More than just horns

Few animals exemplify the weird and wonderful creatures that evolved in South America like horned armadillos. But having horns is one of only several features that make these ancient armored animals so atypical.

FIg. 0 Peltephilus.jpeg
Reconstruction of the best-known horned armadillo, Peltephilus. By Velizar Simeonovski, copyright D. Croft, from Horned Armadillos and Rafting Monkeys: The Fascinating Fossil Mammals of South America.

For one, horned armadillos – technically known as peltephilids – have a heavily built skull and well-developed teeth. This isn’t particularly strange for most mammals, but it is quite odd for an armadillo, most of which have a long, narrow snout, slender lower jaws, and simple, peg-like teeth. By contrast, the snout of a horned armadillo is quite broad, its lower jaw is stoutly-built, and its teeth are relatively large and triangular in cross-section. What’s more, horned armadillos have teeth at the front of their jaws in addition to along the sides, an arrangement unlike that of any other armadillo (but typical of most other mammals). Some paleontologists once thought that these features meant that horned armadillos fed on meat and preyed on other mammals, but a detailed biomechanical study concluded that they more likely dined on underground roots and tubers and perhaps also worms.

Fig. X Cranium.jpg
The cranium of the early Miocene (~18 million-year-old) horned armadillo Peltephilus ferox as viewed from the side (left) and below (right). Note how its teeth are chisel-shaped and form a complete arc from one side of the mouth to the other. Other armadillos have no front teeth, only teeth along each side of the jaw. Modified from Ameghino (1894:fig. 66).

Another peculiar feature of horned armadillos is their armor. Like other armadillos, their shell (carapace) was made up of hundreds of individual skin bones known as osteoderms. (This term should be familiar to you if you have read other Rafting Monkey posts!) But unlike the shells of other armadillos, the carapace of a horned armadillo lacked solid plates of osteoderms at the front and back (known as bucklers) as well as broadly-overlapping bands of osteoderms between them. Instead, a peltephilid shell consisted entirely of osteoderms that overlapped one another to a small degree, creating a very flexible armor. To put this in medieval terms, modern armadillos have something akin to plate armor, whereas peltephilids had something more like mail (chain) armor. This alternative arrangement probably worked just fine for to protect peltephilids from predators (or at least as well as other types of armadillo armor), but it has an unfortunate consequence for paleontologists: no one has yet found a complete peltephilid armadillo shell. This is probably because the bones of their armor separated from one another much more easily after death than those of more heavily armored species.

Fig X Osteoderms.jpg
Six isolated peltephilid osteoderms, from the early middle Miocene (~ 15 million-year-old) site of Cerdas, Bolivia. Note how nearly the entire surface is rough, with only a very small smooth area along the top edge where it would have been overlapped by another osteroderm. The two large pits in each osteoderm would have housed a bundle of hairs and associated glands. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

So what’s the deal with those horns?

“Horn” is one of those words that can have a very specific or a very general meaning depending on the context. For most people, horns are simply structures that stick off an animal’s head. We zoologists haven’t helped matters by using the term horn to refer to a wide variety of such appendages. In addition to long-horned cattle and horned armadillos, there are horned beetles, horned toads, and horned larks, all of which have horns that are made of very different materials.

Even within mammals, structures called horns can vary significantly. In the strictest sense, horns are structures with a core of bone and a covering of keratin (the material that makes up hair and fingernails), neither of which is lost naturally during an animal’s lifetime (unlike antlers, which are shed annually). These type of horns are only found in one family of mammals, Bovidae, the largest living family of ungulates, which includes many familiar animals such as cows, sheep, goats, and antelope. Rhinos are also described as having horns, but these are made purely of keratin. It doesn’t make them any less dangerous, but it does mean they evolved independently from the horns of bovids. Which leads us to horned armadillos…

Fig 3 Ronwolffia.jpg
Side view of the skull of the late Oligocene (~ 25 million-year-old) horned armadillo Ronwolffia pacifica from Bolivia. Note how the horn is merely sitting on top of the skull and is not attached to it. Image modified from Shockey (2017:fig 7F).

The horns of these armadillos are unlike those of any ungulate: they are pointed osteoderms (!). Osteoderms form in the deep layer of the skin (the dermis) and are covered in life by epidermal scales made of keratin. These scales are not preserved in fossils – only the osteoderms themselves – and it is likely that the horns of these armadillos were probably also covered by keratin in life. This would have made them somewhat larger than the osteoderms themselves. In terms of tissues, this makes them similar to the horns of bovids. However, bovid horns are extensions of bones of the skull and remain fused to the skull when an animal dies. The horns of horned armadillos were only connected to the skull by soft tissue. After death and decomposition of the soft tissues, the horns detached, just like the rest of its osteoderms. Thus, their exact position (and even number) has been a point of discussion among paleontologists. Florentino Ameghino believed Peltephilus had two pairs of horns (a larger pair in back and a smaller pair in front), but subsequent workers, such as William Berryman Scott, have not been able to verify that this was the case. Since these horns are solid pieces of bone, they are found more commonly than skulls themselves.

Why did this particular group of armadillos evolve horns? That is difficult to say, mainly because there is no horned mammal alive today that is similar in size and habits. However, one other group of extinct mammals does compare favorably: a North American group of mountain beaver relatives known as mylagaulids. Like peltephilid armadillos, mylagaulid rodents were medium-sized, accomplished burrowers, and sported a pair of horns on their snout. A study of mylagaulid paleobiology concluded they used their horns for defense and suggested that peltephilids probably used them for the same purpose.

Fig. 4 Epigaulus 2.jpg
The horned North American rodent Ceratogaulus hatcheri (previously known as Epigaulus hatcheri) in the Evolving Planet exhibit at Chicago’s Field Museum. Note how its horns are solidly fused to its skull, in contrast to the the horns of peltephilids. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Despite the specialized horns of peltephilids, it seems they represent a very early branch of the armadillo evolutionary tree, maybe even the first branch for which we have fossil evidence. This is because many of their more unusual features – such as their well-developed teeth and loosely articulating armor – seem to be ancestral traits for armored mammals. Unfortunately, we know very little about the evolutionary history of peltephilids. Most species are known solely from osteoderms or poorly-preserved skull fragments. Some well-preserved skulls of Peltephilus are known from the early Miocene (about 15-20 million years ago), but this represents just a fraction of the group’s history, which probably spanned most of the past 66 million years.

One thing we can definitively say about peltephilids is that humans had nothing to do with their extinction. They are last recorded during the late Miocene, around 10 million years ago, long before humans arrived in South America (or had even evolved in Africa). Whether their extinction was due to changing habitats in South America, some unknown interaction with other animals, or simply bad luck will likely never be known. Nevertheless, thanks to their extraordinary headgear, they are one group of splendid South American mammals that will not soon be forgotten.

References Cited:

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