How Dinosaur Brains Led to Modern Birds

by AMNH on

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The extinction of non-avian dinosaurs holds a stubborn place in our imagination, but in reality dinosaurs never vanished from the Earth.

Many died out, certainly, but their evolutionary legacy lives on all around us in birds. Whether it’s the pigeon on your windowsill or the chicken on your dinner plate, chances are you don’t go a day without encountering dinosaurs.

Artist’s rendering of the dinosaur Anchiornis huxleyi.
Zhao Chuang; courtesy of Peking Natural Science Organization

Starting March 21, the Museum presents the new special exhibition Dinosaurs Among Us, detailing the unbroken line between these ancient animals and modern birds, marked by features from feathers to fused clavicles. (That’s right, turns out that, like the turkey, Tyrannosaurus rex had a wishbone—though we wouldn’t recommend tugging on it.) Much of the work behind identifying and understanding these shared characteristics is being done by scientists working or trained at the Museum, including Dr. Amy Balanoff, whose work compares the brains of ancient dinosaurs and modern birds.

Dawn of the Bird Brain

The occasional feathered-but-flightless specimen aside, the defining characteristic of birds is their ability to take to the skies. Flying, though, is tricky business. It demands synchronized activity in different areas of the brain, and that, in turn, means it requires a lot of brainpower.

“Birds are doing a lot of different things with their brains during flight. For example, they have big optic lobes to help coordinate the visual information that they are collecting with the movements of their wings,” says Dr. Balanoff, a Museum research associate who studied with Mark Norell, Macaulay Curator and Chair of the Division of Paleontology, and is now a research instructor at Stony Brook University. “It’s not surprising that they have really big brains.”

Melanerpes aurifrons
This CT scan shows a modern woodpecker (Melanerpes aurifrons) with its brain cast rendered opaque and the skull transparent. The endocast is partitioned into the following neuroanatomical regions: brain stem (yellow), cerebellum (blue), optic lobes (red), cerebrum (green), and olfactory bulbs (orange).
©AMNH/A. Balanoff

While many of us grew up learning that dinosaurs were physical giants with puny brains, that’s actually not the case, says Balanoff, especially for the animals that turn out to be most closely related to birds, such as tyrannosaurs and velociraptors. For the past eight years, Balanoff has worked to map the big brains of long-dead dinosaurs to find connections to modern birds.

Looking at dinosaur brains and comparing them to modern bird brains would be the ideal way to learn how the former evolved into the latter. Unfortunately, dinosaur brains were just as soft and mushy as our own, and thus not great candidates for preservation in the fossil record. What did get preserved, though, were dinosaur bones—including their skulls.

Scanning Skulls

Balanoff and her colleagues can learn a lot about the volume and shape of dinosaur brains by examining their skulls using computed tomography (CT) scanners. CT scans allow scientists to create digital endocranial casts—detailed, 3D reconstructions of the interiors of fossilized skulls—for the first time, a thrilling feat that sheds new light on the evolutionary road from dinosaur brains to those of modern birds.

Citipati osmolskae
The transparent skull and opaque brain cast of Citipati osmolskae, an oviraptor dinosaur, is shown in this CT scan. The endocast is partitioned into the following neuroanatomical regions: brain stem (yellow), cerebellum (blue), optic lobes (red), cerebrum (green), and olfactory bulbs (orange).
© AMNH/A. Balanoff

“Recent advances in medical imaging allow us to see what resided in the skull of an array of extinct animals,” says Balanoff. “What is even more exciting is that once we know what the brain looked like, we can begin to make at least some broad inferences about their behavior.”

Already, CT scanning has offered paleontologists a detailed view of the dinosaur cerebrum, a center for cognition and coordination in the brain. As it turns out, the cerebrum tends to be very large in dinosaurs that are closely related to birds.In the endocast image of the dinosaur Citipati osmolskae, above, the cerebrum (shown in green) is already becoming more prominent 

Ready for Takeoff

Balanoff’s research strongly suggests that these ancient avian relatives developed big brains long before flying was in the picture, laying the cerebral foundation that made the eventual development of powered flight possible. This means that, similar to the way bigger brains in primates served as a precursor to walking on two legs, bigger brains in dinosaurs primed them for flight.

Artists Rendering of Archaeopteryx
Animals like Archaeopteryx show the development of birdlike brains in dinosaurs. 
© AMNH/Z. Chuang; courtesy of Peking Natural Science Organization

In non-avian dinosaurs that are thought to mark the transition from dinosaur to bird, such as Archaeopteryx, the cerebrum is huge compared to the rest of the brain, a trend we see continued in modern birds whose brains are 40 to 60 percent cerebrum, compared to about 25 percent in tyrannosaurs. And the distinction between flight-ready animals like Velociraptor and a flying Archaeopteryx seems to be shrinking. 

“For a long time, Archaeopteryx was considered the first bird, but all of those bird-like features appeared slowly, over the course of millions of years,” Balanoff says, though pointing out that there is no longer a meaningful line demarcating where dinosaurs end and birds begin. “That’s how evolution works. It can be a slow and messy process, but eventually we end up with the amazing diversity of things flying around us today.”

Balanoff CT Scanner
Museum researcher Amy Balanoff imaging fossils with a CT scanner.
© AMNH/D. Finnin

Balanoff hopes that future research in this field could shed light not only on the shape and size of dinosaur brains, but on how they influenced dinosaur behavior. Paleontologists already know that these prehistoric titans shared some behaviors with modern birds, such as brooding their eggs. Additional research may only make these ancient animals seem more familiar. Just picture a Velociraptor, dotted with feathers, sitting on a nest, its brain, if not its body, ready for takeoff.

A version of this story originally appeared in the Winter 2016 issue of the Member magazine Rotunda.