This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Paleontologists are always straining against the pull of the recent. In short, this means that the closer in geologic time you get to the present, the better the fossil record is - we have a better picture of life 50 million years ago than 100 million years ago, and we understand life 100,000 years ago better than 50 million years ago. Yet, despite over two centuries of dedicated searching and study, there's still a great deal left to uncover in even the relatively recent fossil record. A 4.9 million year old giant sloth is one such example.
Zacatzontli tecolotlanensis, named by paleontologists Gregory McDonald and Oscar Carranza-Castañeda, is a sloth that seems out of place. Known only from a lower jaw and two broken teeth, this beast from Jalisco, Mexico is a messenger from another place. That's what makes it so special.
The new sloth lived right in the middle of a critical window for animals of Americas. Between 9 and 1.6 million years ago, as North and South America edged ever closer and eventually joined through prehistoric Panama, various animals moved across the gap. That's why there are jaguars in Brazil and armadillos in Texas. Giant sloths were part of this Great Biotic Interchange, too, and Zacatzontli suggests that the trundling beasts came in waves.
In terms of family relationships, McDonald and Carranza-Castañeda point out, Zacatzontli more closely resembles giant sloths of South American origin than those that eventually evolved in the north. This is despite the fact that giant sloths had already been moving north for millions of years prior. What this means, the researchers note, is that there was an earlier wave of sloths moving from south to north and a later one now represented by Zacatzontli and another giant sloth called Meizonyx in El Salvador. These sloths didn't have an easy time of it. At 4.9 million years ago there was still a significant stretch of water between the continents. But somehow Zacatzontli made it, and now bears the name of the Aztec god of travelers.
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Three views of the Zacatzontli jaw. Credit: McDonald and Carranza-Castañeda 2017
Name: Zacatzontli tecolotlanensis
Meaning: Zacatzontli is named for the Aztec god of travelers, and tecolotlanensis is named after the town Tecolotlán where the sloth was found.
Age: Miocene, about 4.9 million years ago.
Where in the world?: Jalisco, Mexico.
What sort of organism?: A giant ground sloth.
How much of the organism’s is known?: A partial lower jaw and two broken teeth.
References:
Royo-Torres, R., Upchurch, P., Kirkland, J., DeBlieux, D., Foster, J., Cobos, A., Alcala, L. 2017. Descendants of the Jurassic turiasaurs from Iberia found refuge in the Early Cretaceous of western USA. Scientific Reports. doi: 10.1038/s41598-017-14677-2
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