Kevin,
horn material is Keratin. But it is not growing in thin air and needs something to adhere to - this is where the bones (more or less Calcium-Phosphate) come in handy
Try to cut through your fingernail sometime and look what is underneeth?
Every claw, nail, hoof or horn (headgear) for that matter will have a bone inside - except in Rhinos, which don't grow horn per se. The horn is growing as a sheat, as an outer layer on bone and soft tissue. It will quite easily decompose (spelling) and in fact is gone in the pic above.
If I assume it is what I think, then you could also make some adjucated guess as far as it's age is concerned and what had happened after all:
From the angle the point or shaft has entered we can assume it was shot by an indian from horseback as he had already passed the buffalo. If we could determine if it is right or left and front or rear, we could also tell at which side the Indian rode along. To me it looks like a left phalange, from left rear foot. So, I assume he undershot the belly of the buff as he passed on the right and hit the hind hoof with his arrow.
So, we need a right handed Indian on a horse and a buffalo to shot at. I have no idea about when horses where available to Indians in greater numbers nor when Buffalo Bill Cody shot the last, but IMO a timeframe between maybe 1600 and 1865 sounds plausible.
[A ballistic head on attac on a buffalo seems unlikely to have been tried. Shooting from a cliff or elevated stand would be an other, less romantic possibility to explain direction and angle the shaft had entered.]
Small remains can tell big stories, not?!
Falk