Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences have found part of the fossil skeleton of a 120 million-year-old miniature extinct bird that can be placed in the palm of the hand and preserves a unique skull that mixes dinosaur and bird features.
The fossil's two-centimeter-long (0.75-inch) skull has many nearly identical structural and functional features to the giant T. rex, suggesting that early birds retained many of the features of their dinosaur ancestors, and that their skulls functioned much like dinosaurs rather than modern birds. Their findings were published in Nature Communications on June 23, 2021.
The bird was deposited 120 million years ago in a shallow lake in today's Liaoning Province. Through detailed reconstructions of the bird family tree, the researchers demonstrated that this new bird fossil belonged to an extinct group of birds known as enantiornithines, or "anti-birds." They were the most diverse group of birds of the Cretaceous dinosaur era and have been found all over the world.
Bird skulls have a unique mobility, this unique feature of birds is called "kinetic energy skulls", and many birds have upper jaws that can move independently of the lower jaw and skull. However, compared to live birds, the skulls of this new "anti-bird", as well as those of close relatives of dinosaurs and birds like T. rex (such as thorodontids and dromaeosaurs), are not "kinetic". Instead, its bones are "locked" and unable to move.
The temporal region (on both sides) of the skull of this bird fossil is very different from that of a live bird. This new species has two bone arches for the attachment of the jaw muscles, as found in reptiles such as lizards, crocodiles and dinosaurs, making the back of the skull stiff and resisting movement between the bones.
Dr Min Wang, lead author and corresponding author of the study, said: "When reconstructing all parts of the skull in three dimensions from a high-resolution CT scan of the fossil, I had a problem with figuring out a particular piece of bone. His colleague Dr. Thomas Stidham proposed that the bone was a pterosaur that looked "exactly like the bones of a river-lining thief."
The researchers compared ct scans of the bird's skulls with those of the famous River Robbers from Inner Mongolia, China. The results showed that many other features of the back of the skull, including the shape of the basal nasal bone and its connection to other skulls, also resembled dinosaurs, rather than live birds.
"Bird fossils and dinosaurs also lack the discrete contact of pterosaurs and limbs near the palate, which is used in the movement of the skulls of live birds." Dr Stidham, co-author of the study, said: "Combined with the 'locked' temporal bone, differences in palate structure also indicate no motility in early birds. ”
In addition, the team's findings and meticulous anatomical studies helped reinforce the already well-supported argument that birds evolved not only from living dinosaurs, but also from branches of dinosaurs, based on many different pieces of evidence. Dr Wang Min said: "This is a bird's head embedded in a dinosaur skull, and certainly did not stop anti-birds or other early birds from achieving great success around the world during the Cretaceous period. ”