The earliest step of this evolutionary process is still unknown due to the relatively sparse and spatio-temporally limited fossil record, despite theropod dinosaurs from the Late Jurassic period being the ancestors of non-avialan birds. Knowing the early-diverging species along the avialan line is essential to understanding the evolution of the distinctive avian bauplan and settling phylogenetic arguments regarding the origin of birds.

A new avialan theropod that was discovered in Zhenghe County, Fujian Province, 150 million years ago was described and studied by a team of researchers from the Fujian Institute of Geological Survey (FIGS) and the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences in Beijing.

Fujianvenator prodigiosus, a recently found species, shares a strange assortment of morphologies with various avian, troodontid, and dromaeosaurian taxa. It illustrates how evolutionary mosaicism affected the development of early birds.

Leading and corresponding author of the study, Dr. WANG Min from IVPP, stated that “our comparative analyses show that marked changes in body plan occurred along the early avialan line, which is largely driven by the forelimb, eventually giving rise to the typical bird limb proportion. However, Fujianvenator is an odd species that diverged from this main trajectory and evolved bizarre hindlimb architecture.”

The exceptionally long lower leg and other morphologies of Fujianvenator suggest to a previously unknown ecology for early avialans, one in which the species was either a quick runner or a long-legged wader, together with other geological finds.

"Besides Fujianvenator, we have found abundant other vertebrates, including teleosts, testudines, and choristoderes," said XU Liming from FIGS, the study's primary author.

Southeast China had a high level of tectonic activity as a result of the paleo-Pacific plate's subduction during the Late Jurassic and Early Cretaceous. Due to contemporaneous fault-depression basins and widespread magmatism, the Fujianvenator was found in these areas. The earlier Yanliao Biota was still existent in north and northeastern China throughout the Late Jurassic in this geological setting.

Dr. ZHOU Zhonghe from IVPP, co-author of the study, said, “The extraordinary diversity, unique vertebrate composition, and paleoenvironment strongly indicate that this locality documents a terrestrial fauna, which we named the Zhenghe Fauna. In-situ radioisotopic dating and stratigraphic surveys constrain the Zhenghe Fauna to the 150–148 Ma period. Therefore, Fujianvenator documents one of the Jurassic avialans’ stratigraphically youngest and geographically southernmost members.”

The discovery of the Zhenghe Fauna offers a novel perspective on the Late Jurassic terrestrial ecology of the planet, and the joint research team from IVPP and FIGS plans to continue investigating Zhenghe and the surrounding area.