Laelaps

i-27acb84a396be24b210f81d4c9a0df39-baseball-player-and-glyptodont-thumb-500x226-20777.jpg

My new favorite illustration from the technical literature; a baseball player compared to the glyptodont Doedicurus clavicaudatus. From the Proceedings of the Royal Society B paper.

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.

ResearchBlogging.org

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.

i-2d4c1f3d7a74e64a7310bb46699f9f50-glyptodont-tail-assortment-thumb-310x500-20780.jpg

A lineup of rigid glyptodont tails. A) Neosclerocalyptus, B) Pseudoplohophorus absolutus,C) Panochthus tuberculatus, D) Castellanosia, E) Doedicurus clavicaudatus. From the Proceedings of the Royal Society B paper.

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

Comments

  1. #1 DD
    October 15, 2009

    Glyptodont Golf?

    They were a bit low to the ground. Cricket perhaps?

  2. #2 Jared
    October 15, 2009

    I think this is where “just-so stories” of natural history are often confused with evolutionary history. Precisely as you point out about making statements of inferring behavior from morphology. Some of it can be inferred however, given conditions such as the armor damage discussed in this paper, or, for another example, evidence of teeth of a certain predator on the bones of another animal. I think being careful not to overstate the evidence is important, but animal interactions, to some extent, can certainly be understood. You brought up Eustenopteron recently, and this is an example of overstating behavior from anatomy (the image of this particular group on land).

    It is unfortunate that we will never be able to observe their behavior, but we can get some clues, and I think these are useful in understanding the interactions of these wonderful organisms with others.

  3. #3 Zach Miller
    October 15, 2009

    Megan Arbour just published a similar paper regarding ankylosaur tails. It’s surprising how much ankylosaur and glyptodont tails have in common: both have fairly mobile bases, very stiff “handles” (though some mobility between the caudal vertebrae is likely), and large clubs at the end.

    Given the proposed intraspecific combative purpose for glyptodonts here, I wonder if ankylosaurs used their clubs in a similar manner. There is circumstantial evidence to suggest that ankylosaurs used their tails for defense against predators, however–note Sue’s badly broken tibia.

  4. #4 afarensis, FCD
    October 15, 2009

    That would make a great t-shirt…

  5. #5 Zen Faulkes
    October 15, 2009

    For those interested in another summary and analysis this paper:

    http://neurodojo.blogspot.com/2009/08/were-glyptodonts-clubbed-tails-weapons.html

  6. #6 Laelaps
    October 15, 2009

    Jared; When I averred to “just so stories” I was not talking about inter-glyptodont combat so much as the idea that the tail clubs and armor evolved as a result of that combat. I have no problem with the idea that glyptodonts fought each other, but I just wanted to stress that we should take care when formulating adaptive scenarios to explain a particular feature.

  7. #7 John McKay
    October 15, 2009

    Zack beat me to the ankylosaurus comparison. While I doubt there is a ecological niche that is best filled by large armored grazers with nasty clubs on there tails, it seems that that particular defensive arrangement must provide a special enough advantage for it to pop up twice.

  8. #8 Tanystropheus
    October 15, 2009

    I seem to recall reading that certain glyptodont specimens were interpreted as having been overturned in life, allowing certain predators to devour their unprotected underbellies. Some people have suggested that Megatherium may have occasionally gone after these beasts. Is this actually supported by the evidence, or is it mere unsubstantiated speculation?