Extra Armor Gave Glyptodon an Edge

Killing a glyptodont was no easy task. Prehistoric, bad-ass cousins of modern armadillos, these large mammals were protected by bony shielding on almost every part of their body. Some, such as Hoplophorus, even had modified tail clubs tipped with mace-like arrangements of spikes. Saber-toothed cats like Smilodon were surely formidable predators, but even they would have had a difficult time preying upon glyptodonts and shucking them from their shells.

At least one sabercat found a way around all that armor, though. Stored within the American Museum of Natural History’s massive Frick Collection of fossil mammals is the busted-up skull of a juvenile Glyptotherium texanum designated F:AM 95737. Tiny fractures run over the entire skull – damage done after death but before fossilization – but most remarkable are two oblong holes sunk into the frontal bones. These holes were likely made by a large saber-toothed cat (though a jaguar is another possibility), and, as assessed by paleontologists David Gillette and Clayton Ray, the apparent ease with which the predator dispatched its victim suggests that this Glyptotherium was stuck. Rather than a sabercat jumping out from nowhere and biting the glyptodont on the head, they reasoned, “It seems more likely that this juvenile was stranded, perhaps in mud, or was otherwise debilitated, unable to avoid an approaching predator.”

The single, perforated skull represents a lucky catch for a saber-toothed hunter, as well as paleontologists. Traces of predation on glyptodonts are rarely found. Juveniles – in which the armor plating had not fully ossified  – may have been more vulnerable than adult glyptodonts, but predation on these animals was probably more common than the small collection of damaged bones suggests. The recent discovery of additional armor accessories hints that some of these shelled mammals were in an arms race with the predators of their time.

Among the first of the glyptodonts to become known to scientists was, not surprisingly, Glyptodon. It lacked the tail club seen among some of its relatives – instead possessing a short tail encircled by spiny rings – but species of Glyptodon were just as well-armored. In addition to the head shield, bony shell, and tail rings, bits of bony armor were embedded on its underside, along its hind limbs, and in its face. Now, thanks to some well-preserved shells from two South American species, we know that Glyptodon had an extra fringe of spines along the margins of its carapace.

The accessory structures were described by Alfredo Zurita, Leopoldo Soibelzon, and colleagues last year from shells of the large, recent species Glyptodon munizi and Glyptodon reticulatus. Found in northern Argentina and bordering countries, these two species lived after the great interchange of American mammals that brought Smilodon, bears (the predecessors of the immense Arctotherium), and other large predators to South America. (Glyptodon munizi is older, dating from the early-mid Pleistocene, while Glyptodon reticulatus lived during the last 100,000 years before disappearing about 12,000 years ago.)

Well-preserved shells of both species had extra rows of outwardly-oriented bony knobs along the sides, culminating in a set of larger, forward-oriented spines in front of the head. These were not tightly-connected to the rest of the shell, explaining why they have been so rarely found, but in life they may have been wrapped in a tough sheath that would have made them even longer and spikier. These fringes of mini-spikes – so far as we know – are unique to these species.

The spiky shell curtains of the two Glyptodon species appear to have been defensive structures. They would have made it difficult for predators to attack the neck and forelimbs of the glyptodonts without risking injury, and Zurita and co-authors propose the additional spines were an adaptation spurred by new carnivores on the landscape. The origin of the spike fringes, the increase in glyptodont size, and the addition of extra osteoderms elsewhere on the body all follow the interchange of American mammals. Before this time the predatory mammals were smaller and not as diverse, and so it would be expected that new predators from the north would influence the evolution of the native South American herbivores.

Curiously, though, the glyptodonts that traveled to North America – like Glyptotherium – do not appear to have possessed the sharp shell fringe despite having lived alongside some of the same predators. Perhaps these structures have yet to be found intact among North American species. After all, it took over 170 years for them to be recognized in the South American Glyptodon. If the difference is real, though, then the same group of mammals – encountering similar predators on two different continents – became adapted in different ways. Either way, the glyptodonts were literally some of the toughest mammals of all time, and I can only imagine how a saber-toothed cat or short-faced bear would overcome all that armor.

Top Image: The glyptodont Doedicurus clavicaudatus, with a baseball player for scale. From Blanco et al., 2009.

References:

Blanco, R., Jones, W., & Rinderknecht, A. (2009). The sweet spot of a biological hammer: the centre of percussion of glyptodont (Mammalia: Xenarthra) tail clubs Proceedings of the Royal Society B: Biological Sciences, 276 (1675), 3971-3978 DOI: 10.1098/rspb.2009.1144

Gillette, D., and Ray, C. (1981). Glyptodonts of North America Smithsonian Contributions to Paleobiology, 40, 1-255

Zurita, A., Soibelzon, L., Soibelzon, E., Gasparini, G., Cenizo, M., & Arzani, H. (2010). Accessory protection structures in Glyptodon Owen (Xenarthra, Cingulata, Glyptodontidae) Annales de Paléontologie, 96 (1), 1-11 DOI: 10.1016/j.annpal.2010.01.001