Fossil Friday – sloth jaw

Yesterday was International Sloth Day (with sloth in this case being a noun, not an adjective)! That’s a nice day to celebrate at Western Science Center, because Diamond Valley Lake is the only locality in California with three different species of ground sloths. 

By far the most common sloth at DVL is Harlan’s ground sloth, Paramylodon harlani. Shown above is a Paramylodon partial lower jaw. This is the anterior end of the right dentary seen in dorsal view, with anterior to the left. The long straight edge at the lower left is the mandibular symphysis, where the right and left dentaries attach to each other (essentially the chin). The bases of two teeth are visible at the upper right.

Here’s a lateral view:

This time the anterior end is on the right. In this view it’s clear that the teeth are broken off, and barely protrude above the bone. This is presumably breakage that occurred after the animal died, since the breaks are jagged.

This fragment makes an interesting comparison with one of the other DVL sloths, Megalonyx jeffersonii. Paramylodon seems to have a longer, more slender jaw than Megalonyx and lacks the latter’s enlarged, chisel-like incisors. That’s not entirely surprising; while Paramylodon and Megalonyx are both sloths, they are only distantly related to each other.


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