Cratonavis zhui, a new species of pygostylian bird from the Early Cretaceous Jehol Biota of China, had a unique combination of a dinosaurian skull with an avialan post-cranial skeleton, revealing the key role of evolutionary mosaicism in early bird diversification.

Life reconstruction of Cratonavis zhui. Image credit: Chuang Zhao.

Cratonavis zhui lived in what is China during the Early Cretaceous epoch, approximately 120 million years ago.

On the bird evolutionary tree, it sits between members of the bird clade Ornithothoraces and the more reptile-like Archaeopteryx.

“The Cretaceous period is a critical time interval that encompasses explosive diversifications of terrestrial vertebrates, particularly the period when the earliest-branching birds, after divergence from their theropod ancestors, evolved the characteristic avian bauplan that led eventually to their global radiation,” said Dr. Min Wang, a researcher with the Institute of Vertebrate Paleontology and Paleoanthropology and the Center for Excellence in Life and Paleoenvironment at the Chinese Academy of Sciences, and colleagues.

“This early diversity is overwhelmed by the Ornithothoraces, consisting of the Enantiornithes and Ornithuromorpha, whose members evolved key derived features of crown birds.”

“This disparity consequently circumscribes a large morphological gap between these derived clades and the oldest bird Archaeopteryx.”

Holotype of Cratonavis zhui. Image credit: Li et al., doi: 10.1038/s41559-022-01921-w.

In the research, Dr. Wang and co-authors studied the skull and the post-cranial skeleton of Cratonavis zhui using the high-resolution computed tomography.

Their results demonstrated that the skull is morphologically nearly identical to that of dinosaurs such as Tyrannosaurus rex.

“The primitive cranial features speak to the fact that most Cretaceous birds such as Cratonavis zhui could not move their upper bill independently with respect to the braincase and lower jaw, a functional innovation widely distributed among living birds that contributes to their enormous ecological diversity,” said Dr. Zhiheng Li, also from the Institute of Vertebrate Paleontology and Paleoanthropology and the Center for Excellence in Life and Paleoenvironment at the Chinese Academy of Sciences.

“The scapula of Cratonavis zhui is functionally vital to avian flight, and it conveys stability and flexibility,” Dr. Wang added.

“We trace changes of the scapula across the theropod-bird transition, and posit that the elongate scapula could augment the mechanical advantage of muscle for humerus retraction/rotation, which compensates for the overall underdeveloped flight apparatus in this early bird, and these differences represent morphological experimentation in volant behavior early in bird diversification.”

“Our study shows that the first metatarsal was subjected to selection during the dinosaur-bird transition that favored a shorter bone.”

“It then lost its evolutionary lability once it reached its optimal size, less than a quarter of the length of the second metatarsal.”

“However, increased evolutionary lability was present among Mesozoic birds and their dinosaur kins, which may have resulted from conflicting demands associated with its direct employment of the hallux in locomotion and feeding,” said Dr. Thomas Stidham, also from the Institute of Vertebrate Paleontology and Paleoanthropology and the Center for Excellence in Life and Paleoenvironment at the Chinese Academy of Sciences.

“For Cratonavis zhui, such an elongate hallux likely stems from selection for raptorial behavior.”

“The aberrant morphologies of the scapula and metatarsals preserved in Cratonavis zhui highlight the breadth of skeletal plasticity in early birds,” said Dr. Zhonghe Zhou, also from the Institute of Vertebrate Paleontology and Paleoanthropology and the Center for Excellence in Life and Paleoenvironment at the Chinese Academy of Sciences.

“Changes in these elements across the theropod tree show clade-specific evolutionary lability resulting from the interplay among development, natural selection, and ecological opportunity.”

Source: sci.news