Paper summary: Semprebon et al (2016). An examination of the dietary habits of Platybelodon grangeri from the Linxia Basin of China: Evidence from dental microwear of molar teeth and tusks.
Paper is available here for all of you playing along at home!
The part with the backstory:
Platybelodon is an extinct genus of proboscidean (elephant relative) that lived in Africa and southwest Asia about 10-15 million years ago. You might know of it from certain tumblr posts.
It’s famously been called a “shovel-tusker”, because its lower jaw looks like...well...a shovel.
(Image: The fossil skull of a Platybelodon. Its upper tusks are very small, but its lower jaw is extended and has two large rectangular “tusks”. [Source])
Take a closer look at that jaw!
(Image: The lower jaw of Platybelodon next to a typical coal shovel. They’re the same size and shape.)
Wow! That’s quite some jaw. You could clear your driveway off with that jaw, though I suspect it might make the Platybelodon a little grumpy. No wonder it’s often called a “shovel-tusker” — surely it must have been using that jaw for shoveling! And that’s exactly what palaeontologists immediately figured it did with it: shoveled water plants out of the bottoms of ponds, or shoveled roots out of the ground.
(Image: 1931 reconstruction of Platybelodon shoveling plants out of a pond. [Source])
The part with the actual paper
It’s pretty much been taken for granted that Platybelodon and its relatives (collectively called amebelodontids) used their shovel jaws for shoveling plants. Consequently, not much research has been shoveled into seeing if that was true. Sure, there were some studies throughout the 1900s (notably Lambert’s 1992 paper), but none of these looked quantitatively to determine what they were doing with the tusks.
That’s one thing to say. How the heck do we do that?
Well, as it turns out, teeth get worn down differently depending on how they’re used. They’ll be scratched from debris, or pitted from chemicals, or whatever. And these scratches are really small — so small that you need a microscope to see them. They’re called microwear, and they’re different depending on what teeth are used for.
In modern animals – like pigs – that use their tusks to dig, we see large scratches (from encountering grit in the soil) and large pits (from erosion due to chemicals in the soil). So we’d expect to see that on the tusks of Platybelodon, especially on the underside of the tusks.
Instead, they found that the lower tusks had really fine scratches and small pits, and they weren’t very different between the top and bottom of the tusk. The scratches were parallel to each other and parallel to the length of the tusk.
(Image: Microscope view of the microwear on the lower tusk of Platybelodon. There are a lot of very fine parallel scratches. Scale bar is 0.4mm.)
That’s weird! So what the heck were they using the tusks for then? We might be able to get a clue by looking at the cheek teeth, using the same method of looking at scratches and pits and comparing to modern animals.
Using databases of microwear of living herbivores, the researchers graphed living elephants based on the microwear of their molars. The results showed what was expected: African Bush Elephants and Asian Elephants were in the space ranging from grazers (animals that eat grass) and browsers (animals that eat mostly leaves and other brushy vegetation), while African Forest Elephants were squarely in the space that includes browsers. This matches with what’s known about the diets of each species. Good! We know that this method works for elephants!
(Image: Microwear graph of modern elephant species, showing average pits vs average scratches. Each dot represents an individual elephant. All three species of modern elephant are shown: Loxodonta africana, the African Bush Elephant, Loxodonta cyclotis, the African Forest Elephant, and Elephas maximus, the Asian Elephant. The Forest Elephant falls entirely within the space marked B for browsers, while the other two are scattered between browsers and grazers.)
What happens if we do this for Platybelodon? It falls completely within the range of browsers!
(Image: Microwear graph of Platybelodon, showing average pits vs average scratches. All individuals fall within the region marked B for browsers.)
When the size of the scratches on the molars were examined, they turned out to be much larger than those on the tusks. The authors interpret this as being similar to modern animals that eat a lot of bark. Thus, based on the scratch patterns of the tusks and of the molars, they propose that Platybelodon was not using its jaw as a shovel, but instead to strip bark from trees! (This doesn’t mean that it only ate bark, of course, just that it could.)
(Image: A more modern reconstruction of Platybelodon, feet planted firmly on the ground, standing near a tree that has its bark stripped off. Image by Julio Lacerda @paleoart. [Source])
What this tells us: We can’t always take “common knowledge” for granted in palaeontology! New research on old specimens can completely overturn what we thought we knew about animals, and a shovel-tusked swamp dweller can become a scraper-tusked landlubber when we examine the scratches on its teeth.
