Fossil Friday – Harlan’s ground sloth jaw


For this week’s Fossil Friday we have a partial lower jaw of Harlan’s ground sloth, Paramylodon harlani, collected near the eastern end of Diamond Valley not far from the museum’s current location.

This particular fragment is the back half of the left dentary (the left and right dentaries are the bones that make up the lower jaw in mammals). The image above is the lateral (side) view with the front to the left. The small projection sticking out on the middle of the back edge is the mandibular condyle, the part of the jaw that forms the joint with the rest of the skull.

Here’s the medial view of the same jaw:


And here’s the dorsal (top) view:


In this view we can see that, while the teeth are missing, the tooth sockets for the 3rd and 4th molars are preserved, as well as the back and inner edges of the socket for the 2nd molar. The mandibular condyle also appears much larger from this angle, because while it is relatively short vertically it is quite wide transversely (from side-to-side).

There are several different species of sloths known from the Diamond Valley Lake region and that are represented in the WSC collections. Paramylodon harlani is by far the most common, making up over 93% of the individual sloth bones from the valley (Springer et al. 2010).


Springer, K., E. Scott, J. C. Sagebiel, and L. K. Murray, 2010. Late Pleistocene large mammal faunal dynamics from inland southern California: the Diamond Valley Lake local fauna. Quaternary International 217:256-265.


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