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Instead of Thrown, Spears Were Planted in the Ground to Kill a Charging Mammoth
(www.discovermagazine.com)
The grooves carved into each point could allow it to slide down the shaft upon impact. A fixed point, by contrast, would be more likely to shatter when it hit dense material, especially bone.
This is really interesting. And to further illustrate just how much we have no idea and might be wildly wrong, there's an incredible book, All Yesterdays, which reimagines prehistoric animals in interesting new ways. The second half of the book shows possible recreations of contemporary animals based solely on their skeletons to really drive home the point at how much guessing is involved in this field. Some of the images can be found here.
This is a rhino skeleton (wtf):
