TODAY’S BIRDS HAVE a more crouched leg posture compared to their ancestors, which are thought to have moved with straighter limbs similar to the typical postures of humans.
In a joint study by researchers from the UK and China, experts reveal how birds gradually shifted towards this more crouched posture.
Led by professor John R. Hutchinson at the Royal Veterinary College (UK) and professor Baoyu Jiang at Nanjing University in China, the team studied fossilised tissue of a lower leg belonging to a crow-sized Confuciusornis bird, which was fossilised in volcanic ash and lake sediments in China between 125-145 million years ago.
They found that the fossil had amazingly well-preserved soft tissues around the ankle joint, including cartilages and ligaments.
Prof Jiang explained: “These soft tissues were not just preserved as an ashen replacement of the former tissue, as sometimes happens. Rather, the cellular and fibrous structure of the tissues was preserved at a microscopic level.”
Imaging methods showed that the detailed anatomical preservation extended to the molecular level, and some of the original chemistry of the bird’s tissues was still there.
In particular, the team found evidence of fragments of the collagen proteins that made up the leg ligaments, which matched the preservation at the microscopic tissue level of detail.
These findings support existing evidence that, under special conditions, some biological molecules can survive over millions of years.
Once the team were confident that some of the remnants of the soft tissues around the ankle joint still remained around the ankle, they reconstructed parts of the ankle beyond just the bones.
The University of Manchester’s professor Roy Wogelius, one of the collaborators of the study, explained: “The preservation in this fossil was exceptional and allowed us to resolve subtle but important chemical and structural details within this critical early species of bird.”
Prof Hutchinson added: “The new information we gained about the anatomy of the cartilages and tendons show that this early bird had an ankle whose form fit an intermediate function between that of early dinosaurs and modern birds.
“Overall this reinforced other lines of evidence that the more crouched, zigzag limb posture of birds evolved gradually from early dinosaurs to birds, with even these early birds having limbs that were built and worked differently from those of living birds, but were approaching the modern condition.”
The study has been published in the online journal Nature Communications.