The Great Discovery

Mr Gray needs something spectacular to show for the grand reopening of his museum. So when he receives a call about a 'very old thing' that has been discovered in a garden, he is very keen to investigate.
What he finds there is a ginormous bone. Then another, and another.
Could the bones be from an elephant? A rhinoceros? Or even a hippopotamus?
When a little dog helps him uncover an enormous antler, Mr Gray thinks he might have solved the puzzle and found a very special exhibit for his grand reopening, too!