How did the dinosaurs become birds? The nose knows Premium

Relative to the sizes of their heads, warm-blooded animals have larger nasal cavities than cold-blooded ones. Herein lies a clue.

A reconstruction of a velociraptor skeleton. | Photo Credit: Eduard Solà Vázquez

In a famous 1859 book, ‘On the Origin of Species’, the British naturalist Charles Darwin presented the world with the theory that life-forms evolved through natural selection.

A decade later, after he noticed that a bone in a turkey he was eating resembled a fossil of the theropod Megalosaurus that lay in his office, the anatomist Thomas Henry Huxley suggested that the birds of today could be the descendants of the extinct dinosaurs.

The proof of Huxley’s idea came a century later, yet some mysteries persisted as well. One was that both birds and dinosaurs have and had brains of a similar size – even as other parts of dinosaurs evolved to become smaller. The brain generates heat that needs to be moved away, so how did the smaller bird cranium manage this?

A study published in the journal Royal Society Open Science suggests the nasal cavity might have the answer.

“The evolution of the skull of dinosaurs to birds has been one of the main focuses in dinosaur palaeontology for a long time,” Seishiro Tada, a researcher at the University of Tokyo and National Museum of Nature and Science, Japan, whose team conducted the study, told this writer.

“By focusing on the nose, this study helped us improve our understanding of the cranial evolution of dinosaurs to birds.”

Were dinosaurs hot or not?

In 1998, two fossils discovered in China provided paleontological evidence that modern birds had evolved from theropod dinosaurs. The 120-million-year-old fossils, of the species Protarchaeopteryx and Caudipteryx, depicted different stages in the evolution of birds from terrestrial, two-legged dinosaurs with feathers.

The dinosaurs had slowly developed bird-like features, such as wings, wishbones, and feathers. Yet even as they got smaller, their brains didn’t. Scientists were soon curious: As theropods became birds, what cranial adaptations helped birds cool their brains efficiently?

Even today, scientists are not sure whether dinosaurs were warm-blooded or cold-blooded. The word  ‘dinosaur’ comes from the Greek words ‘deinos’ and ‘sauros’, meaning “terrible lizard”, and lizards are ectotherms. On the other hand, dinosaurs are also related to birds, which are warm-blooded.

The location of non-avian dinosaurs in the phylogenetic tree – a diagram depicting how different life-forms are related to each other by evolution – is somewhere between animals that depend on environmental conditions to regulate body temperature (e.g. lizards and crocodiles) and those that can regulate it on their own (e.g. birds and humans).

Since the dinosaurs are extinct, some scientists tried to look for answers in the nose.

Ask the nose

The nasal cavity of warm-blooded animals houses a complex scroll-like structure made of thin bony plates called the nasal, or respiratory, turbinate.

Nasal turbinates are found only in warm-blooded creatures. They are responsible for regulating heat and moisture exchange during respiration.

While researchers have elucidated their influence on the metabolism rates of dinosaurs, they don’t yet fully understand the actual physiological function of the turbinate.

Because of their gelatinous composition, nasal turbinates rarely survive fossilisation. As a workaround, Tada & co. examined whether the size of the nasal cavity was smaller or larger based on the presence or absence of the nasal turbinate.

The team obtained computed tomography (CT) scans of 51 present-day species: 21 birds, eight mammals, four Crocodylia (crocodiles and alligators), three Testudines (turtles and tortoises), and 11 Lepidosauria (snakes, lizards, iguanas, etc.). The scan data was then used for 3D reconstruction of their nasal cavities.

The team also digitally reconstructed a three-dimensional nasal cavity of a velociraptor (a type of theropod) based on fossils. The reconstruction accounted for physical characteristics like the fleshy nostril, the inner structures of the maxilla (lower jaw bone), and other soft tissues around the nose that are likely to have been lost in the course of fossilisation.

World of the dinosaurs

Comparing these 3D scans, the researchers found that, relative to the size of their heads, warm-blooded animals had much larger nasal cavities than cold-blooded animals.

The reconstruction and some analysis also shed light on a lesser known physiological function of respiratory turbinates: brain-cooling. “Our study discovered that one of the primary functions of the respiratory turbinate and the bigger nasal cavity of [warm-blooded animals] is to cool their larger brains, not for whole-body metabolism, which was thought to be true but remained unheralded,” said Dr. Tada.

The researchers also found that the velociraptor had a smaller nasal cavity than that of modern birds, and that the theropod didn’t possess a fully developed cooling system that would be required for a brain that could ‘operate’ a warm-blooded animal. Birds and mammals on the other hand had large nasal cavities that in turn accommodated a well-developed respiratory turbinate, and that in turn cooled their brains efficiently. That is the hypothesis, at least.

They also found that in the velociraptor, the maxilla had a significant influence on the shape of the nasal passage. Based on this, they have proposed that a “great reduction of the maxilla on the theropod lineage resulted in the nasal cavity becoming an important apparatus for their thermal regulation strategy”.

Dr. Tada said that more and better research will be required to validate their hypotheses but also that their current study would encourage further research into uncovering the changes in respiration and thermoregulation as dinosaurs became birds.

“Organisms do not evolve in a vacuum but in relation to their surrounding environment,” he said. “We hope to gain more such insights into the evolution of dinosaurs and the environment of the earth they walked on.”

Sanjukta Mondal is a chemist-turned-science-writer with experience in writing popular science articles and scripts for STEM YouTube channels.

• In 1998, two fossils discovered in China provided paleontological evidence that modern birds had evolved from theropod dinosaurs. The 120-million-year-old fossils, of the species Protarchaeopteryx and Caudipteryx, depicted different stages in the evolution of birds from terrestrial, two-legged dinosaurs with feathers.
• Even today, scientists are not sure whether dinosaurs were warm-blooded or cold-blooded. Comparing some 3D scans, the researchers found that, relative to the size of their heads, warm-blooded animals had much larger nasal cavities than cold-blooded animals.
• The reconstruction and some analysis also shed light on a lesser known physiological function of respiratory turbinates: brain-cooling. “Our study discovered that one of the primary functions of the respiratory turbinate and the bigger nasal cavity of [warm-blooded animals] is to cool their larger brains, not for whole-body metabolism, which was thought to be true but remained unheralded,” said Dr. Tada.