Armadillos are among the most recognizable of mammals thanks to the bony armor that covers most of their body. Their shell, technically known as a carapace, is made up of hundreds to thousands of small bones known as osteoderms. These form in the skin – just like some of the skull bones that cover and protect our brain – but unlike skull bones, osteoderms remain in the skin and do not fuse to or have tight attachments with other bones of the skeleton. In this regard, armadillo armor also contrasts with that of turtles. In these animals, the bones of the shell completely fuse with ribs and other bones of the skeleton and cannot be separated from them even in a skeletonized specimen.

Armadillo osteoderms have two basic arrangements: they can butt up against one another tightly on all sides, creating a large, rigid area of bone, or they can form strips that partially overlap one another, resulting in a flexible area of armor. The solid areas are known as shields or bucklers (typically an armadillo has a pectoral shield in front and a pelvic shield in back), and the intervening strips are known as bands. An armadillo’s armor also includes a head (cephalic) shield and some sort of tail covering (caudal sheath).

The variable number of bands in the carapaces of living armadillos clearly made an impact on early taxonomists, as the specific epithets of the first five named species were all based on this feature: Cabassous unicinctus (one band), Tolypeutes tricinctus (three bands), Euphractus sexcinctus (six bands), Dasypus septemcinctus (seven bands), and Dasypus novemcinctus (nine bands). In some cases, the number of bands is also reflected in the common name of a species. For example, D. novemcinctus, the armadillo that can be found in the southeastern US, is known as the nine-banded armadillo. But Cabassous unicinctus, a much larger species from tropical South America, is known as the southern naked-tailed armadillo rather than the one-banded armadillo.

Although the moveable bands of an armadillo’s carapace provide some flexibility in the carapace, most armadillos are incapable of completely rolling into a ball. This ability is exclusive to three-banded armadillos (genus Tolypeutes) and is reflected in their common name in Portuguese (tatu bola) and Spanish (tatú bolita), meaning “ball armadillo.”

Considering the emphasis early taxonomists placed on the number of bands in naming species, it should come as no surprise that the group that includes armadillos and their extinct relatives is termed Cingulata, which essentially means “belted ones.” This term was originally coined by Illiger in 1811 as a family-level name, but since that time it has gradually risen in Linnean rank. It is now recognized as an order, meaning that armadillos are considered to be as evolutionarily distinct as rodents, bats, elephants, and primates. Until relatively recently, cingulates were placed in the order Xenarthra along with sloths and anteaters, their closest living relatives. The group Xenarthra still exists, but it is now a “supraordinal” group (a Superorder, to be exact).

Although the fossil record of the earliest armadillos is relatively sparse (Gaudin and Croft 2015), it seems that their shell began as a covering of loosely connected osteoderms, all of which could move relative to one another (Simpson 1948; Oliveira and Bergqvist 1998). Only later did osteoderms in the front and back of the shell lose their mobility and start to form solid shields. This trend of shell reinforcement culminated in the glyptodonts: large to giant extinct relatives of modern armadillos whose shells lacked mobile bands and thus were essentially immobile – though a few glyptodont species did retain partial rows of overlapping osteoderms along the edges of their shell near their shoulder (Croft et al. 2007). At the other end of the spectrum of shell evolution, the famous horned armadillos of South America (family Peltephilidae) maintained a highly mobile carapace that basically lacked solidly fused areas. It apparently served them well enough until they went extinct about 10 million years ago (González-Ruiz et al. 2013).

It is important to point out that armadillos and other cingulates are not simply covered by a layer of small bones. Just as the bones of our skull are covered by skin and other tissues, osteoderms are embedded in skin and covered by a layer of keratin, the same tissue that forms hair, fingernails, and the outer covering of true horns (i.e., those of cattle and kin). These pieces of keratin are known as scales or scutes, and although they do not fossilize like the osteoderms themselves, they leave patterns of grooves on the surfaces of osteoderms that can be used to identify species in the fossil record and deduce evolutionary relationships. Hairs and glands are also embedded within and between osteoderms (Krmpotic et al. 2009), with the result that some armadillos are quite hairy.

The only other mammals that ever developed a shell remotely similar to that of cingulates are pangolins, a group of social insect-eating mammals now restricted to the Old World (Africa and Asia). However, their armor is composed of overlapping, purely keratinous scales that are periodically shed and replaced; there is no boney component. Pangolins are actually more similar to anteaters than armadillos in general lifestyle, and some are quite adept at climbing trees, an ability that has never evolved in armadillos.

Pangolins were once classified with xenarthrans in a group called Edentata, but it is now known that these mammals are quite distantly related to one another and evolved their resemblances independently because they occupied similar ecological niches on different continents. Thanks in part to their existence on an island continent, South America’s armored mammals evolved into a much wider diversity of species than pangolins, spanning more than four orders of magnitude of body size. There is nothing else quite like an armadillo, and we are fortunate that at least a few branches of the great cingulate evolutionary tree are still around for us to enjoy today.

References Cited:

• Croft, D. A., J. J. Flynn, and A. R. Wyss. 2007. A new basal glyptodontid and other Xenarthra of the early Miocene Chucal Fauna, northern Chile. Journal of Vertebrate Paleontology 27:781-797.
• Gaudin, T. J., and D. A. Croft. 2015. Paleogene Xenarthra and the evolution of South American mammals. Journal of Mammalogy 96:622-634.
• González-Ruiz, L. R., G. J. Scillato-Yané, C. M. Krmoptic, and A. A. Carlini. 2012. A new species of Peltephilidae (Mammalia: Xenarthra: Cingulata) from the late Miocene (Chasicoan SALMA) of Argentina. Zootaxa 3359:55-64.
• Krmpotic, C. M., M. R. Ciancio, C. Barbeito, R. C. Mario, and A. A. Carlini. 2009. Osteoderm morphology in Recent and fossil euphractine xenarthrans. Acta Zoologica 90:339-351.
• Illiger, J. K. W. 1811. Prodromus Systematis Mammalium et Avium. Berolini: Sumptibus C. Salfeld, Berlin.
• Oliveira, E. V., and L. P. Bergqvist. 1998. A new Paleocene armadillo (Mammalia, Dasypodoidea) from the Itaboraí Basin, Brazil; pp. 35-40 in S. Casadío (ed.), Paleógeno de América del Sur y de la Península Antártica. Asociación Paleontológica Argentina, Publicación Especial 5, Buenos Aires.
• Scott, W. B. 1903. Mammalia of the Santa Cruz Beds. Volume V, Paleontology. Part I, Edentata. 2. Glyptodonta and Gravigrada; pp. 107-227 in W. B. Scott (ed.), Reports of the Princeton University Expeditions to Patagonia, 1896-1899. Princeton University, E. Schweizerbart’sche Verlagshandlung (E. Nägele), Stuttgart.
• Simpson, G. G. 1948. The beginning of the age of mammals in South America. Part I. Bulletin of the American Museum of Natural History 91:1-232.
• Soibelzon, E., and D. C. Leon. 2017. Effects of climatic oscillations on the faunas. The Holocene Thermal Maximum and the displacement of armadillos in Argentina: Anatomical features and conservation. Journal of Archaeological Science: Reports 11:90-98.