Ichthyosaurs in Nevada: The mystery of an ancient reptile fossil graveyard : Short Wave This mystery begins in 1952, in the Nevada desert, when a self-taught geologist came across the skeleton of a massive creature that looked like a cross between a whale and a crocodile. It turned out to be just the beginning.

Ichthyosaurs were bus-sized marine reptiles that lived during the age of dinosaurs, when this area of Nevada was underwater. Yet paleontologists found few other animals here, which raised the questions: Why were there so many adult ichthyosaurs, and almost nothing else? What could have killed them all?

Paleontologist Neil Kelley says that recently, there has been a major break in the case—some new evidence, and a hypothesis that finally seems to fit. Neil talked with Short Wave co-host Aaron Scott about his theory of the case, and why it matters to our understanding of the past.

Fossil CSI: Cracking the case of an ancient reptile graveyard

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EMILY KWONG, HOST:

You're listening to SHORT WAVE from NPR.

AARON SCOTT, HOST:

Today, a mystery. It begins in 1952. A self-taught geologist by the name of Margaret Wheat was giving an old quarry in the middle of the Nevada desert a sweep - you know, like geologists do - when she uncovered an ancient skeleton in what would turn out to be an enormous graveyard. Neil Kelley has seen it firsthand.

NEIL KELLEY: It is incredible how many ichthyosaur fossils are present at this site. They're just tumbling out of the hills, basically.

SCOTT: Neil is a paleontologist at Vanderbilt University. And he says ichthyosaurs were marine reptiles that lived during the age of dinosaurs. Basically, picture a giant dolphin with four flippers.

KELLEY: I mean, they really are whale-sized animals. They're like gray whales or sperm whales.

SCOTT: At the time in the '50s, ichthyosaur fossils had been found in a few places around the world. And so it wasn't such a surprise to find them in Nevada. It might be a desert now, but in the late Triassic, it was a shallow seaway. So when Wheat reached out to some paleontologists at UC Berkeley...

KELLEY: And said, I found these giant ichthyosaur fossils in Nevada, they said, that's cool. We already have a lot of those, so we're not that interested.

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SCOTT: She didn't give up and convinced the paleontologist Charles Camp to visit this reptile graveyard out in the middle of nowhere.

KELLEY: And quickly, they realized that this was not just the same thing that they already had. This was something very new.

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SCOTT: The sheer number of fossils was incredible. Some were nearly 50 feet long, larger than any known ichthyosaurs at the time. Camp described them as monsters and went on to spend years excavating the site. He named this new ichthyosaur species shonisaurus popularis. And he and Wheat lobbied to get Nevada to make it the state fossil and to recognize the site as a state park. Neil Kelley first learned about the Berlin Ichthyosaur State Park as a fossil-loving kid. But he didn't get to visit until 2014 when he was a postdoc at the Smithsonian.

KELLEY: And when I saw it, I was just like, wow, this is incredible. How did so many of these big animals end up on this one rock layer, which represents one interval of time? So it really does call for some kind of explanation.

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SCOTT: In fact, it presented a mystery so big that paleontologists are still trying to solve it. Why are there so many adult ichthyosaurs in this one location, what killed these ancient underwater giants, and what can we learn from it? So today on the show, we take up our best magnifying glass, and we set out to solve one of the coldest cases on the continent - like, hundreds of millions of years cold. And there's been a big break in the case. Consider this true crime, SHORT WAVE style. I'm Aaron Scott, and you're listening to SHORT WAVE, the daily science podcast from NPR.

To begin, we need to step back to the date in question - Earth, the late Triassic period, some 230 million years ago.

KELLEY: This is really the sort of peak of Pangea - of the one global supercontinent, which sometimes people imagine was just, like, the base state of the planet, and then it changed. But that's not the case at all. In fact, the continents have been cruising around the planet for billions of years, and they lock together and form giant mountains and then they fall apart. And so at this time, all the continents had basically stuck back together again, and we are on the sort of western coast of Pangea, just above the equator, probably.

SCOTT: So these giant, meat-munching crocodile whales are swimming about, minding their own business, living their best Triassic lives until presumably, something happens that kills a whole bunch of them. The question was, what was it? The scientists had a few suspects.

KELLEY: The initial hypothesis by Charles Camp, who first excavated the site, was that it was a mass stranding, which is a great hypothesis because we see that in whales today, which are the closest sort of evolutionary analogues for ichthyosaurs. So he had imagined that these large animals had gotten trapped by receding tides, and the smaller animals had escaped. And so he was already realizing there was something strange about this site with it being all one thing without the smaller things that we would expect to find.

SCOTT: Neil and his Smithsonian colleagues began by focusing on the rock layer that first captured so much attention. The area is now protected by a building, which means they couldn't actually excavate the fossils. So the team used high-tech scanners to create a 3D data set.

KELLEY: I was not prepared for how abundant fossils would be all over the canyon at many, many layers outside of the site. And that really transformed my thinking that this was not just about one group of animals that had an unlucky day but really about longer term cycles of animals doing their thing, living in an environment over thousands of years. And that's the biology that was being preserved at the site.

SCOTT: And so that was kind of your first clue that maybe it's not just a single mass stranding because there's other creatures outside of this single moment in time that are concentrated here.

KELLEY: Right. Yeah. I mean, for the mass stranding, the rock evidence basically had already shown us that that wasn't going to be a good model because the rocks don't support an idea of it being on a beach. It's clearly deeper water, although not really, really deep. But yeah, I was really getting interested in the fact that ichthyosaurs had clearly been living in this habitat for a long interval of time, and I felt like there was more to figure out.

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SCOTT: And so suspect No. 1, mass strandings, turned out to be innocent. Neil quickly ruled out suspect No. 2 - a voracious, eight-armed serial killer believed to arrange its prey just so.

KELLEY: There was a widely hyped but undersupported idea that there might have been a giant squid that somehow was involved with this. There's never any evidence to support that.

SCOTT: So on to suspect No. 3 - a natural disaster.

KELLEY: So volcanic eruptions leave a chemical signature in rocks, especially with the element mercury. And we tested it and found no good evidence to support that that was the mechanism.

SCOTT: So next up, Neil investigated suspect No. 4 - toxic algal blooms.

KELLEY: We also looked at variations in the amount of organic matter - carbon, basically - in the rocks, which can tell you something about oxygen levels. Possibly could be an index for algal bloom or a red tide, but we couldn't find any evidence to support that, even though I would have loved for that to be the story.

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KELLEY: It really was disappointing at first not to have some smoking gun to explain these particular skeletons. But then again, as I sort of ruminated and thought we might be asking the wrong questions in just trying to focus on this particular layer. There's something else going on here which should help us to understand why there's so many of these animals in this area for, you know, thousands of years, at least.

SCOTT: So he decided to pursue the lead that also struck Charles Camp as odd - why weren't there fossils of other animals?

KELLEY: Most, or really almost all other places in the world where we find ichthyosaur fossils, they're living alongside many other marine reptiles. We didn't really find many other fish or sharks, which we also would expect to find in the same types of environments 'cause they're all living in the same places in general, feeding on the same types of food sources. So we were really surprised not to find those other creatures.

SCOTT: Complicating that is that you also didn't find young ichthyosaurs. I mean, that this was mostly just adults.

KELLEY: Right. So it seems like there's a lot of these animals living around, so we would expect if they're staying put in this place all the time, that we would get the entire sort of segment of the population. That we would have maybe some babies, some young, some fully grown adults. There shouldn't be a bias towards just one particular size class.

SCOTT: But then Neil discovered something while going through Charles Camp's original collection of fossils at UC Berkeley.

KELLEY: This particular specimen was actually mixed in with some invertebrate fossils, which are really useful for reconstructing geologic age and environment. And so I was looking through those collections and I saw this small jaw fragment. And I realized that looks like a shonisaurus jaw fragment, but it's two centimeters long. It's really small. That's weird.

SCOTT: So he went back and reread Camp's original paper and noticed something that he had missed before.

KELLEY: He said, oh, yeah, and there's embryos in these fossils. But he just says that, and he never drew it or photographed it or described how many embryos or what sizes. And I can understand why he wouldn't think that was significant because embryos had already been discovered from other ichthyosaurs from Germany, for example. And this actually proved that ichthyosaurs gave birth to live young, which was a surprise, maybe, because they're reptiles and was an important piece in understanding their biology.

SCOTT: This clue that had barely piqued the interest of his predecessors would break the case open for Neil, especially after he found more tiny fossils at the site that looked to be from embryos or newborns.

KELLEY: And that started to reawaken my understanding of how large marine vertebrates work today. When the babies come out, they're not that big. They're still at risk to predation by other animals living in the environment. So one adaptation that many, many marine vertebrates have evolved is to try to find a secluded place that there aren't very many other things living in and give birth in those areas.

SCOTT: An ancient ichthyosaur birthing ground. This was the hypothesis that seemed to fit, particularly with all the rock evidence showing that these ocean giants returned over and over again.

KELLEY: Probably on the order of hundreds of thousands to a million-plus years, which is a huge range for normal people. For geologists studying 230 million years ago, it's about as good as we can get.

SCOTT: But that they're coming here for at least hundreds of thousands of years.

KELLEY: It has to be. It just has to be. Yeah. Because it's just layers and layers and layers and layers in the hillside. And these are being deposited over many seasons, many millennia.

SCOTT: What are the ramifications of the data and your hypothesis and conclusion? I mean, what does it change to know that these giant marine reptiles were migrating to give birth hundreds of millions of years ago?

KELLEY: One important thing to keep in mind is just that we really can't understand the modern landscape without understanding history. I think people recognize this for human history and society, that we have to know sort of how we got to where we are to really understand where we're at. And I think...

SCOTT: Right, right.

KELLEY: ...The same applies for the natural world as well, that if we don't really know when things evolved, when behaviors and relationships in the ecosystem evolved, we don't fully understand how important they are and how much impact they have on, you know, the stability and future of ecosystems today.

SCOTT: Thank you so much for taking us back in time, Neil.

KELLEY: Absolutely. It was great talking to you.

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SCOTT: If you have a science question or an idea for the show, we want to hear it. Send us an email at shortwave@npr.org. This episode was produced by Brit Hanson, edited by Gabriel Spitzer and Gisele Grayson and fact-checked by Anil Oza. Our audio engineer was Maggie Luthar. Rebecca Ramirez is our supervising producer. Brendan Crump is our podcast coordinator. Beth Donovan is our senior director of programming, and Anya Grundmann is our senior vice president of programming. I'm Aaron Scott. Thanks for listening to SHORT WAVE from NPR.

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