This is a very nice, 5.1" long metatarsal (toe bone) of the dinosaur Pachycephalosaurus ("thick headed lizard"). It was collected from the Hell Creek Formation in Montana. The bone is in very good condition with just some minimal restoration to fill some cracks.
Pachycephalosaurus was a bipedal dinosaur that lived during the Late Cretaceous period and probably was about 15 feet in length.
Pachycephalosaurus had a distinctive, large, bony, dome on top of it's skull up to 10 inches thick to cushion it's brain from impacts.
Some paleontologists believe this thick skull may have been used for head-butting, much like rams while others contend it may have been a sexual display.
It was one of the last non-avian dinosaurs before the K-T extinction event, 65 million years ago.
There is only one known species of Pachycephalosaurus, Pachycephalosaurus wyomingensis. It's remains have been found in Montana, South Dakota and Wyoming.
It was either a herbivore or an omnivore with small leaf shaped teeth which would have been very effective at shredding plants.
An artists reconstruction of Pachycephalosaurus. By Jordan Mallon