Author: Eric Malikyte

Have you ever wondered how Dinosaurs managed to evolve into birds?

I remember the first time the concept was introduced to me. I was sitting in my elementary school auditorium in fifth grade for an assembly. One of the teachers presenting at the front of the auditorium pointed to a bird that was stuck up in the metal rafters and told us all that it was a dinosaur.

I remembered as a kid being confused by this. I knew what dinosaurs looked like, I had several books on them after all, how could a bird be a dinosaur?

But it’s true. Birds are dinosaurs, or at least, they are what dinosaurs evolved into after the Cretaceous Paleogene extinction event, and a newly discovered fossilized bird brain may hold some answers as to how they survived and evolved into our feathered avian friends.

Surviving the K-Pg Extinction

The types of birds available today are extremely diverse. In fact, as a new research paper published in Science Advances states, they’re the most diverse clade of terrestrial vertebrates.

A clade is a group of organisms that are thought to have evolved from a common ancestor according to the principles laid out in cladistics. For example, chimpanzees and humans would be from the same clade.

In order to have a complete picture of how life continues to evolve on Earth, we need to understand why avian dinosaurs (what the paper calls extant birds, which basically just means birds that are alive today) alone among all the other kinds of dinosaurs that lived during the late Cretaceous were able to survive the impacts (yes, impacts, as in plural, more on that in a future article) that caused the K-Pg extinction event.

Traits thought to influence the differential patterns of those dinosaurs who managed to survive the K-Pg extinction event are thought to relate to neuroanatomy and feeding ecology.

Birds living today feature brains with relative volumes and neuronal densities that surpass all other reptile species (and yes, birds are considered to be reptiles). In fact, birds today have greater neuronal density than mammals. (That’s why crows are so damn smart, and why you never want to mess with them…they will remember you, they will find you).

Credit in Image Sources section.

It’s traits like this that scientists think would have given avian dinosaurs a major advantage over other dinosaurs in the face of a climate that was becoming increasingly more inhospitable to life.

In addition to these traits, changes in avian dinosaur diet that can be seen in the evolution of dinosaur jaws and palate (the roof of the mouth) have also been suggested as potential drivers for both surviving the K-Pg extinction and how they were able to diversify so quickly in the Early Paleogene.

However, in order to explain these patterns in evolution, hypotheses were developed that demanded the identification of unique traits specific to avian dinosaurs, a tall order since it’s very hard to find complete fossils of birds. Sadly, this means that for a long time now there hasn’t been enough data to paint a coherent picture of how birds today survived the cataclysm.

But a new remarkably preserved fossil of a 70-million-year-old bird called Ichthyornis is providing some new insights in how avian body sizes (and more importantly, brain sizes) varied in the Late Cretaceous and the Cretaceous-Paleogene boundary.

Ichthyornis

A Total Bird Brain

It is extremely rare to find a near-complete fossil for species like Ichthyornis. One reason for that is how brittle avian bones are when compared to other species. They just don’t typically survive the fossilization process.

So this latest find is a potential game-changer for scientists trying to unlock the secrets of how birds came to rule our skies, particularly when it comes to what it can tell us about cranial anatomy in avian dinosaurs that lived around this time period.

Early reconstructions (much like most early dinosaur reconstructions) of Ichthyornis were shown later on to be nothing more than speculative illustrations that lacked anatomical basis.

However, since the discovery of this new fossil, Christopher R. Torres, Mark A. Norell, and Julia A. Clarke were able to create a new reconstruction of the skull using x-ray computed tomography (basically the same a CT scan).

And the result is quite stunning.

The New Reconstruction

The new reconstruction of Ichthyornis reveals that it lacked the expanded cerebrum and ventrally shifted optic lobes found in extant birds.

Reconstruction of Ichthyornis fossil using CT scanner. Credit: Christopher R. Torres, Mark A. Norell, and Julia A. Clarke.

Though Ichthyornis went extinct around the same time as non-avian dinosaurs, this unique brain shape may be why birds were able to win the game of natural selection and survive to modern times. But Ichthyornis isn’t the only ancient bird we’ve discovered. There are two others so far. One with another nearly complete skull that we called Turducken, or Wonder chicken, (yes, that was what they called it…though it’s now being called Asteriornis.) And Vegavis iaai, which predated Turducken and Ichthyornis by about 200,000 to 300,000 years, whose bones were discovered in Antarctica.

Christopher Torres, who conducted the research while earning his Ph.D and now a National Science Foundation postdoctoral fellow at Ohio University and research associate at the UT Jackson School of Geosciences said this about the investigation, “Living birds have brains more complex than any known animals except mammals. This new fossil finally lets us test the idea that those brains played a major role in their survival.”

Ichthyornis is a fusion of modern bird-like features and those more common in non-avian dinosaurs. Its beak is lined with sharp teeth. Remember that birds began losing teeth as a whole about 100 million years ago. The skull almost resembles a baby crocodile in some ways, but the cranium is clearly avian.

But the team didn’t stop at just scanning the Ichthyornis fossil, they wanted to know how this bird was related to birds today, so they compared it to 2003 different birds (oh, and Archaeopteryx too) as well as seven different non-avialan dinosaurs.

Archaeopteryx. Credit: Ballista
Archaeopteryx lithographica, Oxford Museum. CC BY-SA 3.0

Avialan dinosaurs are a clade (hey, there’s that word again) that contains the only living dinosaurs, birds. They’re usually defined as theropod dinosaurs with ancestors closer to birds than say deinonychus (the dinosaur portrayed in the Jurassic Park movies as Velociraptor…no, I won’t let it go!)

The great thing about birds is that their skulls wrap tightly around their brains, so the CT-imaging data was able to model a replica of Ichthyornis’ brain. The team called it an endocast.

The Endocast

The results from creating the endocast of Ichthyornis revealed that the bird lacked the expanded cerebrum and ventrally shifted optic lobes that we see in birds living today.

While most of the skull is intact, unfortunately, the dorsal-most and ventral surfaces as well as the rostral and caudal ends were crushed by whatever killed the bird prior to the fossilization process.

Ichthyornis endocast. Credit: Christopher R. Torres, Mark A. Norell, and Julia A. Clarke.

But, what is there has shed new light on the timing of the evolution of the shape of the modern bird’s brain.

Now it’s clear that theropods like Tyrannosaurus and Alioramus continued to sport very linear brains with unexpanded cerebella and cerebra and non-avialan dinosaurs like Zanabazar and Incisivosaurus featured relative expansion of both of those regions along with simultaneously developing ventral deflection of the midbrain and optic lobes and dorsal deflection of the cerebrum.

Dinosaur and bird brains. Credit: Christopher R. Torres, Mark A. Norell, and Julia A. Clarke.

But avialae like Archaeopteryx saw the cerebellum expanded, which also caused the midbrain to expand from the dorsal brain margin by contact between the cerebellum and the cerebrum.

And finally, a period of brain and cerebrum expansion happened an undetermined amount of time after Avialae like Archaeopteryx diverged. During this expansion, the optic lobes showed up in the ventral position in relation to the cerebrum.

And while the paper fully admits that the endocast isn’t complete, the position of the optic lobes relative to the mediolaterally widest point of the cerebrum strongly suggests that Ichthyornis had a brain shape very close to Archaeopteryx.

The paper suggests that this phase of brain evolution must have happened in tandem with the origin of modern birds.

But the team isn’t done yet. No, their reconstruction of this endocast was quite extensive.

Incipient Wulst and Modern Birds?

According to the analysis of the endocast, Ichthyornis also possesses an incipient wulst, which is a derived cerebral structure previously thought to only exist in modern birds. In modern birds, the wulst is a dorsal expansion of the cerebrum that does not exist in other reptiles that live today. This actually corresponds to a subregion of the cerebrum thought to be homologous to parts of the neocortex found in mammals.

You’ll notice on the right dorsolateral surface of Ichthyornis’ cerebrum, there is a shallow sulcus (a groove or furrow, which usually exists on the surface of the brain), and this is associated with a change in slope of the endocranial surface. Among living birds, the existence of features like this sulcus in the dorsolateral surface of the brain is related to the vallecular groove separating the lateral margin of the wulst.

Different types of birds. Some birds incapable of flight do not possess a wulst. From left to right, Emu (diurnal, flightless); b, Kiwi (nocturnal, flightless); c, Barn Owl (nocturnal, flying), and d, Pigeon (diurnal, flying). Credit in Image Sources section.

A similar feature was thought to exist in Archaeopteryx, but this was later to be shown to be an artifact left over from the fossilization process.

But because this fossil is so well preserved, that isn’t the case with Ichthyornis.

So, what is a wulst anyway?

Well, it’s basically a visual processing center that most researchers think has something to do with flight, though its primary function appears to be somatosensory integration related to sensing pressure, pain, or warmth. In avialan’s like Archaeopteryx, no Wulst is present, so it’s thought that if the wulst was developed in species related to Ichthyornis then powered flight must have evolved first. Archaeopteryx was capable of powered flight and definitely predates Ichthyornis and Turducken!

Christopher Torres went on to say, “If a feature of the brain affected survivorship, we would expect it to be present in the survivors but absent in the casualties, like Ichthyornis. That’s exactly what we see here.”

And Julia Clarke, a professor at UT Jackson School of Geosciences and co-author of the study says, “Ichthyornis is key to unraveling that mystery. This fossil helps bring us much closer to answering some persistent questions concerning living birds and their survivorship among dinosaurs.”

What it’s looking like now is that these animals may have evolved just the right traits for survival before impending doom struck, which is pretty amazing if you ask me.

There’s a lot more in the paper linked in the sources section, so be sure to give it a read if you want to know more about this fossil and the evolution of birds.

Sources:

https://advances.sciencemag.org/content/7/31/eabg7099

https://www.sciencedaily.com/releases/2021/07/210730165436.htm

https://en.wikipedia.org/wiki/Avialae

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0000198

Image Sources:

Ichthyornis dispar skull and neck.jpg by Ghedo – Own work, CC BY-SA 4.0 I, https://creativecommons.org/licenses/by-sa/4.0/deed.en

Bird Diversity 2013.png by Concerto, CC BY-SA 3.0 Unported, https://creativecommons.org/licenses/by-sa/3.0/deed.en

Archaeopteryx lithographica.JPG by Ballista, CC BY-SA 3.0

Brain of Barn Owl.png by GR. Martin, KJ. Wilson, J. Martin Wild, S. Parsons, CC BY 2.5, https://creativecommons.org/licenses/by/2.5

Bird fossil in Canada.jpg by Daniel Thornton – Ottawa – Museum of Nature, CC BY-A 2.0 Generic, https://creativecommons.org/licenses/by/2.0/deed.en

Ichthyornis BW.jpg by Nobu Tamura – Own work, CC BY-A 3.0 Unported, https://creativecommons.org/licenses/by/3.0/deed.en

Jeholornis prima NMNS.jpg by Tiouraren (Y.-C. Tsai) – Own work, CC BY-SA 4.0, https://creativecommons.org/licenses/by-sa/4.0

Deinonychus FMNH.jpg by Jonathan Chen – Own work, CC BY-SA 4.0

Visual processing areas of the brains of four species of birds.png by Graham R. Martin ,Kerry-Jayne Wilson,J. Martin Wild,Stuart Parsons,M. Fabiana Kubke,Jeremy Corfield, CC BY 2.5, https://creativecommons.org/licenses/by/2.5

Archaeopteryx lithographica (Berlin specimen).jpg by H. Raab (User: Vesta) – Own work, CC BY-SA 3.0