To Get on the Same Frequency as Early Reptiles, this Scientist Compares Tiny Fossilized Bones to Modern Lizard Ears

Using the fossil record and modern cold-blooded critters, paleontologist Kelsey Jenkins recreates the hearing capabilities of ancient animals

During the Late Permian period more than 255 million years ago, early reptiles were in the midst of a sensory revolution. Around this time, species evolved the ability to regulate their internal body temperature and honed senses like smell and sight. “They were going from simply surviving on land to actually thriving there,” said paleontologist Kelsey Jenkins, a Peter Buck postdoctoral fellow at the National Museum of Natural History.

Jenkins’ research revolves around how these early reptiles finetuned their hearing. Recently she examined fossils of the Permian reptile Youngina capensis, a lizard-like animal from South Africa. Youngina was one of the earliest relatives of modern reptiles and some fossilized skulls preserve stapes, the tiny middle ear bone that transports sound vibrations to the inner ear.

To gauge how Youngina heard the world around it, Jenkins and her colleagues recently compared these fossilized stapes to the middle ear bones of modern geckos, which boast some of the best hearing of any reptile. The team, which also included Hans Sues, one of the museum’s curators of vertebrate paleontology, discovered that Youngina was likely capable of hearing relatively high frequencies.

Smithsonian Voices recently asked Jenkins about her interest in the hearing capabilities of ancient reptiles and the evolutionary implications of her team’s new findings, which were published in December in the journal Nature Communications.

How did you first become interested in investigating the evolution of hearing in early reptiles?

I was initially given a research project on reptiles and I didn't think I was going to become a reptile person. But the more I learned about them, the more interested I became. I've now been in the reptile world for over ten years, and I notice the gaps that are occurring in our research and our knowledge on this group.

For example, we have a ton of research on early mammal hearing — I've heard multiple professors call it the greatest evolutionary story ever told. It's this classic textbook example of studying evolution through the fossil record because we see the lower jaw bones of early mammals become smaller and migrate over time to the back of the jaw and eventually become the bones of the middle ear.

But when you look at the reptile side, there's almost nothing. And that’s because we have a mammal bias. We see a lot more paleontologists study mammals than reptiles because we are mammals and find our evolution really interesting. But reptiles are amazing and are way more diverse than mammals with around 10,000 different species. I was struck by why we don't know very much about this major evolutionary transition in this huge group of animals that have been around for hundreds of millions of years.

Why was the Permian period such an important time period for auditory evolution in amniotes?

There's always been people interested in the Permian but this time period tends to get overlooked. People, including researchers, tend to gravitate towards the times when dinosaurs started to rise up or when you see the first fishy thing with legs crawl up onto the land. And so a lot of people have treated the Permian like this in-between time where things aren't that interesting, and it's just totally not true.

If we think about that first fish with legs that crawled onto land around 385 million years ago, it had to deal with this host of first order problems, like gravity and not drying out and breathing air. And once it dealt with all those problems, there's a whole series of second order problems that you have to contend with.

This is the interval of sensory evolution. So you begin to see senses like smell start to get really good and the ability to regulate your body temperature starts to appear. During the Permian, you have all these changes to your eyes, because now you have to see in air instead of seeing in water. And of course, hearing is a part of that sensory evolution too. And we see throughout the Permian, reptiles are getting better and better at hearing until they can hear really well in the Triassic.

Why was Youngina such an interesting test subject for understanding the early evolution of hearing?

Youngina is a pretty well-known fossil that was discovered more than 100 years ago. It’s one of the oldest stem reptiles that evolved right before the evolution of modern lineages of reptiles. So you get this diversification of crocodiles and lizards and turtles and Youngina comes right before that. So it's a good way to look at the ancestral states of these early reptiles.

We also knew that Youngina had this middle ear bone preserved that doesn’t preserve well in ancient reptiles because that bone is so small and delicate. So Youngina fit our two main criteria: something that lived close to the origin of reptiles and had this important ear bone preserved.

How did your team utilize modern lizard specimens to help gauge this ancient reptile’s hearing capabilities?

To figure out how Youngina’s ear actually works, we need to know how reptile ears work. So we looked at ear bones from a couple different geckos, which probably hear the best of any modern lizard. They're one of the few lizards that make noises and even bark at each other.

We made models of the gecko ears and we could actually watch mechanically what happens when a sound wave hits that eardrum. How does that make the rest of the system move? And once we know how each little component moves, like the eardrum and the cartilage and the ear bone, we can start applying that to fossils.

And so we used Youngina and a couple of other older fossils because our hypothesis was that some of these ancient reptiles hear at least decently and some don't hear very well at all. And we see that exact thing happen in those fossil ear bones: Youngina wasn’t hearing as well as a gecko but it's definitely hearing a bit better than its ancestors could. So we found that nice transitional form right in the middle.

Do you have a sense of what kind of noises Youngina and its relatives would have been able to hear?

That’s a great question that we’re still trying to figure out. We looked at how much the middle ear bone could move, which is a proxy for slightly better hearing. But we don’t actually know the precise frequency that Youngina could hear.

If we think about our own hearing as mammals, we can generally hear really well. For example, we can hear the entire range of keys on a piano. Even though the highest keys may sound slightly fainter, we can still hear them. Most lizards can probably only hear the bottom half of these keys and early reptiles are probably only getting the bottom octave or two. We think Youngina is probably in the middle of that range.

Were you surprised to discover that relatively high-frequency hearing was likely present in the common ancestor of reptiles?

I was not surprised because if we look at the different ear bones in modern reptiles and all these ancestors, there's something about Youngina that always looked a little bit different. It is a proper transitional form: it's a little longer and a little thinner. And so it made sense to me that it could probably hear better than people thought because hearing is a spectrum.

But I do think this alters what people have been thinking for a really long time. Some have suggested that different lineages of reptiles — like crocodiles, lizards and turtles — evolved high-frequency hearing independently. While that's not impossible, it seems unlikely because usually the simplest explanation is the most appropriate. It makes much more sense to me that the ancestor of reptiles probably heard at least decently, and maybe these various lineages further refined their hearing along the way.

Hopefully this research shows that good hearing was potentially ancestral to reptiles. And new fossil discoveries are also supporting this notion that other early reptiles also had semi-good hearing. It’s an exciting time to be in this little region of research, because everyone's finding the same thing at the same time.

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Jack Tamisiea | | Read More

Jack Tamisiea is a Science Communications Specialist at the Smithsonian's National Museum of Natural History. In addition to covering all things natural history for the museum's blog, Smithsonian Voices, he tracks media coverage and coordinates filming activities for the museum's Office of Communications and Public Affairs. Jack recently completed his masters in science writing at Johns Hopkins University and his writing has appeared in the New York Times, Scientific American, National Geographic and other science-focused publications. In his free time, he loves exploring the outdoors with a sketchbook and camera. You can read more of Jack's work at https://jacktamisiea.com.