The Belted Ones

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).

Armadillo Armor.jpg
Parts of an armadillo carapace separated into different components and viewed from above (upper image) and as a single structure viewed from the side (lower image). In the lower image, note how the carapace can be separated from the rest of the skeleton. In life, it is connected by muscles, ligaments, and other soft tissues. Two different species are represented in this figure; a three-banded armadillo (Tolypeutes sp.; above) and a pichi (Zaedyus pichiy; below). Modified from Soibelzon and Leon (2017:fig. 2).

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.”

Tolypeutes.jpg
Tolypeutes matacus, the southern three-banded armadillo. Note the near-perfect ball it forms when it rolls up. Left photo by Brian Gratwicke; reused under CC BY 4.0. Right photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

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).

Propalae Hinged Carapace.jpg
Lateral view of the carapace of the 17 million-year-old glyptodont Propalaehoplophorus australis from Santa Cruz province, Argentina. The red arrows indicate areas of overlapping osteoderms in the shoulder area (left is toward the front of the glyptodont). Note that they do not extend across the entire carapace, unlike the bands of armadillos. Modified from Scott (1903:pl. XVII).

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.

Zaedyus and Chaetophractus Shells.jpg
Left: Most of the carapace of an immature pichi (Zaedyus pichy). Nearly all of the keratinous scales have fallen off, though a few are visible near the holder’s thumb. What remains are the osteoderms themselves and the pattern of grooves and small foramina left by the scales and hair follicles. Right: Close-up view of a taxidermied specimen of a screaming hairy armadillo (Chaetophractus vellerosus). Note the prominent long hairs and the many small scales covering the bony osteoderms. Photos by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

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:

2 thoughts on “The Belted Ones

  1. Thanks Darin, this is so interesting! Question: on the cellular level, what is the structure of the carapace? If the osteoderms are embedded in skin cells, how do cells attach to one another, and how does the bony carapace structure remain intact when the skin decomposes?

    Like

  2. Hi Catherine! The osteoderms are embedded in the dermis, and adjacent osteoderms are held together by small interdigitations of bone and Sharpey’s fibers (connective tissue) in addition to the skin itself. Like skin, the Sharpey’s fibers eventually disintegrate after death. If an armadillo is fossilized relatively quickly and not disturbed, its shell will be preserved intact with all the osteoderms in position. But the carcass is on the landscape for an extended period, the carapace will usually break down into individual osteoderms before being fossilized. Even if a carapace enters the fossil record complete, it must be collected as soon as it is exposed again, or it will begin to break apart. Finding a fossilized armadillo skeleton at exactly the right time requires a lot of luck, and for that reason, complete carapaces of armadillos are quite rare in the fossil record. Even large portions of carapace are relatively uncommon.

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s