Jim several folks on another site guessed prehistoric beaver and thats actually not bad. The huge beavers that roamed north America had massive jaw bones and muscles very similar to this........but much smaller.
Jon Piper (goodturns) hit it on the head! This is the lower jaw bone of a walrus. This particular specimen died before whites arived in Alaska but it's not much more than 100 years old. Giant beaver would be 10,000 and it's a beautiful chocolate brown color. Walrus jaw is very dense and a lovely cream/brown. It has very little spongiform matrix and turns wonderfully! Walrus eat mostly Mollusks and have evolved into powerful shell crushing machines!
Male walrus can reach 3500 pounds females 2000.......jr will be a 150 pound new born and quadruple his weight in just a few weeks.