The reason is a mystery. Blick speculated that the arm may have been a war trophy, buried with the dog “to symbolically keep the enemy at bay in the afterlife”

I love this haha I just got a book called “Motel of the Mysteries” and its about a civilization 2000 years in the future finding archeological artifacts from our time and they try to explain all of these. An example is looking at the highway system and claiming it was used for extraterrestrial landing.