study by researchers Wang Min and Zhou Zhonghe of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (Institute of Vertebrate Paleontology and Paleoanthropology) of the Chinese Academy of Sciences on the evolution process of dinosaurs and birds found that limb bones evolved more slowly and changed less in terms of deep temporal changes in their morphological diversity and rate of evolution.

The rate of limb bone evolution of Mesozoic theropods, including birds. Photo courtesy of Wang Min

This important achievement paper of "dinosaur to bird" research was published online in Springer Nature's professional academic journal Nature Ecology & Evolution on the night of June 6, Beijing time.

Wang Min, the first author and corresponding author of the paper, said that the most significant thing in the evolution of "dinosaurs to birds" is the change in body size represented by the length of limb bones, for example, from the overall trend, theropod dinosaurs that are closer to birds on the evolutionary tree have relatively longer forelimbs. Therefore, the systematic quantitative analysis of the dynamic evolutionary trajectory of limb bones in the origin of birds is the key to understanding the important transformation of "land-running dinosaurs" into "dinosaurs (birds) flying in the blue sky".

In this study, by constructing a morphological evolution space containing a large number of limb bones of theropods and Mesozoic birds, and quantifying the limb bone diversity of the three groups of birds, non-avian parabirds and non-paraavian theropods, it was found that the overall diversity of limb bones in early birds was the lowest, and the non-paraavian theropods were the highest, and similar differences were reflected in the forelimbs. However, when the study subjects focused on the hindlimbs, the difference was not significant.

Evolution of two morphofunctional indices of Mesozoic theropod dinosaurs (including birds). Photo courtesy of Wang Min

The researchers also estimated the evolution rate of the limb bones of Mesozoic theropod dinosaurs, including birds, and the results showed that the overall evolution rate of the front and rear limbs slowed down when approaching the origin node of birds, and the same trend also appeared in the forelimbs, but there was no similar slowdown trend in the evolution of the hind limbs. This suggests that the slowdown in the rate of evolution in primitive birds is still a result of forelimb action.

The study also quantified two morphological and functional indicators, namely the arm index reflecting flight mode and the foot index reflecting ground habitat capacity, and still found that both indices were the lowest in birds, and the rate of evolution was relatively slower in birds.

"The findings of these studies are very different from the common sense of evolutionary biology." Wang Min pointed out that when a feature with the meaning of "evolutionary innovation" (such as upper and lower jaw) or morphological function (such as flight, chewing) appears in a certain taxon, it is generally believed that the rate of evolution of the taxa will become faster and the diversity will increase, because these "innovations" will help organisms enter new ecological niches. For this phenomenon, this study believes that the barrenness of the morphological diversity of limbs and bones in early birds, as well as the reduction of the rate of evolution, were mainly "dragged" by the forelimbs, because under the selective action of flight, the forelimbs of primitive birds were only able to undergo limited changes in the framework suitable for aerodynamics. In other words, primitive birds have "skeletal blueprints" that have appeared in some non-avian theropod dinosaurs.

Evolution rates of arm index and foot index of Mesozoic theropod dinosaurs (including birds). Photo courtesy of Wang Min

Wang Min said that with the emergence of many flight-related advanced features, including muscles, ligaments, and joint patterns of bones related to the completion of flight fine motor in the later stage of evolution, the changes in the forelimbs can break through the "bottleneck" and eventually evolve the diverse forelimb structures presented by modern birds.

He revealed that this study also provides quantitative data support for the hypothesis that "shoulder straps evolve first, belts behind" at the beginning of bird evolution. (End)