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In addition, the fossil specimen, named Cratonavis zhui, retains a strikingly elongated shoulder and first metatarsal, which distinguishes it from all other birds, including fossil.
The study, published in the journal Nature Ecology & Evolution on January 2, was conducted by paleontologists from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences.
Cratonavis is located between long-tailed reptiles such as Archeopteryx and Ornithothoraces (which have already evolved many of the traits of modern birds) on the bird evolutionary tree.
To study the fossil skull, scientists for the first time used high-resolution computed tomography (CT). Then they digitally extracted the bones from their rock-cut tomb and reconstructed the skull’s original shape and function.
The result shows that the skull of Cratonavis is nearly identical morphologically to that of dinosaurs such as Tyrannosaurus rex, rather than being bird-like. “The features of the primitive skull speak to the fact that most Cretaceous birds like Cratonavis could not move their upper beak independently of the brain and mandible, a functional innovation widely distributed among living birds that contributes to their enormous size.” ecological diversity,” said Dr. LI Zhiheng, lead author of the study, in a statement.
Regarding the scapula and rare shoulder metacarpals in Cratonavis, Dr. Wang Min, chair and corresponding author of this study stated: “The scapula is functionally vital to bird flight, as it imparts stability and flexibility. We have tracked changes in the scapula throughout theropod transition, and hypothesize that An elongated scapula may increase the muscle mechanical advantage of humerus retraction/rotation, compensating for the general underdevelopment of the flight apparatus in this early bird, and these differences represent morphological experiences in flight behavior in early bird diversification.”
The new study shows that the first metatarsals underwent selection during the transition from dinosaurs to birds, which preferred shorter bones. It then lost its developmental ability once it reached its optimal size, less than a quarter of the length of the second metatarsal.
“However, there was more evolutionary progression between Mesozoic birds and their dinosaur relatives, which may have been due to conflicting demands associated with the direct use of the hallux for locomotion and feeding,” said study co-author Dr. Thomas Stidham. In the case of Cratonavis, this long thumb likely resulted from selective predatory behaviour.
Study co-author Dr ZHOU Zhonghe said the aberrant scapular and metatarsal shapes preserved in Cratonavis highlight the breadth of skeletal plasticity in early birds.
Changes to these elements in the theropod tree show the specific evolutionary potential of each clade, the result of the interaction between evolution, natural selection, and environmental opportunities.
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