Flight is a rare skill in the animal world. Among vertebrates, it evolved only three times: in bats, birds, and the long-extinct pterosaurs. Pterosaurs were the pioneers, taking to the skies more than 220 million years ago, long before early bird relatives such as Archaeopteryx appeared, around 150 million years ago. While scientists have a detailed fossil record that sheds light on how birds' brains evolved for flight, the same story for pterosaurs has been far less clear. Until now.
In a new study published in Current Biology, an international team now reveals how pterosaurs evolved the neurological structures required for powered flight.
"The breakthrough was the discovery of an ancient pterosaur relative, a small lagerpetid archosaur named Ixalerpeton from 233-million-year-old Triassic rocks in Brazil," said Mario Bronzati, an Alexander von Humboldt fellow at the University of Tubingen in Germany and lead author of the study.
"We've had abundant information about early birds and knew they inherited their basic brain layout from their theropod dinosaur ancestors," added co-author Lawrence Witmer, professor of anatomy at the Ohio University Heritage College of Osteopathic Medicine. "But pterosaur brains seemed to appear out of nowhere. Now, with our first glimpse of an early pterosaur relative, we see that pterosaurs essentially built their own 'flight computers' from scratch."
How researchers mapped brain evolution
To piece together this evolutionary story, the researchers used high-resolution 3D imaging techniques, including microCT scanning, to reconstruct brain shapes from more than three dozen species. These included pterosaurs, their close relatives like Ixalerpeton, early dinosaurs and bird precursors, modern crocodiles and birds, and a wide range of Triassic archosaurs, the larger group that includes all these animals.
"Then, using statistical analysis of the size and 3D shape of their cranial endocasts, we were able to map the stepwise changes in brain anatomy that accompanied the evolution of flight," said co-author Akinobu Watanabe, associate professor of anatomy at the New York Institute of Technology College of Osteopathic Medicine.
Flight is a physiologically demanding form of locomotion and has long been assumed to require major neurological adaptations including enlargement of the brain to coordinate the complicated sensory and motor information required for powered flight. Previous studies of pterosaur brain structure had shown that they indeed shared some neurological similarities with bird precursors like Archaeopteryx, such as some enlargement of brain regions like the cerebrum and cerebellum involved with sensorimotor integration, as well as enlargement of visual centers like the optic lobes.
Ixalerpeton, the lagerpetid close relative of pterosaurs showed some but not all neurological traits of pterosaurs. For example, as Bronzati notes, "lagerpetids were probably tree-dwellers, and their brains already show features linked to improved vision, such as an enlarged optic lobe, an adaptation that may have later helped their pterosaur relatives take to the skies, but they still lacked key neurological traits of pterosaurs."
Lagerpetids like Ixalerpeton had brains intermediate in shape between more primitive archosaurs and pterosaurs but retain greater similarity to early dinosaurs. Other than the enlarged optic lobe that occupies a position in the brain similar to that in pterosaurs and birds and their close theropod relatives, there is little in Ixalerpeton that indicates what was to come in pterosaurs.
A unique feature of the brain of pterosaurs is a greatly enlarged flocculus, a structure of the cerebellum likely involved in processing sensory information from their membranous wings to keep their eyes fixed on a target while in flight. The flocculus in Ixalerpeton wasn't expanded like pterosaurs, instead resembling the modest flocculus of other archosaurs, including early birds and their close nonavian theropod relatives.
Likewise, the new analyses show that pterosaurs retained modest brain sizes.
Comparing pterosaur and bird brains
"While there are some similarities between pterosaurs and birds, their brains were actually quite different, especially in size," said co-author Matteo Fabbri, assistant professor of Functional Anatomy and Evolution at the Johns Hopkins University School of Medicine. "Pterosaurs had much smaller brains than birds, which shows that you may not need a big brain to fly."
Surprisingly, the overall brain shape of pterosaurs most closely resembled that of small, bird-like dinosaurs such as troodontids and dromaeosaurids, animals that had little or no powered flight ability. Yet pterosaurs and birds still represent two entirely independent experiments in the evolution of flight. Birds inherited a brain already adapted from their non-flying dinosaur ancestors, while pterosaurs evolved their flight-ready brains at the same time they developed their wings.
Birds' notably large brains, the authors note, likely came later and were tied more to increasing intelligence and complex behaviors rather than the act of flying itself. A key takeaway from the study is that, according to Witmer, "it apparently doesn't take a large brain to get into the air, and the later brain expansion in both birds and pterosaurs was likely more about enhancing cognition than about flying itself."
Another important takeaway is that paleontological fieldwork remains an engine for new breakthroughs.
"Discoveries from southern Brazil have given us remarkable new insights into the origins of major animal groups like dinosaurs and pterosaurs," co-author Rodrigo Temp Müller, a paleontologist at Universidade Federal de Santa Maria, Brazil, noted. "With every new fossil and study, we're getting a clearer picture of what the early relatives of these groups were like, something that would have been almost unimaginable just a few years ago."
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