​The Distribution of Dermal Ossicles in

​ Giant Ground Sloths
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By Harrison Sturgeon

Among extinct giant ground sloths a select few species exhibit integumentary armor. These sloths had small bones called dermal ossicles distributed within their skin, providing defense. The bones vary in shape, size, and distribution.

Dermal ossicles are rarely found directly associated with other skeletal elements, making the study of their distribution relative to the body difficult. This project aims to use digital 3D modeling to examine the distribution of dermal ossicles within ground sloth skin.

​Click on the buttons below to see the abstract and poster presented at the 2024 Zzyzx Desert Symposium! 
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First, let's look at the origins of ground sloths and their close relatives to understand where this bizarre trait comes from...

Meet the Modern Xenarthrans...

Anteaters (Vermilingua)

Giant Anteater (Myrmecophaga tridactyla), © MarcosHalemFelix (CC BY-NC 4.0)

Armadillos (Cingulata)

Six-Banded Armadillo (Euphractus sexcinctus), photographed at Safari de Peaugres

Sloths (Tardigrada)​

Three-Toed Sloth (Bradypus variegatus), © SergioDelgado (CC BY 2.0)

​Anteaters, armadillos, and sloths are extant members of the order Xenarthra. Xenarthrans are categorized mainly by extra contact points on their lumbar vertebrae, which give them extra support and strength in their lower bodies. 

...And Their Extinct Relatives

Pampatheres

Pampatheriidae holmesina, photographed at PUC de Minas Gerais

​​Pampatheres first appear in the fossil record during the middle Miocene. Much like armadillos, their carapaces feature banded plates that allow them to flex.

Glypodonts

Glyptodon clavipes, © JJonahJackalope (CC BY-SA 4.0)

​Glyptondonts appear earlier, during the middle Eocene. Their carapaces were composed of immobile scutes like those in a turtle shell. Great for defense!

Ground Sloths

Paramylodon harlani, © Nikhil Iyengar (CC BY-SA 4.0)

​Ground sloths arise around the Eocene - Oligocene boundary. It may appear that ground sloths are missing the armor that their relatives exhibit, but that isn't quite the case...

Ground Sloths in Anza-Borrego

Giant ground sloths appear in the fossil record around 35 million years ago in South America. As their name implies, they were a lot larger than any extant sloths. Some species of ground sloth are estimated to have been 10 feet long and weighed over 2000 pounds! Unlike modern sloths, these gentle giants roamed at ground level rather than taking to the trees.

Starting approximately 8 million years ago, three main genera made their way into North America in waves. Fossil ground sloths are found throughout most of the continent, reaching as far as Northwest Canada. 

Megalonyx, Paramylodon, and Nothrotheriops are the main genera represented in the Anza-Borrego fossil record. By around 11,000 years ago giant ground sloths had gone extinct in North America, likely due to changes in climate and human hunting. 

What are Ossicles?

​Dermal ossicles (or osteoderms) are pieces of bone or concretions within the skin of an animal. They are present in extant armadillos and reptiles. Dermal ossicles function as a suit of armor, protecting the wearer against the environment and predators. In some species, they also help to regulate body temperature. Among living mammals, only armadillos exhibit this trait.

The extinct Pampatheres and Glypotodons shown earlier have carapaces made from dermal ossicles. Certain dinosaur species exhibited this trait as well. The heavily armored Ankylosaurs and Nodosaurs were covered in ossicles, while certain sauropods like Saltasaurus had more sparse coverage. The saurischian carnivore Ceratosaurus had ossicles along its neck and back, so this trait may have occurred more often than we currently have evidence for. 

Some ground sloths also had armored skin, though the expression of their ossicles differs from the examples above.

Crocodylus porosus, © Lando25 (CC BY-SA 4.0)

Ceratosaurus nasicornis, © Jens Lallensack (CC BY-SA 4.0)

Borealopelta markmitchelli, © Etemenanki3 (CC BY-SA 4.0)

Crocodiles use highly vascular ossicles under the scales for both defense and thermoregulation.

The line of bones hovering over Ceratosaurus'​ spine are dermal ossicles. Their function is still unclear!

An outstanding fossilized Nodosaur was recently uncovered during a mining project in Alberta, Canada. 

Dermal Ossicles in Ground Sloths

Fossilized ground sloth dermal ossicles look like normal rocks at first glance.

Dermal ossicles within a mummified ground sloth skin found in Cueva del Milodon, Chile.

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Dermal ossicles mostly appeared in a family of ground sloths called Mylodontidae. ​There are still a lot of unknowns surrounding the expression of this trait, due in part to the following challenges: 
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1. As with any long-extinct species, knowledge is limited to fossilized material. There could be more ground sloths that exhibit this feature waiting for us to find!                       
2. Sloth ossicles are held in place by skin, and don't connect to each other or any other bones. As a result, they're usually found separated from the rest of the animal. This makes attributing them to a species or studying their function and location in the body difficult.

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Another big question involves the distribution of ossicles throughout the ground sloths skin. 

​Studies to date hypothesize that dermal ossicles demonstrate patterns such as rows around elongate skeletal elements like the ribs and spine, and mosaics (stars and rosettes) around rounded ones like the pelvis.

Definitive evidence of this correlation requires a specimen with ossicles in their original orientation and a clear link to a portion of the ground sloth’s skeleton. 

​Our Paramylodon Specimen
​

​In November of 1960 paleontologists from Los Angeles County Museum collected a fossil specimen of Paramylodon harlani from Anza-Borrego Desert State Park (ABDSP). The fossil was stabilized and stored until 1997, after which it was returned to ABDSP.

Consisting of a partial innominate (pelvis) resting on a mat of fossilized dermal ossicles, this find is noteworthy due to its preservation of the ossicles in their original orientation (without the skin) and the innominate in life position relative to them.

This fossil satisfies the requirements mentioned above, and could teach us a lot about how exactly dermal ossicles were distributed throughout the body. 

Step 1: 3D Modeling​

First, the fossil was photographed and a 3D model of it was created. Over 200 photos of the specimen were taken from different angles and uploaded into a 3D modeling program called Metashape. The program analyzes the photos and stitches them together, creating a manipulatable, three-dimensional, digital object.  
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LACM_1568_V77700 Ground Sloth by anzaborregopaleo on Sketchfab

Step 2: Combining​ and Fitting
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Next, the specimen was superimposed over a complete skeleton of Paramylodon harlani. The large piece of pelvis on our specimen was rotated and aligned with the pelvis of the model skeleton. 
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Step 3: Analyzing
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Finally, we can