Sloth anatomy challenge

Species evolve in the most astonishing ways! That idea was reinforced for me recently when I was rereading Greg McDonald’s thesis and stumbled across a note about Megalonyx teeth that I hadn’t caught previously. I’ve written before about sloth teeth and the error of assuming differences between sloths and other mammals are evidence they were inept or stupid and maladapted.  Some remarkable mammals owe much of their success to abandoning the “normal” patterns of the so-called “higher” mammals and following the path of ground sloths. Greg cites an amazing example . . . as my generation would say, the implications are mind-blowing (image borrowed from)

 

So here’s the challenge:  There’s a living mammal with teeth that bear a remarkable functional resemblance to those of Megalonyx (excluding other Xenarthrans). That is, its teeth are made up entirely of dentin wrapped in a layer of cementum, and as in sloths they are self-sharpening and ever-growing. What animal is it and what special advantage does this adaptation give it to survive in its unique niche?  

 

No fair running out and getting Greg’s thesis.  Think about it and send in your guesses–remember to include your rationale. . . you are in for a surprise.  The answer in two weeks. . .  . Dave 

  1. Pete Eyheralde’s avatar

    Given the self sharpening, ever-growing clue, my first guess would be something in the Order Rodentia. Beavers, woodchucks, squirrels and the like. But I’m pretty sure that the orange outer layer of material on a rodent’s incisors is enamel, not cementum. So I’ll have to keep looking….

  2. David’s avatar

    hi Pete, right you are. . . it wouldn’t be that easy! You’re the real naturalist in this bunch–is that orange enamel on all sides of the teeth or just the out-side? A different kind of enamel? Different hardness? It’s not merely a different color is it?–what would be the point?

  3. Pete Eyheralde’s avatar

    If you look at a rodent’s four incisors, the orange enamal is mostly on the front surface of the teeth. There’s a little overlap on the sides, but not by much. The enamel is much harder than the white dentin, which makes up rest of the tooth. When chewing down trees or through walnut shells, the softer dentin wears away faster than the enamel. This creates a chisel-like (and self sharpening) cutting surface to the teeth as they wear.

  4. David’s avatar

    Thanks Pete, I didn’t know the non-orange part was 100% dentin. That makes sense with what I’ve read about the sloth’s teeth. I understand the rodent arrangement creates a very sharp but ragged edge, with the dentin wearing down faster and leaving the enamel layer unsupported and easily chipped. If true, that may be a helpful clue to the puzzle.

  5. Amanda’s avatar

    I would have had the same initial guess as Pete. You really didn’t give much of a clue for novices like me.

    I saw you in the paper today (http://www.press-citizen.com/apps/pbcs.dll/article?AID=/20081017/NEWS01/810170340/1079). Shoot me an email when you get a chance. We need to catch up!

  6. David’s avatar

    Hi Amanda. good hearing from you. Hint huh? Any combination of enamel and dentine is going to produce a jagged edge because of the big difference in hardness. 100% dentine (the cementum has a very similar hardness) means an edge which can be self-ground smooth and sharp. Think about who would need sharp teeth but not necessarily hard and why.

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