The hidden “armor” found in the skin of a giant ground sloth

The hidden “armor” found in the skin of a giant ground sloth

Argentinian scientists used X-rays on a 13,000-year-old sloth skin and discovered thousands of tiny bones on the inside.

After having managed to determine the precise age of the famous mummified skin of a Mylodon (an extinct species of giant ground sloth) the same scientists continue their quest to further understand this ancient animal.  This well-preserved skin was found on a scientific expedition, led by the Museo de La Plata (UNLP) at the end of the 19th century and continues to shed some fascinating results.

The scientists applied a rare diagnostic technique in the study of paleontological remains: X-rays. The results of the analysis, which have just been published in the Journal of Morphology, allow us to infer previously unknown aspects of the biology and evolution of the huge sloth Mylodons.


giant sloth mylodon
Weighing between 1,000-2,000 kilograms (2204-4409 lbs), the mighty Mylodon reached lengths of about three meters long (10 feet). / Illustration: Nestor Toledo

The skin is 13,200 years old and is only a portion of an animal whose largest specimens reached more than one ton in weight and three meters in length. It is believed that the ideal conditions of the cave in which it was found led to its excellent degree of preservation (which includes mummified hair and other soft parts).


mummified giant sloth
Although there is talk of “mummified” skin, in reality, it is unclear if the term exactly applies to the way the famous “sloth leather” was preserved. / Photo: Nestor Toledo

The pattern of the bones

Embedded in the skin is a layer of thousands of small bones ranging from the size of a lentil to 2 centimeters in diameter. These well-ordered structures caught the attention of the study authors, who decided to X-ray it using portable equipment for veterinary use. “This feature of the skin of the giant sloths was already known about, but it was thought that these bone pieces were scattered at random, but we discovered that they are arranged according to a pattern,” says Néstor Toledo, CONICET researcher at the Faculty of Natural Sciences Museum of the National University of La Plata (FCNyM, UNLP) in Argentina.


giant sloth armor skin
This Image shows the disposition of the ossicles of the Mylodon and comparison in size with a human. | Image: CONICET


Thus, in the four radiographs taken, these tiny bones (called ossicles) – were shown to form rows or bands in some areas, and rosettes or stars in others. When looking for bibliographic references that could complement these observations, they found that, without knowing it, they applied the same technique that Wilhelm von Branco had used over a century before.

Wilhem von Branco was a German scientist who published a report on X-ray analysis in 1906.  X-ray technology had just been discovered a decade before, and he applied this new technology to different paleontological pieces, among which, coincidentally, was another Mylodon skin preserved in the Natural History Museum in Berlin.

To the great surprise of Toledo and his colleagues, von Branco had also noted the same patterns in the skin bones of the Mylodon that they were now seeing.

“By superimposing von Branco’s radiographs with ours, we came to understand that the rosette structures would be located in the area of ​​the spine and its surroundings, while the row ones would be located on the sides and near the legs.”

“With this information, we began to discuss the probable functional reasons for these positions, thinking of those bones as a kind of armor that would still enable movement,” describes Toledo.

Therefore, the skin over the parts of the body that needed greater flexibility (such as the armpits or the belly) could fold or wrinkle, while other parts benefitted from a more rigid structure. All these conjectures raised a new question: Why did these animals have such thick skin that was reinforced with these small bones?


armor skin

An actual armor?

“The strongest hypothesis regarding the function of this dermal skeleton is related to defense against other organisms: firstly, possible predators, but also individuals of the same species during combat (such as competing males) based on the behavior of some current mammals,” explains Alberto Boscaini, CONICET researcher at the Institute of Ecology, Genetics, and Evolution of Buenos Aires (IEGEBA, CONICET-UBA).

Although the first option casts some doubts as to which species would attack animals of this size, the main argument would be evolutionary in nature: the oldest records of ossicles in the skin date back to other Mylodonts of the Mylodontini group, which lived about 10 million ago and were quite a bit smaller.

“They could have been eaten by carnivorous marsupials and large birds, so having an internal shell would be a great advantage. It may be that this structure was inherited by subsequent generations, even as they increased in size,” says the expert.


ancient sloth
Artist’s rendition of a giant caiman (Purussaurus) attacking a giant ground sloth (Pseudoprepotherium)./Image by Jorge A. González


Related to the issue of evolution, another question appears, but this time it looks to the present. “In paleontology, resorting to current animals as a key to understanding those that no longer exist is very common and is known as the ‘principle of actualism’.

In this case, we can look to the only living mammal that still has bones in its skin: the armadillo, which has similar bone patterns as the sloths of the past, that is to say, the rosettes and rows of ossicles,” Boscaini describes.

Although for the researchers this could answer the purely functional questions related to the necessary rigidity and mobility in the different areas of the body, these similarities could also suggest a shared development pattern, “perhaps related to the expression of the same genes,” he points out.

Handling and cultural significance

The application of X-rays to the mummified skin was a delicate procedure that required the participation of specialized technicians to be able to use the equipment without moving it from the display case in which it is exhibited. More than a century prior, von Branco also used X-rays to examine a preserved skin but he went about it in a very different way.

To do so, he exposed the preserved “leather” to steam to soften it and then stretched it on a smooth surface on which he took X-rays. “The positive thing is that those plates have an extraordinary quality and were very useful for our study, but in truth, it is an unthinkable thing to do today: it is practically an attack on a piece that is unique and invaluable,” he explains. Leandro M. Pérez, also a CONICET researcher at the FCNyM and one of the authors of the work.


giant sloth skin armor
Leandro Pérez, one of the researchers, next to the showcase where the skin is exhibited. Photo: Courtesy of researchers.


According to Leandro, the enactment of the Argentinian Law for the Protection of Archaeological and Paleontological Heritage in 2003 was very important to prevent the manipulation of these types of remains or fossils. “The sloth skin that we have in the local museum is a real treasure: it is likely that nothing remotely similar will be found again.”

“The mummified skin has some very deep cuts and even some parts missing because in the ’70s samples were extracted for the first radiocarbon dating, a method that looks for the presence of isotopes of a certain chemical element within the sample. Nowadays, even though it is wrinkled and dehydrated, we know that it should not be modified (even though that is how we usually take X-rays) which is why in many areas the bones are superimposed on each other,” added Pérez.


-By Mercedes Benialgo, CONICET La Plata

Bibliographic references:

Nestor Toledo, Alberto Boscaini, Leandro Martín Pérez. The dermal armor of mylodontid sloths (Mammalia, Xenarthra) from Cueva del Milodon (Ultima Esperanza, Chile). Journal of Morphology. DOI:

Original text: