A Bird the Size of Your Fist Had a Head Like a T-rex

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A digital reconstruction of the new Mesozoic bird fossil skeleton.
Cute Mesozoic bird that had akinetic dinosaurian jaw featuresCredit: WANG Min

A teacup-size early bird got its worm using a rigid jaw similar to that of the great and terrible T-rex, Chinese paleontologists reported this week. More precisely, it had skull structures similar to the ancestral dinosaurs from which it arose – a group to which Tyrannosaurus rex belonged.

The little thing, which could have nestled comfortably in the palm of your hand and probably would have tried to bite it, lived 120 million years ago by a paleolake in today’s northern China. It retained the archaic arrangement of a fixed jaw, like its ancestors the therapod dinosaurs (a group including velociraptors and said tyrannosaur), reported researchers from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences in Nature Communications.

In other words, it was a sort of intermediate between the rigid, toothed and massive dinosaurian skull to the lightweight kinetic skull of today’s birds.

Birds as we know and eat have what anatomists call kinetic skulls, which means the upper jaw can move independently of the brain and the lower jaw, explain researchers Min Wang, Thomas Stidham, Zhiheng Li, Xing Xu, and Zhonghe Zhou.

Well-preserved fossils of early bird crania are rare, the team observed. However, this one was in marvelous condition, still possessing the palatal elements, which is how they could deduce the state of its cranial kinesis. To their surprise, their 3D reconstruction of its skull showed that this bird had a dinosaur-like, akinetic arrangement.

Visitors looking at a 67-million-year-old skeleton of a Tyrannosaurus rex, at a museum in Paris.Credit: PHILIPPE WOJAZER/REUTERS
A bevy of dromaeosaurids: Microraptor gui, Velociraptor mongoliensis, Austroraptor cabazai, Dromaeosaurus albertensis, Utahraptor ostrommaysorum, and Deinonychus antirrhopusCredit: Fred Wierum

The long-dead bird belonged to the extinct avian group called enantiornithines, aka “opposite birds,” which thronged the planet in the dinosaurian heyday of the Cretaceous. They were enormously successful and their remains have been found all over the world, but certain fossil beds in China are famed for the “quality” of preservation.

Dinosaurs on the evolutionary lineup toward birds, such as troodontids and dromaeosaurs, also had locked-up jaws. Long story short, this mini-proto-bird has skull structures including a pterygoid bone “exactly like that of the dromaeosaur Linheraptor” found in Mongolia, not like those found in today’s birds, which have kinetic jaws, bless them – in all sorts of ways and shapes, because birds diverse enormously, but kinesis in the skull, they have.

This birdlet didn’t; it had bony arches for jaw muscle attachment like those found in dinosaurs and reptiles such as crocodiles, the team explains. These arches render the rear of the skull rigid and resistant to movement among the bones.

“The fossil bird and dinosaurs also lack the discrete contact between the pterygoid and quadrate near the palate that is used in skull kinesis in living birds. In combination with the ‘locked up’ temporal bones, the difference in the palate structure also points to the absence of kinesis among early birds,” stated co-author Stidham.

Their study reinforces the already thoroughly accepted thesis that birds arose from dinosaurs, are in fact living dinosaurs, arising from the branch that included everyone from lumbering troodontids and the “four-winged” microraptor to The Big One.

“Having a ‘dinosaur’ skull on a bird body certainly did not stop the enantiornithines, or other early birds, from being highly successful in places all around the world for tens of millions of years during the Cretaceous,” Wang stated.

Lastly, the team concluded that the bird died young: it was a baby, going by a host of indicators, including incomplete fusion of the knee and ankle elements. And that rigid jaw is just one of many dinosaurian features it had. However, its tender age may explain why its head looks so big: With all due respect to its dinosaurian lineage, babies have big heads.

Digital reconstruction of the new Mesozoic bird skull with expanded detail of the dinosaur-like palatal bones, i.e., the basisphenoid (red), pterygoid (pink), and quadrate (purple).Credit: WANG Min

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