The School of Hard Knocks: Finding the "Sweet Spot" on Glyptodont Tails

In the introduction to his most famous work, On the Origin of Species by Means of Natural Selection, the Victorian naturalist Charles Darwin began by writing;

WHEN on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species--that mystery of mysteries, as it has been called by one of our greatest philosophers.

Darwin would not turn his full attention to the "geological relations" of present and past life until much later in the book, but this opening passage makes it clear that the fossils he had dug out of the South American strata had made quite an impression upon him. Although he was not trained as a paleontologist, Darwin knew enough about anatomy and geology to recognize that the fossilized bones he found were the remains of large, extinct mammals. Among the remains he puzzled over where bits of bony armor, not unlike the armor of living armadillos, and Darwin wondered what kind of animal they could have come from.

Other naturalists were just as perplexed by the shards of ancient armor as Darwin was. Some thought that they had come from the extinct giant ground sloth Megatherium, an imposing animal whose remains had also been found in South America, but this did not seem quite right. As more of the armor was discovered and shipped off to Europe during the 1820's and 1830's other naturalists affirmed that the armor and associated bits of skeleton had come from an extinct variety of giant armadillo. Even though he stressed the importance of finding more complete specimens, in 1833 the German naturalists E. D'Alton formally interpreted the fossils in just this way.

By the time Darwin returned to England in 1836 there were serious doubts that the armor belonged to Megatherium (despite William Buckland's association of the two in his contribution to the Bridgewater Treatises). The armor had come from a different kind of animal that lived alongside the giant sloths, and in 1837 the Danish naturalist Peter Wilhelm Lund placed some of the controversial remains into the genus Hoplophorus on the basis of a few of the osteoderms and a few other bones. It was envisioned as an enormous armadillo, and its description was followed by the Richard Owen's restoration of the related genus Glyptodon in 1839.

(Up to six names were proposed for similar fossils at the time of Owen's description, but not all of these names are treated as valid today. Hoplophorus and Glyptodon are among those that remain in use.)

The resolution of this paleontological debate confirmed what Darwin had begun to suspect. He had seen living sloths and armadillos while in South America, but the modern forms had enormous, prehistoric precursors that lived in the same region.There had clearly been a "succession of types" in South America over time; the history of life on earth had been marked by major changes.

What Darwin went on to do is well known, but what of the glyptodonts? Discoveries of new types did not simply cease with Owen's description of the genus that gives the group their name. Many more species and genera have been named in the time since, and some of these ancient armadillo-relatives carried some pretty formidable weapons.

All glyptodonts had tails ringed by mobile bands of armor. This arrangement protected the tail while still allowing it to move. In some species, however, the bands towards the end of the tail were fused to make a long club similar to a baseball bat. (In some species the bony clubs even bear depressions where knobs or spikes would have jutted out, as in Doedicurus clavicaudatus.) Clearly these structures were weapons, but how were they used? In a new study published in the Proceedings of the Royal Society B paleontologists R. Ernesto Blanco, Washington Jones and Andres Rinderknecht looked to tools we use for fun to figure out how some glyptodonts swung their tails.

What the team of researchers was looking for was the center of percussion in a series of glyptodont tails. This would be the part of the rigid tail club that would have done the most damage without injuring the joints being used to swing it. Using baseball bats as an analog, they were basically looking for the "sweet spot" on the glyptodont tails.

What the scientists found was that the center of percussion was not at the very tip of the tail but on the side, about 3/4 to 2/3 the way up the tail club in the species examined. This is where the largest depressions for the attachment of knobs or spikes were situated, a finding consistent with the idea that this was the area that exerted the greatest amount of force on a target. The mammals would have thus swung their tails from side-to-side, and the upper part of the tail remained flexible to allow them to do this.

But why did the glyptodonts carry these weapons? The researchers argue that they would have been of relatively little use against fast-moving predators like saber-toothed cats. The glyptodonts would have needed pretty good aim to KO a Smilodon or similar threat. (Then again, such a club would have certainly been a deterrent to hungry predators. An angry glyptodont, hunkered down and swinging its tail from side to side, would have been anything but an easy meal.) Instead, the scientists suggest, the glyptodonts were fighting each other.

Since the late 19th century scientists have occasionally found glyptodont armor with fractures in it. What could have caused this damage? It probably was not a botched attack by a predator, the authors of the new study suggest, but wounds from confrontations between glyptodonts. Their outer armor, which was harder than bone, would have allowed them to withstand a fair amount of punishment, and perhaps some species engaged in ritualized competitions in which they beat on each other with their tails. If such competition was for mating opportunities then the selection of bigger spikes or knobs in the "sweet spot" of their tail clubs could have been selected for; whoever could do the most damage the fastest would win more bouts and perhaps more mates.

Inferring behavior directly from anatomy is tricky business, and unfortunately we will never be able to directly observe the habits of the glyptodonts. They probably did fight each other, but whether these confrontations selected for more elaborate and deadly tail clubs remains uncertain. It is an evolutionary hypothesis that will have to be confirmed by more evidence. Indeed, we should be wary of evolutionary storytelling. Just because we can think of an adaptive scenario does not mean that the particular trait in question evolved for that reason.

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