Fossil Friday – Harlan’s ground sloth femur

There are three different species of ground sloths known from the Pleistocene Diamond Valley Lake fauna. By far the most common of the three is Harlan’s ground sloth, Paramylodon harlani. For this week’s Fossil Friday we have a Paramylodon femur.

This is the right femur, seen from the back (posterior view), with the distal end on the right. The articular surfaces for the knee joint are visible on the right, but the proximal end of the bone is not preserved so the ball joint where the femur attaches to the pelvis is missing (it would have been at the lower left corner of this image). Even though the bone is crushed, it has maintained its shape pretty well. Like many of the larger bones in the WSC collection, this one has only been prepared on one side.

I’ve always found sloth femora fascinating, because they have a really strange shape when compared to most of the other mammals we find in Pleistocene deposits in North America. They appear to be very wide for their length, and generally kind of overbuilt for their size. To get an idea, compare this femur to the juvenile mastodon femur I wrote about a few months ago. These animals were probably roughly the same size, but the difference in their femoral proportions is striking.


On another note, we are busily preparing for tomorrow’s Inland Empire Science Festival. If you’re in Southern California we hope you can make it out to the Western Science Center tomorrow for this exciting event.


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