Dinosaur Photo: VCG

Analysis of large amounts of dinosaur and bird fossils has suggested that the evolution of primitive birds was slow and the diversity of body shapes dropped, which is opposite to the common belief that quick and major changes occur when a new species is taking shape.

The discovery was made by Wang Min and Zhou Zhonghe from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences. An article about the research has been published by Nature Ecology & Evolution, a sub-journal of Nature. 

Vertebrate evolution from dinosaurs to birds was an epic moment in natural history and the process involved many changes in bones, muscle and skin, which are related to flying, according to a press release the institute sent to the Global Times on Monday. 

One of the most notable changes was in body shape, represented by the length of the limb bones. Theropod dinosaurs, which are closer to birds in the evolutionary tree, have relatively long forelimbs. Therefore, a systematic quantitative analysis of the dynamic evolutionary trajectory of limb bones during the origin of birds is key to understanding the important transition from "dinosaurs running on land" to "dinosaurs (birds) flying in the blue sky."

Researchers established a model to analyze the limb bones of avialans (birds), non-avialan paravians (dinosaurs similar to but not the same as birds) and non-paravian theropods, finding that diversity of avialans was the lowest while for non-paravian theropods it was the highest. An estimate of limb bone evolution speed indicated the evolution "slowed down" among avialans, or primitive birds.

Analysis also found two other indexes, which represented the flying pattern and cursorial pattern, were also the lowest among birds, indicating a low evolution speed. 

These findings go against the common sense that the diversity and evolution speed increase at an epic evolutionary juncture. 

One hypothesis is that birds' forelimbs can only have limited changes within the aerodynamic frame, and many characteristics related to flying had already appeared among theropods. 

Global Times