National Geographic

Toxodonts Traveled North

Not so very long ago, giant ground sloths trundled through the forests and over the grasslands of North America. Like the sabercats and mammoths of their time, the shaggy beasts and their armored glyptodont cousins were part of the recently-lost Pleisotcene megafauna. Yet the sloths and their kin had a very different backstory from the rest of the characteristic, charismatic megamammals that inhabited Ice Age North America.

Whereas mammoths and sabercats migrated in from Eurasia, and horses and camels evolved in North America itself, the sloths and glyptodonts were southern pioneers whose lineages had evolved in the “splendid isolation”, as paleontologist George Gaylord Simpson called it, of South American before the Isthmus of Panama closed the gap between the Americas around 3 million years ago. The entire transition was more haphazard, with some species jumping continents earlier and others later, but the “Great American Interchange” appears to have peaked when prehistoric Panama formed. As North America’s elephants, carnivorans, camels, horses, and other lineages pushed south, armadillos, terror birds, possums, and their neighbors made their way north. Paleontologists are still discerning the pattern of this dispersal, and a short report in the latest Journal of Vertebrate Paleontology places yet another South American lineage in ancient North America.

Toxodon was one of the first of South America’s Pleistocene mammals to come to the attention to western scientists. Charles Darwin himself purchased a fine skull of this sturdy quadruped from a farmer in Uruguay, later described and named by English anatomist Richard Owen. The identity of Toxodon wasn’t immediately clear. The creature had the bulky stature of a rhino and a hippopotamus-ish head, but the details of its anatomy didn’t match either group. In time, paleontologists realized that Toxodon was part of a unique, totally extinct group of South American mammals called notoungulates. Superficially, many of these mammals resembled forms found elsewhere – from rodent-like varieties to hippo-mimics such as Toxodon – but they were a distinct radiation that evolved when South America was a remote island continent.

Until now, paleontologists thought that toxodonts never made it to North America. Their northernmost occurrences were pinned in southern Mexico. As Ernest Lundelius, Jr. and coauthors report, however, an isolated toxodont tooth found in Harris County, Texas indicates that these mammals traveled much further north than researchers previously expected.  Dated at 74,000 to 16,000 years old, and assignable to the genus Mixotoxodon, the upper third molar is a singular clue that toxodonts had a significant geographical reach.

If toxodonts reached North America, though, why didn’t they flourish like some of their other southern neighbors? That’s hard to say, and the toxodonts were not alone in having a tenuous hold on northern habitats. The formidable terror bird Titanis represented another southern lineage that has been found in Florida and Texas, but apparently never gained a strong foothold. After arriving in Texas about 5 million years ago, Titanis disappeared by about 1.8 million years ago.

In the big picture of the Great American Interchange, the northern species seem to have been more successful in the south than the southern species in the north, with sloths and their kin representing an abnormality. The reason for the imbalance has often been attributed to climate and evolutionary history, but, since we can’t watch these species interact directly, the uneven victories of the different faunas remains a puzzle made all the more confounding by the loss of North America’s ground sloths, South America’s sabercats, and their fantastic extinct neighbors.

Reference:

Lundelius, Jr., E., Bryant, V., Mandel, R., Thies, K., Thoms, A. 2013. The first occurrence of a toxodont (Mammalia, Notoungulata) in the United States. Journal of Vertebrate Paleontology. 33, 1: 229-232

There are 6 Comments. Add Yours.

  1. Ross
    January 15, 2013

    Fascinating stuff. Do any other South American endemics make it further than Megalonyx jeffersoni in Alaska? http://pubs.aina.ucalgary.ca/arctic/Arctic53-3-213.pdf

  2. Brian Switek
    January 15, 2013

    Great question, Ross! I don’t think any of the South American mammal lineages made it as far as the sloths, although I’m not entirely sure of their entire distribution. It seems that some SA lineages – toxodonts, terror birds – stuck to coastal areas in southern North America. I wonder why that is, and why the sloths were so much more widespread.

  3. Ross
    January 16, 2013

    Thanks for the reply! I guess sloths are just weird. The image of Megalonyx in the snowy North is just bizarre. I guess it works the other way as well. Coelodonta never made it over Beringia, while Mammuthus, Cervus, Alces and others did. Without knowledge of the environmental and ecological constraints on the living animals we’ll probably never know.

  4. Hai~Ren
    January 16, 2013

    I think the North American porcupine counts as one of the more successful southern species. It seems like most other southern species, such as capybara, terror birds, and other xenarthrans like anteaters, glyptodonts, pampatheres and armadillos did not manage to spread very far north.

  5. Steve
    January 16, 2013

    Great post.
    One thing to bear in mind when writing about southern US coastal areas is that what constitutes the coast would have been completely different at the height of the last ice age. For example, a lot of the area between Yucatan and Florida would have been dry.

    Therefore the S American beasts found from Texas to Florida would have had a lot more land to roam at that time.

  6. Stefanus Sutopo
    January 18, 2013

    I think that the reason why toxodonts and phorusrhacids never make it up north is that they just weren’t adapted for high latitude life from the start (AFAIK, no toxodont or phorusrhacid fossils have been found from South America’s far south; correct me if I’m wrong)

    Still, this is still a very big “IF”, and perhaps some of the more established, adult, scientists can enlighten me on this :)

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