Fossil Friday – sloth ulna

IMG_3670 Last Saturday the Western Science Center was visited by Eric Scott from The Cooper Center and high school student Santiago Hernandez. They came with a specific goal in mind, to look at fossil horses from a small, understudied collection, called the Harveston Collection, from western Riverside County.

While the Harveston Collection is dominated by horses, there are several other kinds of animal present, including bison, camels, deer, and mammoths. As we examined each drawer looking for horses, Eric noticed the bone shown above, which was labeled in the field as a juvenile bison humerus. We both immediately agreed that this bone was not a humerus, as is lacked the distinctive articulations found in the humerus at both the proximal and distal ends. After a few minutes of debate, we decided that the bone looked more like a damaged ulna (a bone from the forearm), perhaps from a ground sloth. Conveniently, the ground sloth Paramylodon was common at Diamond Valley Lake, and we have a cast mounted skeleton on exhibit. So, with the horses temporarily forgotten, we rushed off to the exhibit hall to compare the Harveston specimen to Paramylodon (thanks to Santiago for the photo):

It turned out that our suspicion was correct; allowing for breakage, the bone is a perfect match for the right ulna of Paramylodon. This was a neat discovery, as this is the only sloth bone currently identified from the Harveston Collection (although I think sloths had previously been found at nearby localities). There are new discoveries to be found in museum collections all over the world.


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