Fossil Friday – Megalonyx tooth

megalonyx-jeffersoniiOn January 17, Western Science Center is kicking off a new monthly lecture series. Our first speaker will be H. Greg McDonald from the Bureau of Land Management, who is going to be talking about ground sloths, animals which make up a substantial part of the Diamond Valley Lake fauna.

One of the the more rare sloths from Diamond Valley Lake is Megalonyx jeffersonii, which is known from fewer than a dozen bones including the isolated tooth shown above. Sloth teeth are unusual compared to most other mammals. The teeth have no enamel, and instead are composed of different layers of dentine and cementum. There are no deciduous teeth, and the teeth that are present grow continuously through the sloth’s life. With no deciduous teeth, no enamel, and very little variation between different tooth position (with the exception of the first tooth and the last tooth in some species), it’s actually very difficult to tell which teeth are actually represented in the sloth’s jaw.

The Megalonyx tooth shown above is a cheek tooth, with the occlusal surface of the tooth at the top. When the tooth first starts growing, it is a simple peg at the the occlusal end. The sculptured surface is entirely the result of wear from food and from occlusion with the opposing tooth, so the usual methods of tracing tooth evolution by examining variation in the occlusal surfaces don’t work with sloths.

If you’re in the area, I encourage you to come by the museum on January 17 for Dr. McDonald’s lecture to learn more about sloths.


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