Horse teeth! Answer to FoW #37

Horse teeth

Several of you recognized that these two teeth are from a horse. Horse teeth are very distinctive; note the squareish shape of the chewing (occlusal) surface of the tooth. In side view, this gives the tooth an overall rectangular appearance. These are upper premolars (PM2 & PM3); lower cheekteeth will have a more rectangular occlusal surface. A “tall” tooth like this, with a high crown, is called “hypsodont”. This means that the enamel of the tooth extends down the length of the tooth, well past the gumline. In the side view (below) I drew in the approximate location of the gumline, about one-quarter of the way down from the occlusal surface. This type of tooth is different than a low-crowned tooth, such as in humans, dogs, cats, etc., where the enamel stops at the gumline.

Gumline in high-crowned (hypsodont) horse teeth.

Also you can see how complicated the cusp pattern is in the top photo! There are ridges of enamel running down the length of occlusal surface, and some of the ridges are infolded and run across the tooth surface as well. This is called a lophodont crown pattern, and is found in mammals that are primarily grazers. If you’re eating coarse vegetation such as grasses all day long, you need a tooth with a tough occlusal surface made up of ridges of very hard enamel. Grasses contain a large number of phytoliths, microscopic granules of silica, which are very abrasive and can wear down a tooth pretty quickly. So a hypsoodont tooth is an adaptation to a grazing diet because of the phytoliths in ….

WHOA, HOLD EVERYTHING! WE INTERRUPT THIS BLOG FOR A SCIENCE UPDATE!!! Recent research has shown that the adaptation of a high-crowned tooth probably happened as a result of the large amount of grit ingested from the soil when grazing, rather than from the grass itself! Paleontologists looked at when the extensive grasslands originated in the Great Plains of North America, and discovered that high-crowned teeth evolved in different mammal groups either millions of years before or after the grasslands developed. So the teeth likely didn’t evolve in response to the abrasive grass but from another cause, the grit itself! Think of how dirty your hands get when weeding your garden or lawn, and imagine you’re eating that stuff! There’s a lot of grit and minerals in the soil that would wear down even the hardest material in an animal’s body: tooth enamel.

So back to our two teeth; how do we know that these aren’t from a cow, cows are grazers and have high-crowned dentition!? Well, if we compare the crown pattern of cattle teeth and horse teeth we can see the difference (below). Cattle have a selenodont crown pattern which means that the crown is composed of a series of “half moon” shapes, called selenes, which run the length of each tooth. If you know your Greek mythology (which I don’t!), Selene is the goddess of the moon.

Upper toothrow of domestic cattle


Upper toothrow of a horse

OK, we’ve established that they are horse teeth, but Bob brought up a good point: how old are they!? We know that horses lived in Ohio during the Ice Age, and became extinct in North America after the end of the Pleistocene. They were then introduced back into North America by Europeans at about 1500 A.D. and have been here ever since. Because the modern horse and the Pleistocene horse are so similar morphologically, they are very difficult to separate – especially by just a tooth or two. Documented Pleistocene horse remains from Ohio are pretty rare, so most bones and teeth discovered are probably from the modern horse. We would have to find horse bones in Pleistocene deposits to prove that they are indeed from the Ice Age, and we’d have to be sure that it’s not a modern horse that was simply interred into Pleistocene sediments! So the next time you’re out horsin’ around, keep your eye out for these distinctive looking teeth!


Posted June 9, 2015

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