Annual growth lines (arrows) in the thigh bone of Hypacrosaurus show rapid growth when the dinio was young (bottom of the image). The closer together the growth rings are the slower the growth rate. That's because there is less bone being formed between each year at that point in the dinosaur's life. The tightly packed growth lines (bracket) reflect the attainment of full grown size. (Scale bar is 1 mm) Credit: Lisa Noelle Cooper, NEOU and Kent State University.