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How Did Birds Escape from Mass Extinction? NSRRC Discovered the Secret Hidden Within Their Teeth!

The research team consists of Dr. Wang Chun-Chieh and Mr. Chiang Cheng-Cheng from the National Synchrotron Radiation Research Center (NSRRC), Dr. Li Zhiheng  and academician Dr. Zhou Zhonghe from the Institute of Vertebrate Paleontology and Paleoanthropology, Prof. Huang E-Wen from the Department of Materials Science and Engineering, NCTU, and Mr. Hsiao Kiko from Mr. Fossil, spent 3 years on the research and analysis of the tooth evolution from Theropoda, a dinosaur clade that is most related to ancient birds, to ancient birds, using synchrotron Transmission X-Ray Microscopy (TXM). It is the first time in history that the research team discovered the Porous Mantle Dentin of ancient birds has deteriorated and disappeared, which confirmed that the transformation of feeding habits of birds fortunately helped them to escape from a mass extinction event. The research result was published in the international journal BMC Evolutionary Biology on April 21st.

< Cretaceous–Paleogene Extinction Event >

How did birds, descendants of dinosaurs, escape from the mass extinction before 65 Mya, has always puzzled scientists. When meteorites struck the earth, the already frequent volcanic eruptions led to a significant amount of dust entering the atmospheric layer, which blocked the sun and hindered photosynthesis for plants, thus induced further severe impact to the global ecosystem. When plants no longer received energy from the sun, herbivores began dying due to no food sources, which eventually led to the successive extinction of carnivores. This series of food chain collapses resulted in the extinction of 75% of organisms on earth, for which the spotlight lies on the mass extinction of non-avian dinosaurs (Birds is the only survived dinosaur lineage).

Scientists generally believe that omnivores, insectivores, and scavengers survived from this mass extinction due to their extensive feeding preferences. These organisms feed from fruits, seeds, insects, dead plants and animals, or organic biodebris, thus were not affected by the collapse and extinction of plants. It is also possible that birds have evolved into having a diversified feeding orientation, which helped them escape from the catastrophe and survive until today. 

<Deterioration of Porous Mantle Dentin Confirms the Transformation of Feeding Habit for Birds>

In order to verify this inference, the NSRRC conducted an unprecedented research on the teeth of small Theropoda and Avialae, and compared them with the previous experimental results of large carnivorous Theropoda teeth, such as Tyrannosaurus teeth. The analysis discovered that the distinctive Porous Mantle Dentin extensively appeared between the enamel and dentin of the Theropoda teeth were completely deteriorated and disappeared inside the teeth of ancient birds. The Porous Mantle Dentin has been verified with functions of shock absorption and crack resistance, and is able to prevent the teeth from cracking amidst tearing of flesh. The disappearance of this structural layer verifies that a significant transformation had occurred between the feeding habits of ancient birds and their Theropoda ancestors, which substantially elevated the degree of survival adaptability of birds at the time.

In addition to the disappearance of the Porous Mantle Dentin, the research team also discovered that the enamel of the ancient bird teeth became thin, and its crystal structure were simplified, meaning that their teeth no longer needed special protection mechanism, which again verified that there are monumental differences in the feeding habits between birds and Theropoda.

This research collected a total of 9 different teeth, including Ornithurine, indet. Enantiornithine, Sapeornis, and Jeholornis under Avialae, as well as Troodon, Anchiornis, and Microraptor under Theropoda. Apart from Avialae, the research team also discovered that the Porous Mantle Dentin inside a small indet. Microraptor teeth has also disappeared. Though compared to the comprehensive herbivorous or omnivorous evolution tendency for Avialae, despite minor groups of Theropoda also underwent convergent evolution, they were unable to escape from being wiped out during the abrupt catastrophe.

<Transmission X-Ray Microscopy - New Weapon for Paleontological Research>

As expressed by Wang Chun-Chieh, the relatively rare and tiny ancient bird teeth, where some of them are as small as a piece of sand, make it with extremely high criterions in sample preparation and high-resolution imaging. Hence, the synchrotron transmission X-ray microscope played an essential role in this research. Besides, this is currently the research that consists of the most collection of different teeth of ancient birds, and there has been no research teams that have comprehensively and systematically compared the differences in the evolution of internal microstructure for the teeth from non-avian dinosaurs to birds.

The high-resolution synchrotron transmission X-ray microscope at the NSRRC functions similar to that of medical X-ray computed tomography, which establishes 3D tomographies, though it has an image resolution that is over 2000 times better than the medical system, making it one of the best instruments for paleontological research.

BMC Evolutionary Biology volume 20, Article number: 46 (2020)