‘Oh my God, that’s a beautiful bone!’ What a rare sloth fossil tells us about ancient Santa Cruz County

Last spring, when local paleontologist Wayne Thompson received a photo of an object that looked like a bone from Tara Redwood School students and their families, he wasn’t sure what to make of it. 

So he asked the students to bring the item into the Santa Cruz Museum of Natural History, where he’s the paleontology collections advisor. 

“When they came to the museum and rolled back the towel, I was floored,” he recalled. “Because I knew it was significant. I knew right away it was a fossil because it was unlike any living creature.” 

After a bit of analysis and by process of elimination, Thompson realized it was likely the arm bone of an ancient sloth. He asked sloth experts for their opinions and they confirmed it was a Jefferson’s ground sloth, which was likely 11,500 to 300,000 years old. 

The students had unearthed the first recorded sloth bone in Santa Cruz County, and one of very few ever found in California. 

“This discovery is way up there on the scales of human interest and within the field of paleontology, it’s pretty far,” said Thompson. “The colleagues that I’ve shown it to, the first thing that comes out of their mouth is, ‘Oh my God, that’s a beautiful bone!’ Because it’s so well preserved.”

Tara Redwood School teacher Bryn Evans told KSBW-TV that students found it while on a field trip in the Santa Cruz Mountains last spring. Located adjacent to Nisene Marks State Park, Tara Redwood School is a forest school serving toddlers and preschool-aged children through sixth grade. 

They were building a dam and looking for crawdads when one of them brought it to Evans, and said, ‘This isn’t a stick, this is a bone.’”

Thompson told Lookout about the discovery’s significance, how it got its name and what this means for understanding what the Santa Cruz County region was like in the Pleistocene era. 

This interview has been edited for clarity and brevity. 

Lookout: What is the significance of this sloth bone discovery?

Thompson: This is probably the most significant artifact found in Santa Cruz County for the last 40 years. It’s significant in California as well because only a handful of Thomas Jefferson’s ground sloths have been known from California. They’re known latitudinally on the West Coast, all the way from the Yucatan in Mexico, all the way up to the Yukon in Canada. They’re less well-known on the West Coast; they were prominent on the East Coast. The evolutionary stock of this species came from South America 9 million years ago – that was the first migration of sloths into North America. Then, depending on whether the Isthmus of Panama was open or closed, depending on the rise and fall of sea level, there were various waves of migrations from South America. But because of its rarity in California and not being known from Santa Cruz County before, it’s extremely significant.

Lookout: Where does the name “Jefferson” come from?

Thompson: Thomas Jefferson was the first person to find remains of what was then not known to be even a sloth because it was in 1797. And ground sloths were not known until he found them in Cromer Cave in West Virginia. But he thought it was the claw of a gigantic American living lion still roaming around in the forest when he first found it. He published a paper on it, it was one of the first scientific papers in American paleontology. He published that in 1797, so he was the first person in North America to publish on ground sloth fossils. 

He actually carried the arm bones and the claw to Philadelphia in 1797 to accept the vice presidency. He gave a talk to the Philadelphia Academy of Natural Sciences on his trip to accept the vice presidency about the bones that he had found. Then, two years later, he got together with a paleontologist, Caspar Wistar, and confirmed that these are the bones not of a lion, but of what would come to be known as a giant ground sloth. Until that time, nobody knew that something like a ground sloth existed. There were six living species of sloths and they all inhabited trees and they were all tiny. So this was the first evidence that these ground sloths got gigantic and lived on the ground. Since Thomas Jefferson’s time, we’ve found ground sloths in all kinds of various environments. There’s even oceanic sloths that live their life in the ocean and feed on seaweeds and algaes and things. So they’re fascinating.

Lookout: How does this discovery of a sloth bone in Santa Cruz County contribute to our understanding of what the region was like in the Pleistocene age (ice age)?

Thompson: In terms of the habit and habitat of Jefferson’s ground sloths, they were what’s called browsers. They would go from tree to tree stripping the leaves off with their giant claws. Unlike something like a bison or a mammoth, they were not grazers. So browsers go from tree to tree, grazers are like lawnmowers out in the grasslands. 

Where this was found in Santa Cruz illustrates that it was a river environment that was forested, probably spruce and alder, and some hardwoods perhaps. It was cool. It was probably foggy. … The habitat for these creatures probably extended out into the continental shelf much more than we see today. For example, mammoths were walking out to the Channel Islands [off the Santa Barbara coast]. So it probably lived in a river environment, a creek environment with heavy forest, where it browsed on the leaves of the trees that lived in the corridor of the rivers. That’s actually one of the reasons why they would have survived in either drier or colder environments at lower latitudes because they were protected in these protected corridors of the rivers. Whereas in other areas where it was a lot drier or a lot colder, they couldn’t survive. Their migrations – and the fossils that give us evidence of that – indicate that they did have a wide range of latitude and tolerance to cold and rain. But they weren’t impervious to it and so they did migrate northward and southward. 

Lookout: What do you think about the students being the ones to find this? What is the significance of that?

Thompson: The first word that comes to mind, when I think about that, is perseverance – on the part of the kids and their families. Because they had originally reached out to another large museum in California, and they came back with, “Well, it might be a flipper bone of a whale.” And they were not convinced of that, so they contacted me at the Santa Cruz Museum, and they sent me a picture. I was like, “I’m not really sure. I’ve never seen anything like that. In fact, is this a fossil? Could you bring it in?” The family who was part of the school group, who had the bone at the moment, made an appointment with me at the museum and we met up. This was shortly after it was discovered. When they came to the museum and rolled back the towel, I was floored. Because I knew it was significant. I knew right away it was a fossil because it was unlike any living creature. Sloths belong to a group of animals called Xena arthropods. That means strange, or unusual, bones, and their bone structure is unlike any other mammal.

Without the kids, without people out there having fun and finding these things and doing the right thing and bringing them into the museum, paleontologists wouldn’t know what’s out there. 

So if it weren’t for the kids, in then-kindergarten, we wouldn’t know about this sort of thing. It’s not scientists going out there and searching. It’s all the people of the community that are coming together. I’d say 90% of our significant finds in Santa Cruz, and California, really anywhere, [are] because of the community bringing it forward for scientists, for paleontologists to look at. If it weren’t for that, we wouldn’t know about this stuff. I’m very fortunate to have these incredible kids out there, probably budding paleontologists at this point. They’re so excited about what they found and what it means for our community. It’s really heartwarming.

Lookout: How is this discovery more significant than the recent finding of a mastodon tooth?

Thompson: Mastodons and sloths lived in the same environment. The American mastodon, Mammut americanum, was long thought to be living here in California, but recently it was described as a new species, Mammut pacificus. It’s a Pacific mastodon. Mastodons have been known from our area for quite some time, and they were much more prevalent, much more common than the ground sloth, which are much rarer even though they lived in the same environment. We don’t know, in terms of population dynamics, whether they outnumbered or were the same in terms of numbers relative to the mastodon. But, in terms of significance, it’s more significant just because there’s fewer of them. We know very little about them. There’s hundreds of specimens of Pacific mastodons in California, whereas there’s only a dozen or Jefferson sloths that are known from California, and before this summer, we didn’t even know they were in Santa Cruz. I think sloths are in vogue, popular culture right now. Everybody loves sloths. But not many people know that there were these gigantic, extinct ground sloths. Most people love the six species of living sloths that live in trees and have cute faces. But these formidable giants, not many people know about them. So it’s significant in that regard as well. 

Lookout: Anything else to add? 

Thompson: I think we’re just really fortunate to have this fossil at this time. It most likely has been dependent on our current El Niño cycle being back into the erosional part of that cycle. The mastodon tooth came about during the front end of the El Niño cycle, and now this one has come forth. I’m 99% certain that there’s a connection there. The first mastodon skull that was ever found in Santa Cruz County came during the 1980 El Niño cycle that was even bigger than the one that we’re having now. So there’s connections there in deep time that you start to pick up on if you engage with that longer cycle.

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